i | i ae NS ee aero aa es — Sena enn SSS =: SS = - rae ac pap oo a come a : a ——————— : : Se na cc a A a RS A AN : = = _ See OO = —= a == : = . : | —- aS ne ne re nena eee re \ q ee AAC AAT IAN ALAA | HD NILE WT } \ Mit ANY) ik} } ii " i ye ah LAY y aA “1's 47 Agricultural Science Series L. H. BAILEY, Eprror THE NATURE AND PROPERTIES OF SOILS AGRICULTURAL SCIENCE SERIES UNDER THE EDITORSHIP OF L. H. BAILEY THE NATURE AND PROPERTIES OF SOILS, by T. Lyttleton Lyon and Harry O. Buckman THE NATURE AND PROPERTIES OF SOILS A COLLEGE TEXT OF EDAPHOLOGY T. LYTTLETON LYON PROFESSOR OF SOIL TECHNOLOGY, CORNELL UNIVERSITY HARRY O. BUCKMAN PROFESSOR OF SOIL TECHNOLOGY, CORNELL UNIVERSITY jdew Bork THE MACMILLAN COMPANY 1922 All rights reserved PRINTED IN THE UNITED STATES OF AMERICA CopyriGcHT, 1922, By THE MACMILLAN COMPANY. Set up and electrotyped. Published April, 1922. APR 19 1999 Oc aA659657 INnG | TABLE OF CONTENTS CHAPTER PAGE I. Somr CONCEPTIONS OF THE SOIL AND ITS RELATION TO PLANTS ey ee ats eon O5-\ ae aga ee RE eRe he oe 1 II. Som Forming PROCESSES SE Aly plist Seba! BAT eee eee 16 IIT. THE GEOLOGICAL CLASSIFICATION OF SOILS... . . 38 IV. Tue Som PARTICLE AND CERTAIN IMPORTANT RELATIONS 66 Ve Tam OrGAnic MATTER OF THE Som. 5 1. 9. 2. 99 Vi. THe ContomaL. Matter OF THE Som . . . . . . 27 VII. Som StructuRE AND Its MopIFICATION. . .. . . 139 VIII. THe Forms or Som WATER AND THEIR CHARACTERISTICS 151 IX. THE WATER OF THE Sorin IN Its RELATION TO PLANTS . 184 Xe HE CONTROL OF SOM, MOISTURE 3/2 9s) 4 meee « 202 EXO SOLA ELIOAUD os oe iee | Gok a Reems mel tar meu US Vee 2S Rallies SOT pATR ey Ose eS Wee Meare em (RE A mi ats Ph Me Ay XIII. THe ABSORPTIVE PROPERTIES OF SoS... . . . 263 XC REE’ SOM ag OOLULION = sai le. ene ek aren veh apeiron Iecilio XV. THE REMOVAL OF NUTRIENTS FROM THE SOIL BY CROPPING AND PUBACHING cyl. 4c) poate ar yes arte gay cn se 289) ROVE CHEMICAT: -ANATVSIS OR SOILS 9. 5 «4 = |. = 4 toll PRG SAT CAT TE e SOLES! cnt esay Vs citebe seu, Men en) Veil oe) | Pent oon) Sas B28 QV eC SOM ACIDITY 500 ek) we Wie es oe hel eS oa eee, OSD XGOXS eH IMINGS THEE SOM! <2) irc, seus ele) 8%, 6 ee) S62 XX. Som ORGANISMS, CARBON, SULFUR AND MINERAL CYCLES 384 XXI. Som OrGANISMS—THE NITROGEN CYCLE . .. . . 409 XX CoMMERcIAL WertmizeR MATERIAES . . . © . « 442 XXIII. THe PRINCIPLES OF FERTILIZER PRACTICE. ... . 471 NOX Ee ARNO NTAN URE cng, s em Uo eM Soa’ Bh eee ale 409 POXQVAUGRERN: IMIANURE so io a fouls te 3 oe el es te) Hen OSD XXVI. THE MAINTENANCE OF SOIL FERTILITY . . . . . . 502 ENDERGOR PAU THORS ss Wsihyobe | se) 0 feck nae eays ee uae (OGL INDE OF ISUBUEOT MATINR F) 4) 5c ek ee) eh 86T Vv NATURE AND PROPERTIES OF SOILS CHAPTER I SOME CONCEPTIONS OF THE SOIL AND ITS RELA- TIONS TO PLANTS Due to the action of climatic agencies the outer solid por- tions of the earth readily pass into a loose and disintegrated condition. This layer, although superficial and insignifi- eant in comparison to the bulk of the earth, has performed and is still performing a marvelous function. Life on the earth has been slowly but steadily developing and changing until we see about us the forms that characterize our age. This evolution has depended to no small degree on this super- ficial layer of decomposed rock with its admixture of de- caying organic matter which together form the soil. In this medium many and varied organisms have lived and from it have drawn, wholly or in part, their sustenance, leaving as a recompense a contribution of organic debris, which in its turn has given rise to reactions of almost unbelievable com- plexity. Like the life which it has sustained and nourished, the soil has been changing and evolving. The soil of today is not the soil of yesterday nor will it be the soil of tomorrow. It is never still. It is continually seeking a mechanical and chemical adjustment with the forces which surround it or 1 2 NATURE AND PROPERTIES OF SOILS are active within its precincts. Such an equilibrium it never attains and thus the evolution goes on and on. It is this continual change and this endless response to environment that makes the soil useful to plants. The disintegrating rock and the decaying organic additions are thus converted into a mechanical support for plants, while at the same time they are forced to liberate the nutrients essential to plant growth. In the light of its origin and function the soil may be defined as a mixture of broken and weathered fragments of rock and decaying organic matter, which covers the earth in a thin layer and supplies mechanical support and in part sustenance to plants. This debris of rock and plant residue, teeming with its microscopic life and ever restless in its endless efforts at equilibrium, is the arable soil from which man must obtain his bread. As the light of investigation is thrown on it, new changes, new functions and new and unsuspected re- lationships are brought to view until the story of the soil may be retold with a clearer insight into those processes that render it useful to man. 1. Composition of the soil—The soil as defined is com- posed of two general classes of material, mineral and organic. The former in most cases makes up from 90 to 99 per cent. by weight of the dry substance of a soil, the organic matter, except in the case of peat and muck, being in rela- tively smaller amounts. In spite of the low proportion of organic matter its presence is vital, not only because of its influence physically but because of the nutrients, especially nitrogen, that it carries. The mineral portion of a soil functions as a frame-work and as a source of certain chem- ical elements, which are necessary to proper crop growth and development. It must be realized at the very outset that the two main constituents in a normal soil exist in very intimate relation- SOME CONCEPTIONS OF THE SOIL 3 ship, reactions occurring not only within each group but between the groups as well. Unless such interactions take place it is unlikely that the mixture will ever be in a con- dition either chemically, physically or biologically to sus- tain plant growth. These reactions, although very complex, take place with surprising ease and rapidity. As a con- sequence the study of this complex, heterogeneous and highly dynamic mass that we call the soil is often be- set with difficulties that completely baffle our pres- ent facilities for its study. 2. Soil-forming rocks.' Ine any study of ‘soil origin or composition, how- ever cursory, the geological phases of the problem im- mediately force attention. This is due to the bearing that certain geological phe- nomena have on soil condi- tions and crop growth. In fig, 1,.—Volume composition of a the soil we find that the loam soil when in good condition : : : for plant growth. The air and inorganic materials have water in a soil are variable and originated from the com- their proportion determines to a considerable degree the productiv- mon rocks. The best known ity, country rocks are of course involved because they present the greatest outcrop surface and of necessity must contribute most to the mineral fabrication of the soil. They are classified under three heads—igneous, sedi- mentary and metamorphic. The most important types from the standpoint of soil formation are the following: 1For excellent non-technical discussions of rocks and minerals:— Pirsson, L. V., Rocks and Rock Minerals; New York, 1915. Merrill, G. P., Rocks, Rock Weathering and Soils; New York, 1906. + NATURE AND PROPERTIES OF SOILS Igneous Sedimentary Metamorphic Granite Limestone Marble Syenite Dolomite Schist Diorite Shale Slate Gabbro Sandstone Quartzite Basalt Conglomerate Gneiss The mineralogical complexity of rocks has an important bearing on the question of soil formation and soil composi- tion. The fragments of any soil are, for the most part, dis- tinguishable as separate minerals rather than as mineral aggre- gates. For example, a soil from a granite would be char- acterized by separate grains of quartz, orthoclase, micro- cline and perhaps mica rather than by fragments of the orig- inal granite itself. Again, it is the composition of the easily decomposable minerals rather than the composition of the bulk rock that determines what simplifications shall occur, what new substances shall arise in the soil and what elements shall be liberated for plant use. 8. Soil minerals Although hundreds of minerals have been identified, comparatively few are common or important * in rock formation. As a consequence, the list of im- portant minerals found in soils will be correspondingly cur- tailed, although enough are always present, especially in the finer portions, to make the soil very complex mineralogically. The minerals as to origin may be divided into two groups: (1) those that persist from the original rock and (2) those that are produced by the decomposition of the original min- erals, during soil formation. For example, the quartz grains 1The following table indicates the approximate proportions of the common minerals in the earth’s crush to a depth of ten miles: BlelASPALS citer erate BTS Clay. ccivesnd) te ce ohare 1 eee Amphibole and Py- Carbonates: i.ctaectect MOXENE Vee eres Ne eerie UGHO = brie) -oobcadcrc0cc. 3 QUES Sobecosovvence IPAS SN OHS cose teojo06e Cc 8.2 ITT CE pate sshletsiers atts Breve aie 3.6 Recaleulated from Clarke, F. W., Data of Geogr REE U. S. Geol. Survey, Bul. 695, pp. 32-33. 1920. SOME CONCEPTIONS OF THE SOIL 5 of soil almost always come directly from the original rock as do particles of orthoclase, biotite, and apatite. Hematite, the kaolinite group and the chlorite and epidote groups generally originate in soils through weathering. The fol- lowing list of minerals is by no means complete, yet it includes the more important forms from the soil and plant standpoint. A LIST OF THE MOST IMPORTANT SOIL MINERALS. (The elements in bold type are those necessary for plant nutrition.) 1. Quartz Sid, 2. Orthoclase and KAISi,0, Microcline feldspar 3. Muscovite mica KH,A1,8i,0,, 4, Biotite mica KHMgFeAl,Si,0,, 5. Plagioclase feldspar Ca and Na aluminum silicates 6. Calcite and Dolomite CaCO, and (Ca, Mg) CO, 7. Hornblende and Augite Ca, Mg, Fe aluminum silicates 8 9 . Olivine (Mg, Fe).SiO, . Apatite Ca, (PO,). (Ch EP) 10. Kaolinite group Typified by kaolinite. H,Al1,Si,0, 11. Serpentine and Tale Hydrated Mg silicates 12. Chlorite group Hydrated Mg, Fe aluminum silicates 13. Epidote group Hydrated Ca, Fe aluminum silicates 14. Hematite FeO, 15. Limonite group Typified by limonite 2 Fe,0.,. 3) HO 1 Below are some of the most important mineralogical investigations of soil: McCaughey, W. G., and Williams, H. F., The Microscopic De- termination of Soil-Forming Minerals; U. 8. Dept. Agr., Bur. Soils, Bul. 91. 1913. Plummer, J. K., Petrography of Some North Carolina Soils and Its Relationship to their Fertilizer Requirements, Jour. Agr. Res., Vol. V, No. 13, pp. 569-581. 1915. Robinson, W. O., The Inor- ganic Composition of Some Important American Soils; U. 8. Dept. Agr., Bul. 122. Aug., 1914. 6 NATURE AND PROPERTIES OF SOILS 4. Importance of soil minerals.—Quartz is found in al- most all soils, making up often from 80 to 90 per cent. of the composition, although a range from 40 to 70 per cent. is more common. Its universal presence is due to its hard- ness and insolubility. Quartz is a make-weight material, however, as it probably contributes but little to plant nutri- tion. in the form of sand, quartz has a great influence on the friability of soil, improving and maintaining the phys- ical condition to a marked degree. Orthoclase, microcline, muscovite and, to a lesser degree, biotite are important because of their potash content. They decompose, often rather readily, into kaolinite and similar products, thus liberating potassium in soluble form. The plagioclase feldspars also give rise to kaolinite. They carry, however, sodium and calcium. The latter element? plays an important role in soil both as a nutrient and as an amend- ment. When not sufficiently active it must be applied in some form. Calcite and dolomite also carry calcium. Horn- blende and augite bear calcium as well as magnesium and iron. Olivine is a magnesium and iron silicate. The oxida- tion of the iron of the above minerals gives rise to hematite, so common as a red coloring matter of soil. Practically all of the phosphorus of the soil, either organic or inorganic, has its origin in apatite, yet this mineral occurs but sparinely either in rock or soil. It makes up but 6 per cent. of igneous rocks. This accounts for the small percent- age of phosphoric acid in most soils and explains why it is often added in fertilizers.* 1Plummer, J. K., Availability of Potash in Some Common Soil- forming Minerals, Jour. Agr. Res., Vol. XIV, No. 8, pp. 297-315. Aug., 1918. de Turk, E., Potassiwm-bearing Minerals as a Source of Potassium for Plant Growth; Soil Sci., Vol. 8, No. 4, pp. 269-301. 1919. *Shorey, E. C. et al., Calcitwm Compounds in Soils; Jour. Agr. Res., Vol. VII, No. 3, pp. 57-77. Jan., 1917. ’Fry, W. H., Condition of Phosphoric Acid Insoluble in Hydro- chloric Acid; Jour. Ind. and Eng. Chem., Vol. V, No. 8, pp. 664- 665. 1913. SOME CONCEPTIONS OF THE SOIL 7 The members of the kaolinite group are decomposition prod- ucts resulting from the decay of the feldspars and similar minerals. While kaolinite itself shows no nutrients in its formula, it often carries considerable calcium, potassium, magnesium and phosphorus by absorption. Moreover, its close association with other decomposition products such as serpentine, tale, chlorite and epidote tends to accentuate its importance in plant nutrition. The plasticity and cohesion imparted to a soil by the presence of the kaolinite group and its associated minerals are of great practical importance as is also the capacity to hold, either physically or chemically, the bases already mentioned. Hematite and limonite are simple iron compounds and usually occur in the soil as a result of the decomposition of certain iron-bearing minerals such as biotite, hornblende and augite. These iron compounds impart the red and yellow colors so characteristic of certain southern soils. Most of the soluble iron of the soil has its source in these minerals. Hema- tite and limonite are produced by the same general processes as are the kaolinite group and are found in very intimate contact with the serpentine, epidote, chlorite and kaolinite. 5. Soil organic matter.—One of the essential differences between a normal fertile soil and a mass of rock fragments hes in the organic content of the former. The organic matter practically all comes from plants and animals that have in- vested the surface of the soil and the soil material. Through the agency of bacteria and other organisms with which the soil is liberally supplied, this organic tissue yuickly loses its original form, and becomes the dark incoherent material so noticeable in fertile soils. The decay is not one of immediate simplification, as might be supposed. The split-off compounds react not only with materials of a similar origin but also with the decomposing mineral fragments. This tendency pro- vides the intimate relationship between the organic and in- organic constituents of the soil already emphasized as an ex- 8 NATURE AND PROPERTIES OF SOILS ceedingly desirable condition. Incidentally the soil is ren- dered thereby very much more difficult to study, especially chemically. The incorporation of organic matter in any soil, either by natural or artificial means, tends, if the proper decay occurs, to make the soil more friable. The water capacity is markedly increased and the vigor of the bacterial and chemical activ- ities stimulated to a marked degree. As these two latter actions progress, some of the organic matter passes into simple combinations, allowing certain elements to become available to crops. Nitrogen, which is held in the soil largely in organic combination, emerges in the form of ammonia, nitrites and nitrates. It is from a salt of nitric acid that most plants absorb their nitrogen. Small amounts of sulfur, phosphorus, potassium and calcium are liberated from the tissue as decay proceeds. The largest product of organic decay, however, is carbon dioxide (CO,), which in the soil becomes important as a solvent for minerals, thus hastening the decomposition processes. 6. Factors for plant growth.—The growth and develop- ment of a plant depends on two sets of factors, the internal and external. The latter may be classified as follows: (1) mechanical support, (2) heat, (3) light, (4) oxygen, (5) water, and (6) nutrients.1 With the exception of light, the soil supplies, either wholly or in part, all of these conditions. Mechanical support is a function entirely of the soil. The comparatively loose and friable condition presented by most soils allows ample foothold to the ramifying roots. Air and water are easily supplied because of the open condition of the soil, and its large pore spaces. Temperature depends almost wholly on climatic relationships. The water 1Nutrients are materials from which food may be elaborated once they have been absorbed by plants. The energy for this synthetic proc- ess comes from the sun. A food is any substance from which the plant may obtain energy for its normal processes. A large proportion of the materials absorbed by plants are nutrients. SOME CONCEPTIONS OF THE SOIL 9 of the soil acts as a plant nutrient in itself and functions also as a solvent for other materials. By its circulation it not only promotes solution but it continually brings nutrient elements in contact with the absorbing surfaces of the roots. The two prime functions of the soil are thus realized through the factors discussed above—mechanical support and a suffi- cient supply of certain nutrient elements under favorable conditions. 7. Nutrient elements.\—Although the physical condition of the soil exerts a far-reaching influence on plant growth, the relationships involved are more readily understood than those which have to do with plant nutrition. Moreover, the solubility of the necessary nutrients is very closely related to the complex processes of soil formation. Ten elements ” are usually considered as necessary for plant growth. If one is lacking, normal development will not occur. They may be classified as follows: From air or water From the soil Carbon Nitrogen Calcium Oxygen Phosphorus Magnesium Hydrogen Potassium Sulfur Nitrogen Tron Plants obtain most of their carbon and oxygen directly from the air by photosynthesis and respiration. The hydro- gen comes, at least partially, from water. All of the other elements, except a small amount of nitrogen utilized directly from the air by certain plants, are obtained from the soil. It must not be inferred, however, that the bulk of the plant 1For an excellent discussion of the functions of plant nutrients, see Russell, E. J., Soil Conditions and Plant Growth, Chap. II, pp. 30-46; New York. 1915. *It may be possible that manganese and silicon and possibly chlorine and fluorine function as nutrients. They as well as sodium, aluminum, titanium, barium, strontium, and certain rarer elements are found in plant ash. 10 NATURE AND PROPERTIES OF SOILS tissue is fabricated from the soil. Quite the reverse is true. Fresh plant tissue generally carries only from .5 to 2.5 per cent. of mineral material. In spite of this, it is the mineral elements of nutrition that generally limit crop growth since a plant can always obtain, except in cases of drought or disease, unlimited amounts of carbon, hydrogen and oxygen. 8. Primary nutrient elements.—While all of the seven soil nutrients must be available that plants may grow normally, only four or five are likely to become limiting factors. The others are almost always in great sufficiency. These few, nitrogen, phosphorus, potassium, calcium and_ occasionally sulfur, receive as a consequence especial attention. They may limit growth because they are actually lacking or be- cause their availability is low. These conditions often occur in the same soil. Combined nitrogen exists in the soil to a large degree as a part of the partially decayed organic matter present therein.t As decay proceeds, small quantities of this nitrogen appear aS ammonia in combination with some acid radical such as the chloride or sulfate or with the hydroxal group. Later, it is changed through further bacterial action to the nitrate form, united with some bases such as calcium or po- tassium. It is from this latter combination that most plants obtain the greater part of their nitrogen. These inorganic nitrogen compounds, present at any one time in a soil, are but a small proportion of the total soil nitrogen. The air both above the soil and that circulating within its pores has been the original source of all the combined nitrogen. Nat- ural processes have facilitated the combination which has been necessary for such a transfer. The encouragement of such *Certain rocks, particularly those of a sedimentary nature, carry considerable nitrogen. When such rocks weather, this nitrogen tends to become available. The organic matter, therefore, does not absolutely control the amount of nitrogen in a soil, Hall, A. D., and Miller, N. H. J., The Nitrogen Compounds of the Fundamental Rocks; Jour. Agri. Sci., Vol. II, Part 4, pp. 343-345. July, 1908. SOME CONCEPTIONS OF THE SOIL 11 fixation processes, especially those of a biological nature, is a feature of practical soil improvement. Phosphorus has its origin in the mineral apatite (Ca,- (PO,),(Cl,F)) and exists in the soil not only in this form but as tri-calcium phosphate (Ca,(PO,).), iron and alum- inum phosphates (FePO, and AIPO,) and in certain other inorganic complexes. It also exists in organic combinations of a constantly varying nature. It probably is utilized by the plant as a simple phosphate such as the mono- or di-caleium salt (Cal, (PO,), and Ca,H,(PO;),): Potassium, as already stated, occurs in the soil in orthoclase and microcline (KAISi,O,), in mica, especially muscovite (H,KAI,Si,0,,), and in other aluminum silicates, both hy- drated and non-hydrated. These complex forms supply potash to the soil solution and thus to the plant at a more or less rapid rate in the bicarbonate, carbonate, chloride, nitrate, and sulfate forms. Calcium, while necessary in the soil as a nutrient, also functions as an amendment in that it seems to preserve a proper soil reaction. It is possible that this relationship is as much nutritive as strictly chemical. Calcium exists in the soil in many minerals, of which calcite, plagioclase feldspar, horn- blende and augite are perhaps the most important. It is carried as an absorbed compound by kaolinite and similar materials. Calcium becomes available in the soil as the ni- trate, bicarbonate, chloride, phosphate, and sulfate. Sulfur is found in the soil in rather small amounts and generally forms a part of the organic matter. Inorganically it usually occurs as a sulfate combined with the common bases. In this form it is available to plants. The original source’ of most of the soil sulfur has been pyrite (FeS,), the *Considerable sulfur is brought to the soil in atmospheric precipita- tion. From 5 to 150 pounds an acre a year have been reported. Wilson, B. D. Sulfur Supplied to the Soil in Rain Water, Jour. Amer. Soe. Agron., Vol. 13, No. 5, pp. 226-229. 1921, 12 NATURE AND PROPERTIES OF SOILS commonest sulfide of this element. Although sulfur is no more abundant in the average soil than phosphorus, it is generally not considered as an extremely important fertilizing constituent. It is interesting to note at this point the amounts of the above elements in ordinary mineral soils. Generally the nitro- Fig. 2.—Chemical composition of a representative productive soil. gen (N) may range from .1 to .2 per cent., the phosphoric acid (expressed as P,O,;) from .05 to .380 per cent. and the potash (expressed as K,O) from 0.5 to 2.0 per cent. Of the plant nutrients in the soil nitrogen, although usually present in small quantities, is relatively more available than is phosphoric acid or even potash. Phosphoric acid may be in the minimum because of its unavailability as well as because of the small quantity. Potash is commonly present in rela- SOME CONCEPTIONS OF THE SOIL 13 tively large amounts. Its occurrence in complex and insoluble silicates makes its availability of vital consideration. The presence of abundant organic matter may have much to do with the liberation of sufficient potash for vigorous plant growth. The amount of lime (expressed as CaQ) in soils is difficult to state with any degree of satisfaction because of a very wide range in composition. Some soils carry only a fraction of a per cent., while others, especially those formed under conditions where an originally high calcium content has been maintained or where calcium has accumulated, show as much as 10 or 12 per cent. The variability of the sulfur is much less. A range from .02 to .30 per cent. of sulfur (expressed as SO,) will include most soils. It is interesting at this point to note the average composi- tion of thirty-five representative American surface soils’, which were studied by the United States Bureau of Soils dur- ing a systematic investigation of the arable lands of the United States east of the Rocky Mountains. A comparison of these data with those setting forth the composition of the litho- sphere? may be made with profit. (Table I, page 14.) It is immediately noticeable that silicon, aluminum, and iron make up the greater portion of both soil and lithosphere and that the nitrogen, sulfur and phosphorus are particu- larly low in both cases. Magnesium, calcium, sodium, and potassium occur in fair amounts, especially in the earth’s erust. It is noticeable also that the soil is much higher than the lithosphere in silicon, nitrogen, organic matter, and ecar- bon but much lower in all of the other constituents. These differences have developed as a result of the losses and gains during soil formation. *Robinson, W. O. et al., Variations in the Chemical Composition of Soils; U.S. Dept. Agr., Bul. 551. June, 1917. ? The Lithosphere refers to the solid portion of the earth, in this case to a depth of ten miles. Clarke, F. W., Data of Geochemistry; U.S. Geol. Survey Bul. 695, p. 33. 1920. 14 NATURE AND PROPERTIES OF SOILS TABLE I COMPARISON OF THE CHEMICAL COMPOSITION OF AMERICAN SURFACE SOILS WITH THAT OF THE LITHOSPHERE. GOAT TRe 35 AMERICAN COMPOSITION OF SURFACE SOILS LITHOSPHERE Si0, 84.67 DOG Al,O, 6.73 14.89 TiO, . 66 Ba A Fe,0, 2203 6.29 MnO 06 .09 Na,O 49 See K,0 LOS 2.98 CaO 40 4.86 MgO sot 3.74 P.O, .09 28 SO, .09 28 Nitrogen 07 a — Organic Matter 2.61b _ Carbon tele 03 (a) Average of 22 soils only. (b) Average of 13 soils only. (ce) Calculated from the organic matter. 9. The soil and the plant.—aAs the soil considered agri- culturally is essentially a medium for crop production, its rational study has to do with the consideration and applica- tion of such scientific principles as have a bearing on prac- tical soil management. Anything that makes clearer the relationships between soil and crop has a proper place. Un- less a scientific phase has a crop relation, either directly or indirectly, it need receive but scant consideration. The com- position of the soil, its chemical and biological changes, its physical peculiarities and its reaction to certain additions must receive especial attention. More knowledge of the soil 1Soils contain many other elements, although in small amounts, such as chlorine, barium, cesium, chromium, lithium, molybdium, rubidium, vanadium, ete. Robinson, W. O., The Inorganic Constituents of Some Important American Soils; U. S. Dept. Agr., Bul. 122. Aug., 1914. SOME CONCEPTIONS OF THE SOIL 15 will mean better systems of management and will allow the farmer to fulfill to a greater degree his duty to himself and to the State—the production of paying crops and the passing on to the next generation of a soil depleted as little as possible in fertility. CHAPTER II SOIL-FORMING PROCESSES Tue forces which have to do with soil formation are largely climatic in nature. They promote the physical and chemical breaking down of rock masses, they intermix there with the decaying organic matter and they shift the products from place to place. Even after the soil is apparently at rest and has become an effective agency in plant production, these same forces are still much in evidence. The physical and chemical evolutions through which mineral and organic mate- rials at or near the earth’s surface are passing due to natural forces are spoken of as weathering.: Erosion and deposition are terms referring to the natural translocations which soils and soil materials are frequently forced to undergo. If a soil represents a condition more stable than the rock, the rock change is in that direction. If a soil presents con- stituents or conditions not wholly stable to the forces effective at that particular time, it in turn seeks a change by an altera- tion or an elimination. A cycle of development is thus set up proceeding from youth to adolescence and even into old age. According to conditions, soils may age rapidly or slowly. Rejuvenation may even occur, while cases of arrested develop- ment may exist for short periods. 10. Soil-forming processes classified—While weather- ing, with the changes in form and composition which inva- riably accompany it, profoundly affects topography, it is very 1The term weathering is somewhat misleading since it comprehends forces other than those generally considered as weather. All of the forces involved, however, depend upon climatic conditions. 16 SOIL-FORMING PROCESSES 17 superficial in comparison to the earth’s bulk. Nevertheless, the weathered mantle, in spite of its comparative insignifi- cance, presents an effective medium for plant growth. The agencies of formation, therefore, demand more than the brief mention just given. These forces are geologic when the soil is being evolved, but once the soil materials are in place, the actions become localized and the influences may be considered as soil processes rather than more broadly geological. The soil-forming processes', while diverse both in action and product, may be classified under two heads, mechanical and chemical. The former is often designated as disintegra- tion, the latter as decomposition. , SOIL-FORMING PROCESSES I. Mechanical (disintegration) A. Erosion and deposition. Water, ice and wind.? B. Temperature change. Differential expansion of minerals, exfoliation and frost. C. Biological influences. Plants and animals. II. Chemical (decomposition ) A. Oxidation and deoxidation. B. Carbonation and decarbonation. C. Hydration and dehydration. D. Solution. 11. The mechanical action of water.—From the time that that water as rain beats down upon the solid earth until it is finally discharged into the ocean, there to pound as waves upon the bordering lands, it is moving, sorting, and rework- ing the products of weathering. Water to erode must be *For a complete and detailed discussion of soil formation, see Merrill, G. P., Rocks, Rock Weathering and Soils; New York. 1906. Also, Emerson, H. L., Agricultural Geology; New York. 1920. *Gravity is generally included in this group. While indirectly of great significance in soil formation, its direct action is not of great importance and is adequately disposed of in paragraph 27. 18 NATURE AND PROPERTIES OF SOILS armed. Its cutting power, therefore, depends on the amount of sediment that it carries and on its velocity of flow. Erosion by water deserves particular attention, as its denud- ing effects are very rapid when geologically viewed. Most of the changes in topography are due to such activity. The material swept away is partly in suspension and partly in solution.1 The Appalachian Mountains, whose uplift was complete in Carboniferous times, have lost vastly more of their mass than now remains in view. While most of the debris from the ancient erosive cycles has been changed to rock or has become a noticeable charac- teristic of ocean water, remnants persist. To these remnants rivers, lakes and oceans are making, year by year, substantial additions. The cutting, carrying and depositing activity of streams produce alluvial soils of which the Mississippi flood plain is a well known example. Deltas built into oceans, lakes and gulfs represent stream activity under different condi- tions, while uplifted continental shelves are often bedded with erosive products. The delta and marine soils of the Atlantic and Gulf coastal plains afford examples of the latter types of soil production. Even the pounding, grinding and sorting activities of waves in ocean and lake are no mean factors in the mechanics of soil formation. 12. Glacial action.—Ice at the present time, especially in temperate regions, is of little importance in soil forma- tion. Nevertheless, at a comparatively recent date geolog- ically, it had much to do with the preparation and deposition of soil materials over great areas in central and northern North America, northern Europe and the British Isles. Dur- ing the Great Iee Age immense continental glaciers succes- sively invaded these regions, much as the ice cap is over- 1The chemical denudation by streams is generally spoken of as corro- sion. Abrasion is applied to the wear of the stream load upon its channel and of the particules in suspension upon themselves. Erosion is a broader term including corrosion and abrasion as well as trans- portation. SOIL-FORMING PROCESSES 19 riding Greenland to-day. Of great thickness and weight and impelled southward by tremendous pressure, these ice sheets swept away the old soil mantle and ground the underlying rocks with irresistible energy. The heterogeneous debris, im- bedded in the ice, only served to enhance the cutting power of the slowly moving mass. Hundreds of square miles were covered and as the ice was often several thousand feet thick, mountains as well as hills were over-ridden. (See Fig. 3.) In the melting back of these tremendous ice sheets, the accumulated debris was of necessity left behind. When the ice retreat was rapid, the deposit was comparatively thin and uniform. When a halt occurred, the material was left in irregular hummocks. It is hardly necessary to state that the soil developed from the former deposit is the more important agriculturally, due to its level topography and wide extent. The area of the latter is fortunately small. The streams flowing from the ice fronts were no insignificant feature of the glacial phenomena. Such streams were heavily laden with sediment, which was distributed far and wide in regions miles beyond the ice front. In whatever manner the glacial debris was laid down it is necessary to note that such deposits were soil material, not soil. Chemical action in all its complexity and the interven- tion of plants and animals, especially the former, were neces- sary before a true soil could be born, a soil still in its youth and covering in the United States alone over 500,000 square miles. (See Fig. 3, page 20.) 13. The influence of wind.—Wind, like water and ice, has both cutting and earrying power. The fluting of rocks, the polishing of stones, and the undermining of cliffs are of such frequent note as to require but brief mention. There seems no escape from the conclusion that wind is engaged in rock disintegration. Its geological function in arid regions seems similar to that of running water in humid lands. It is, however, as a transporting agency of fine materials NATURE AND PROPERTIES OF SOILS 20 hon IW wy > LABRABOR Grr. SW BS c fre G | Ns NG ac (((rree* ‘Aad 99)))))) i) Wee AsO Se SS S ) f( DS MCE uy Sees iu “ ¥y Wie FG sa ae gil iN A? CLG OLY 30 15 8 5. Very Fine Sand.... il 17 8 Genter ea’ wrk eereislees ee fi 40 36 Of (CHE a ae es Sea ee 5 16 42 The sand group includes all soils of which the silt and clay separates make up less than 20 per cent of the material by weight. Its properties are, therefore, characteristically sandy in contrast to the more open character of gravel and the stickier and more clayey nature of the heavier groups of soil. A soil to be clay must carry at least 30 per cent. of the clay separate. It may even have more silt than clay but, since the silt particles impart clayey characters, as long as the per- centage of clay is 30 or above, the class name must remain elay. *Whitney, M., The Use of Soils East of the Great Plains Region; U.S. Dept. Agr., Bur. Soils, Bul. 78, p. 12, 1911. 82 NATURE AND PROPERTIES OF SOILS The loam class is rather difficult to explain. In mechan- ical composition it is more or less midway between sand and clay. A loam may be defined as such a mixture of sand, silt, and clay particles as to exhibit sandy and clayey properties in about equal proportions. It is a half and half mixture on the basis of properties, although the sum of the sands and the sum of the silt and clay are generally near 50 per cent., respectively. (See Fig. 16.) Because of the marked inter- mixture of coarse, medium, and fine particles, loams are usually soils of good physical character. They generally pos- sess the desirable qualities both of sand and clay without exhibiting those undesirable properties, such as extreme loose- ness and low water capacity on the one hand and stickiness, compactness, and slow air and water drainage on the other. Most of the better soils are some type of loam. It is obvious that in the field not only various kinds of gravelly, sandy, loamy, and clayey soils must occur, but the groups must grade into each other, thus giving rise to a con- siderable number of field names. (See Fig. 16.) These field names are listed below: Common Class Names 1. Gravel 9. Very fine sandy loam 2. Coarse sand 10. Loam 3. Medium sand. 11. Silt loam _4, Fine sand 12. Silty clay loam 5. Very fine sand 18. Clay loam 6. Coarse sandy loam 14. Clay 7. Sandy loam 15. Heavy clay 8. Ine sandy loam 16. Sandy clay The meaning of these names should be clear except possibly those into which the loam group is divided. Loam, as already explained, refers to a soil possessing in about equal amounts the properties imparted by the various separates. If, how- ever, we have practically the same condition but with one THE SOIL PARTICLE 83 size of particle predominating, the name of that particular separate is prefixed, giving still more data regarding the soil in question. Thus, a loam in which clay is dominant will be classified as a clay loam. In the same way, we may have a sandy loam, silt loan, and so on. It is to be noted that the loams make up half of the class names. In fact, the greater proportion of the soils so far classified in the United States are loams, which is fortunate as the loams in general are more favorable for crop production than any of the other class groups. The mechanical analyses of some of the more common classes! are listed in Table XVI: TABLE XVI Fine |Coarse |MeEpIuM | FINE — s Gravet | SAND SAND SAND Syn ILT Cay Coarse Sands....| 12 31 19 20 6 cf 5 SH ats (an 2 5 23 37 al ff i Fine Sands...... iI 4. 10 Sirf 17 {( 4 Sandy Loams....| 4 | 13 12 25 13 21 12 FineSandy Loams| 1 3 4 | 32 | 24 24 | 12 Gamishsct. sien 2 5 5 15 i Ly 40 16 Silt Leams ss: .. 1 2 1 5 11 65 15 Sandy Clays..... 2 8 8 30 12 Se ae Clay Loams...... 1 4 4 14 13 38 26 Silty Clay Loams} 0 2 a 4 i 61 | 25 LOLS Slee a como i a 2 8 8 36 42 It is evident that a mechanical analysis of a soil is nothing more or less than an expression of class, and the inferences that may be derived from either are in general the same. This leads to a consideration of class determination. 45. Determination of soil classes——The common method of class determination is that employed in the field. It con- * Whitney, M., The Use of Soils East of the Great Plains Region; U.S. Dept. Agr., Bur. Soils, Bul. 78, p. 12, 1911. 84 NATURE AND PROPERTIES OF SOILS sists in an examination of the soil as to color, an estimation of its organic content, and, especially, a testing of the ‘‘feel’’ of the soil in order to decide as to the class name. Probably as much can be judged as to the texture and class of a soil merely by rubbing it between the thumb and the fingers or in the palm of the hand as by any other superficial means. This method is used in all field operations, especially in soil survey work. It really consists in sufficiently recognizing the textural composition of a soil that the class name may be determined.* The accuracy of such a determination depends largely on experience. Inaccuracies are likely to occur in distinguishing between the various finer grades of soil; for this reason, more nearly exact methods are necessary at times, especially in checking soil survey work or in carrying out investigations in which absolute accuracy is required. As a mechanical analysis of a soil is really a percentage expression of texture, it presents an exact method for class determination. For detailed work, somewhat complicated tables? have been arranged; but the following diagram 1 Key for the practical classification of mineral soils: I. Soils possessing the properties of one size of particle largely. iS sParticlestvenyalarcesseprsmiseioceceieiettets Gravel 2. Particles apparent to eye; feel gritty and Non=plastles).v2s/ch.)- noes een tes ete Sands 3. Particles very small; soil very plastic when Wet marduwihe nn cinyaraletersrdtaiet risers itelene Clay or Sandy Clay Il. Soils possessing the properties of a number of sizes of particles—a mixture. 1. A fairly equal exhibit of sandy and clayey, Properties... cm aes -)-calye let eee Loam 2. A mixture but with sand predominating. ..Sandy Loam 3. A mixture but with silty character dom- inant. The soil has a floury or tale feel and is quite plastic when wet ............ Silt Loam 4, A mixture but with clayey characters very apparent. Soil is very plastic and ap- proaches a clay in character ............ Clay Loam 2 Bur. of Soils, Soils Survey Field Book, p. 17; U. S. Dept. Agr., Bur. Soils, 1906. Also, Bur. Soils, Bul. 78, p. 12, 1911. THE SOIL PARTICLE 85 (Fig. 17), devised by Whitney,’ presents a simple method for the identification of a soil from a mechanical analysis. The convenience of such a triangular representation is obvious. CLAY 100 90 80| 0 10 20 30 40 50 60 70 60 99 100°” PER CENT Fig. 17—Diagram for the determination of class from a mechanical analysis. In using the diagram the points corresponding to the percentages of silt and clay are located on the silt line (abscissa) and clay line (ordinate) respectively. Perpendiculars at these points are then projected inward until they intersect. The name of the compartment in which the intersection occurs gives the class name of the soil in question. 46. Soil survey classification—soil type—The function of the soil survey is to investigate the nature and occurrence 1Whitney, M., The Use of Soils East of the Creat Plains Region; U. S. Dept. Agr., Bur. Soils, Bul. 78, p. 13, 1911. 86 NATURE AND PROPERTIES OF SOILS of soils in the field. The soils thus studied are classified into areas having approximately the same crop relations and tillage properties. The location of the areas of each kind of soil is represented on an adequate base map, and their character and chief economic and agricultural relations are described in a printed report accompanying the soil map.t (See Fig. 18). In classifying soils six primary factors are considered. These, beginning with the broadest, are as follows: (1) tem- perature, (2) precipitation, (3) agency of formation, (4) kind of material, (5) special properties other than texture, and (6) texture. It is obvious that certain soils may be of different texture but alike in all other ways. Their climatic environ- ments, mode of formation, rock materials, and specific prop- erties, such as color, drainage, organic condition, and lime content may be approximately the same. Such soils are grouped together as series and the series are named, generally from some town, county, or river of the near vicinity. Thus we have the Norfolk series of the Atlantic coastal plain; the Cecil soils of the Piedmont Plateau; the Ontario series arising from the calcareous till of central New York state and the Marshall soils of the loessial region of the Middle West. The soils within each series are approximately the same except for class distinction. The soil type is the unit of classification and may be defined as an area of soil alike in all characteristics, including crop productiveness. Obviously any soil class of any particular series would be a soil type. Norfolk sandy loam, Ontario loam, and Cecil clay are examples of how soil types are designated. The type designation is especially valuable in soil description since the series name expresses in one word a great number of conditions, which otherwise would require detailed explana- tion. The class name establishes in addition the textural con- dition. 1For further information consult one of the numerous soil survey reports as published by the U. 8S. Dept. Agr., Bur. of Soils. PLATE 1 Bul. 60, Bureau of Soils, U. S. Dept. of Agriculture. ——- 7 . \ : ] SI : Volusia Volusia Dunkirk Huntington Dunkirk Muck silt loam loam gravelly doam loam clay Fic. 18.—Part of the Madison County, New York, soil map showing the topography and drainage and the relation of the various soil types to one another. The Volusia series arises from the ground moraine, the Dunkirk from glacial lake sediments while the Huntington is alluvial. Note the varying elevation of the muck. THE SOIL PARTICLE 87 While the principles of series identification are too com- plicated to be expanded farther at this time, enough has been said to establish the importance of accurate soil classification. Unless soils are accurately named in soil survey work, the map and its accompanying report are useless. Soil texture and class are thus the basis for practical soil study, whether regarding some particular property or a gen- eral condition, such as crop adaptation. No matter what the phase of soil study may be, texture and class are sure to have some important influence and must be considered in the in- vestigation. 47. Soil structure.— While texture is of great importance in determining the general characteristics of a soil, it is evi- dent that the arrangement as well as the size of the particles must exert some influence. The term structure is used to refer to this arrangement or grouping. It is at once apparent that soil conditions—such, for example, as air and water move- ment, heat transference, and the like—will be as much affected by structure as by texture. As a matter of fact, the great changes wrought by the farmer in making his soil better suited as a foothold for plants are structural rather than changes in texture. The compacting of a light soil or the loosening of a heavy one is merely a change in the arrange- ment of the soil grains and in the condition and nature of the colloidal complexes! thereof. From the standpoint of size and arrangement of particles there are really two classes of soils, those of single grain struc- ture and those which are complex, the particles both large and small being bound together by indefinite colloidal com- plexes. The former condition is of course best exemplified by a sand. Such a soil is loose and open with large individual pore spaces and ready circulation of air and water. The com- *Material in a colloidal state has a great deal to do with all soil phenomena. Its characteristics and influence must be kept constantly in mind in soil study. 88 NATURE AND PROPERTIES OF SOILS plex structure is best developed in clay. Here the soil gran- ules are made up of many particles, the colloidal material act- ing as a binding agent. Such a soil may be loose, open and friable, if granules of the proper size and nature are developed. On the other hand, improper handling may run the complexes together and an impervious and puddled condition may result. The sand will obviously permit of no very great structural change, while the clay can be modified very materially by certain field manipulations. The ideal structural condition is most likely to occur in a loam soil. In such a soil some of the particles are large and function separately ; others are medium in size and tend to form the nuclei around which smaller particles, both colloidal and non-colloidal, may cluster to form granules, or aggregates. There are thus a few large pore spaces which facilitate drain- age, and numberless small openings in which water is retained. Air, therefore, finds easy movement and sanitation is pro- moted. In promoting such a condition the organic matter plays an important part. It usually exists as a dark, partially decayed material, often colloidal in nature. It pushes apart the grains and lightens the soil, and contributes much in bring- ing about the loamy condition so favorable to plant develop- ment. It is a valuable addition also on account of its water- holding capacity and its nitrogen content. 48. Specific gravity of soils—The texture, as well as the structure of a soil, has considerable influence on certain phys- ical conditions other than those already mentioned. One of these is weight. The weight of a soil is determined by two factors: the weight of the individual particles and the amount of the space occupied by the soil material. The former is determined by the chemical and mineralogical character of the particles, the latter by their structural arrangement. Thus, if the soil particles are heavy and the soil is compact, the weight of any given volume, a cubic foot for example, will be high. THE SOIL PARTICLE 89 The specific gravity’ of a soil is obviously the average spe- cific gravity of the particles. It is unaffected by the structure, remaining the same whether the soil is loose and open or com- pact and unaérated. Although a great range is observed in the specific gravities of the common soil minerals’, the spe- cific gravity of a purely mineral soil varies between the nar- row limits of 2.6 and 2.7. This occurs because quartz and feldspar, whose specific gravities are about 2.65 and 2.57, respectively, usually make up the bulk of the mineral portion of most soils. The fineness of the particles seems to have no appreciable effect on specific gravity as shown by the follow- ing data from Whitney and Smith*: TABLE XVII SPECIFIC GRAVITY OF SOIL SEPARATES SEPARATES WHITNEY SMITH CRAVEN, 24 S/.uie daa ewias< 4 2.64 2.67 @oarseusands 26 ues caaele ites ads 2.65 2.64 PASTS ATG 5. rua, gaan ioserele: =-aNeds 2.64 2.64 TEMAS) FSET 10 DA a eee 2.65 2.69 Were He SANG e265 «3 sevee en! 2.68 2.66 Se ee SER ear ad alee duets. 2 2.69 2.65 Bree ie Sen ak eS ne De8a 2.66 1Specific gravity is expressed as a ratio of the weight of any volume of a substance to the weight of an equal volume of some other substance taken as a standard unit. Liquids and solids are usually compared with water at its maximum density (4° C.). 2The specific gravities of some of the common soil minerals are as follows: (QUIET nl encaie aeckemeeeponaee PAG OE 2 OM weAeutben eer sbsteesi 3.20 Orthoclase A PET Kaolinite yas sean 2.60-2.63 Plogtoclase™.. 2 .....-2.62-2.76 Serpentine... ... 6. . 2.50-2.65 Marscomiteme sae i eaO-oLO00es “Ohloriben se ctekic as sieicc 2.65-2.92 IBTObIe mE creer. be SDs) VM OGKOIEY Good a eo oc 3.25-3.50 Hiornblendes = a.sn. Rbesay lelanteynire: Gooaaudaos 4,.90-5.30 ANI, OS Series Se SSCA SOO) Winrar Soa onocndoc 3.60-4.00 > Whitney, M., Some Physical Properties of Soils; U. S. Dept. Agr., Weather Bur., Bul. 4, 1892. Smith, Alfred, Relation of the Mechanical Analysis to the Moisture Equivalent of Soils; Soil Sei., Vol. IV, No. 6, p- 472, Dec., 1917. 90 NATURE AND PROPERTIES OF SOILS The only marked variation here observed is in the clay separates of the first column. This may be due to the concen- tration of the iron-bearing silicates in this grade and would thus be an apparent rather than a real variation. Only one condition may vary the specific gravity of any soil. This is the quantity of organic matter present. As the specific gravity of organic matter usually ranges from 1.2 to 1.7, the more that is present the lower will be the figure for any given soil. A purely organie soil, such as muck, presents a variable specific grav- ity ranging from 1.5 to 2.0, according to the amount of inorganic wash it has re- ceived from external sources. Some highly organic mineral soils may drop as low as 2.3. Nevertheless, for general calculations, the average arable soil may be considered to have a specific gravity of about 2.65. The specific gravity of a soil is generally Fic. 19.—Drawing determined by means of a picnometer, a showing the type ‘ of picnometer bottle fitted with a perforated ground-glass generally used in stopper and accurately calibrated (Fig. determining the : 2 specific gravity of 19). By comparing the weight of the total soil. The ground- water held by the bottle, usually 50 cubic glass stopper 1s c é 3 perforated. centimeters, with the weight of the water when any given amount of dry soil, say 5 grams, is present in the bottle, the weight of the water dis- placed by the soil can be determined and the specific gravity calculated therefrom.* 1 Below will be found a sample calculation: Weight of picnometer ..........-.-2---seeeeeeees 23.257 grs. Volume of picnometer ......0.2..ees-seeeeseccces 50 ce Wt. of picnometer + 5 grs. soil + X grs. water... .76.347 grs Wt. of picnometer + 5 grs. soil...........----2---- 28.257 grs. Ne OM. fate WEN Ao oe oonouddooncueodoWuuoonouC 48.090 grs, Water displaced (50 —48.09) ..........-0-----e- 1.910 grs. 0 Specific gravity = 5.0 eG) pay THE SOIL PARTICLE 91 49. Volume weight of soils—The actual weight of dry soil in any given volume is generally expressed by volume weight, a figure indicating the number of times heavier the dry soil is than the water that will occupy the same soil vol- ume. Thus, if the dry soil in a cubic foot of space weighs 99.8 pounds, the volume weight would be 99 .8+62.42 or 1.6. The volume weight differs from specific gravity in that it compares the weight of the dry soil to the weight of water that will occupy the total soil volume—that is, the space usually filled by soil particles, soil air, and soil water. Specific gravity, however, compares the weight of the dry soil to that of water that will occupy only the volume of the particles alone, taking no consideration of the normal pore space. It is consequently always the higher figure." This volume weight figure depends on the texture of the soil, the structure and the amount and condition of the organic matter. The particles of sandy soils always tend to le in close contact, thus increasing the weight of soil to a given volume. The particles of the finer soils, such as silt loams, clay loams, and clays, on the other hand, being smaller and lighter, do not lie so closely together. A greater total pore space is, therefore, usually present in the finer soils and the volume weight is correspondingly lowered. Mineral soils may range in volume weight from 1.10 to 1.35 for clay to 1.55 to 1.70 for sand.2 The influence of texture on the volume weight is thus evident. The structural and organic condition of soils often pro- duces wide variation in volume weight. When a soil is loos- 1 As a soil is compacted, its volume weight increases due to the increase volume occupied by the soil particles and the corresponding decrease in pore space. If it were possible to compact a soil to a completely solid condition, its volume weight would approach its specific gravity as a limit. Specific gravity represents, therefore, 100 per cent. soil particles. Volume weight in comparison indicates the proportion of space occupied by the soil particles. *Sandy soils are commonly spoken of as light soils, while clays are called heavy. Such usage refers to working properties and has no reference to actual weights. 92 NATURE AND PROPERTIES OF SOILS ened through tillage, it becomes lighter for any given volume. The addition of organic matter has the same effect, since the particles are spread wider apart and the air and water spaces increased. The specific gravity figure of a sandy loam of 1.55 may readily be lowered to 1.45 by an increase of organic material. Some loams high in organic matter may drop as low as 1.1 in specific gravity while muck often reaches the low figure of .40. In the field the volume weight of a soil may be estimated by driving a cylinder of known volume into the ground and ob- taining thereby a core of natural soil. By weighing the soil and then determining the amount of water that it holds, the amount of absolutely dry soil may be ascertained. Dividing this by the weight of an equal volume of water gives the figure for volume weight." A laboratory determination may be made by putting the soil into a receptacle of known volume and weighing it. From the weight of the absolutely dry soil and the weight of an The rubber tube method has proven very convenient for the field de- termination of volume weight. A hole is bored in the soil to the required depth by a specially constructed auger, the soil being carefully removed and later oven dried. A very thin-walled tubular rubber bag of the size of the auger hole is carefully inserted in the hole previously bored. The tubular bag is then filled with water flush with the surface of the soil. The water is measured and the volume of the soil removed is thus de- termined. Knowing the weight of dry soil and its original volume, the volume weight may be calculated. The experimental error of the method is rather low. Israelsen, O. W., A New Method of Determining Volume Weight; Jour. Agr. Res., Vol. XIII, No. 1, pp. 28-35, April, 1918. The paraffin-immersion is valuable with heavy soils. Small pieces of soil are dried, weighed and then coated very thinly with paraffin, just sufficiently to prevent the entrance of water, yet not enough to intro- duce serious experimental error. The weight of the water displaced by a number of such pieces may be determined easily by the use of a graduated cylinder. Shaw, C. F., A Method for Determining the Volume Weight of Soil in Field Condition; Jour. Amer. Soc. Agron., Vol. IX, No. 1, pp. 38-42, 1917. See also, Trnka, R., Hine Studie tiber einige physikalishchen Higenschaften des Bodens; Internat. Mitt. of Bodenkunde, Bd. IV, Heft 4-5, S. 363-380, 1914. THE SOIL PARTICLE 93 equal volume of water, the volume weight may be calculated. This method will give only approximate results, however, as the structural relationships are more or less artificial. 50. Actual weight of soil—When the volume weight of a soil is known, its weight in pounds to the cubic foot may be found by multiplying by 62.42. Soils may vary in weight from 68 to 80 pounds for clays and silts to 100 to 110 pounds for sands. The greater the organic content, the less is this weight to the cubie foot. A muck soil often weighs as little as 25 or 30 pounds. This weight, of course, is for absolutely dry soil and does not include the water present, which may be much or little, according to circumstances. The actual weight of soil may also be expressed in acre-feet. An acre-foot of soil refers to a volume of soil one acre in extent and one foot deep. In the same way we may have an acre-eight-inches or an acre-six-inches. The weight of an acre-foot of soil usually varies from 3,500,000 to 4,000,000 pounds. The standard usually adopted is 2,000,000 pounds, being the weight of average soil to a depth of 6°/, inches. The value of knowing the actual weight of a soil hes in the possibility of calculating thereby the amount of water, the amount of organic matter, or the actual number of pounds of the mineral constituents present in the soil. Such informa- tion affords another means of comparing two soils. 51. Pore space of soil—The pore space of soil is oceu- pied by air and water in constantly varying proportions. The amount of this pore space is determined by the texture and the structure of the soil. As already emphasized, the coarser +A comparison of the four methods is given by Israelsen, O. W., A New Method for Determining Volume Weight; Jour. Agr. Res., Vol. FEE No. 1, sp. 32,1918. Average Volume Weight of Tehama Clay to a Depth of 60 Inches. Laboratory method on disturbed soil.......... 1.35 + .008 Rubber sa. age skeen 3376 SU PeriOruelavane ame te.s 34.5 4 wpa aieayeneere cea ere 5158 Muck (25% organic matter) 6413 Gay (on Mareen ek: acd 6 PL Rac eAnes ea renee 22185 87. Determination of the hygroscopic coefficient.— The methods for the determination of the maximum hygro- scopicity of a soil, or, in other words, the hygroscopic coeffi- cient, are simple in outline. The soil, in a thin layer, is ex- posed to an atmosphere of definite humidity under conditions of constant temperature and pressure. Complications arise from the necessity of using a very thin layer of soil, from the difficulty of controlling humidity, and from the tendency of capillary water to form in the soil interstices before the hygro- scopic capacity is satisfied. The question of how long the exposure should take place has not been definitely settled. It 1The tremendous heat of wetting is probably due to the latent heat of water, to the attraction that soils have for water and to the condition into which the water is transformed. The heat of condensation is so large as to suggest the probability of a change in the aggregation of the moisture thus absorbed. *Patten, H. E., Heat Transference in Soils; U. S. Dept. Agr., Bur. Soils, Bul. 59, p. 34, 1909. ®Bouyoucos, G. J., Relationship between the Unfree Water and the Heat of Wetting of Soils and its Significance; Mich. Agr. Exp. Sta., Tech. Bul. 42, Mar. 1918. THE FORMS OF SOIL-WATER 155 is evident, therefore, that not only must any method be more or less arbitrary but that its value can only be comparative. In the actual procedure,’ the sample of soil may be air- dried or dried at 100° or 110°C. If the former method is followed, the sample after exposure is heated for four or five hours at 100° or 110° C., the loss being considered as hygro- scopic water. If oven-dried soil is utilized, the gain in weight due to the exposure to the moist air is the hygroscopic mois- ture. If a saturated air is made use of, the gain is maximum hygroscopicity, from which can be calculated the percentage of hygroscopic water based on dry soil, called the hygroscopic coefficient. If a partially saturated air is utilized, a sample of stock soil, the hygroscopic coefficient of which is known, is exposed at the same time. The determination on the known sample shows what proportion of possible hygroscopic water has been taken up. From this the hygroscopic coefficient of the unknown soil sample ean be ealeculated.’ 88. Hygroscopic capacity of soils—Since hygroscopic- ity depends almost directly on the colloidal nature of the soil, it is evident that texture, external factors being under con- trol, will be an important factor in determining the hygro- scopic coefficient. When the organic matter of soils is more or less the same in amount, the inorganie colloids seem to con- 1Hilgard, E. W., Soils; pp. 196-201, New York, 1911. This method is practically the same as that used for the comparative estimation of the colloidal content of the soil, the hygroscopic coefficient being the comparative figure obtained. See note to paragraph 74 of this text. Bouyoucos determines the hygroscopic coefficient in an approximate way by means of the dilatometer method. The dilatometer is an apparatus which measures the expansion of water on freezing. If a given amount of soil and water is reduced below zero, the expansion attained will reveal the amount of water remaining unfrozen, due to its soil relationships. Bouyoucos finds that the amount of moisture unfrozen after supercooling to —4° C. (slightly more freezes at -78° C.) correlates fairly well with the hygroscopic coefficient. Bouyoucos, G. J., A New Classification of Soil Moisture; Soil Sci., Vol. XI, No. 1, pp. 33-47, Jan., 1921. ?Alway, F. J., and Clarke, V. L., Use of Two Indirect Methods for the Determination of the Hygroscopic Coefficients of Soils; Jour. Agr. Res., Vol. VII, No. 8, pp. 345-351, Nov., 1916. 156 NATURE AND PROPERTIES OF SOILS trol the hygroscopicity. The following figures from Briggs and Sehantz,' by whom the hygroscopic coefficient was deter- mined by exposing air-dry soil at 20° C. to a saturated atmo- sphere and then drying at 110° C., illustrate this point. The organic matter was not a serious disturbing factor. TABLE X XIX HYGROSCOPIC CAPACITY OF VARIOUS SOILS EXPRESSED IN PER- CENTAGE BASED ON DRY SOIL? PERCENTAGE HYGROSCOPIC SOILS OF CLAY COEFFICIENT Coarse'sand: 225ee re i 2 eee 16 55) ane: Sadi ook ie ie ote 3.9 Li Satidiys loamy scene. oe. «Sects 7.9 3.0 Mine: sandy loames 280... 4)see 12109 6.6 TES Hira Ware pe ee NG, A ee ec Tce 14.4 9.6 Clay loamy oo. 2 teens thes rege ree 22.0 11.4 Cai sitibtoin Ha eee, Se. ce os Nici es BYARD 132 1 Briggs, L. J., and Schantz, H. L., The Wilting Coefficient for Dif- ferent Plants and Its Indirect Determination; U. 8. Dept. Agr., Bur. Plant Ind., Bul. 230, p. 65, Feb., 1912. See also, Loughridge, R. H., Investigations in Soils Physics; Calif. Agr. Exp. Sta., Rep. of Work of the Agr. Exp. Stations of Calif. for 1892-3-4, pp. 76-77. Ammon, Georg., Untersuchungen tiber das Condensationsvermogen der Bodenconstituenten fur Gase; Forsch. a. d. Gebiete d. Agri.-Physik., Band II, Seite 1-46, 1879. Dobeneck, A. F., von, Untersuchungen tiber das Absorptionsvermogen und die Hygroskopizitdt der Bodenkonstituenten; Forsch, a. d. Gebiete d. Agri.-Physik., Band XV, Seite 163-228, 1892. ? During the many years of soil investigation, especially where the problems had to deal either directly or indirectly with moisture, five methods of water expression have been evolved, their use depending on the nature of the work and on the points to be expressed. They may be listed under two general heads: A. Percentage expression 1. Percentage on a dry basis 2. Percentage on a wet basis B. Volume expression 1. Cubic inches to the cubic foot of soil 2. Percentage by volume 3. Surface inches A soil carrying 25 per cent. of water on the dry soil basis contains 20 per cent. on the moist basis (soil plus water). The former method is THE FORMS OF SOIL-WATER 157 Apparently, the finer the soil, the higher the hygroscopic coefficient. This is due to the fact that most of the inorganic colloidal matter is carried by the finer separates. In consid- ering the hygroscopicity, however, the influence of the organic matter must not be forgotten. Organic colloidal matter has a very marked influence on absorption, and as the organic matter of the soil increases, the hygroscopicity rises rapidly. The following data from Beaumont! is interesting in this respect : TABLE XXX THE HYGROSCOPIC COEFFICIENT? COMPARED TO CERTAIN OTHER SOIL FACTORS HyGro- IGni- SCOPIG 1s Som ae tion | HUMUS| Copper. ? % fe CIENT % Dunkirk silty clay loam, surface} 12.9 | 5.08 | 1.26 3.80 Dunkirk silty clay loam, subsoil] 20.0 | 3.05 20) ly soem Clyde clay loam, surface....... 20.1 |14.54 | 4.34 | 18.90 Vergennes clay, subsoil........ 74.5 | 5.79 AQ | 17.40 In comparing the two Dunkirk soils it is apparent that the colloidal clay is the dominant factor in determining the mag- preferable in that the basis for calculation is not a changeable one as is the weight of moist soil. The dry basis is practically always used in soil work. Where two soils of different volume weight are compared, the per- centage relationship does not give a true idea of the relative amounts of water present. A volume expression should then be used. If a cubic foot of soil, weighing 100 pounds, contains 10 pounds of water it would be carrying (10 x 27.6) or 276 cubic inches of water. This would equal (276 — 1728) x 100 or 15.9 per cent. by volume or (10 + 5.2) = 1.92 surface inches. * Beaumont, A. B., Studies in the Reversibility of the Colloidal Condi- tion of Soils; Cornell Agr. Exp. Sta., Memoir 21, pp. 501-504, April, 1919, * Moisture content in this text unless otherwise indicated will always be expressed on the dry soil basis. 158 NATURE AND PROPERTIES OF SOILS nitude of the hygroscopic coefficient. With the Clyde and Vergennes, however, the organic colloidal matter is dominant, since the Clyde with only 20 per cent. of clay has a higher hygroscopic figure than the Vergennes which carries 74.5 per cent. of that separate. The Clyde clay loam and the Dunkirk subsoil have the same amount of clay, yet the former pos- sesses a hygroscopic coefficient over three times larger. Two external conditions seem to be important in determin- ing the amount of hygroscopic water in soils—(1) humidity and (2) temperature. It has been definitely established that the higher the humidity the higher the content of hygro- scopic moisture. An air-dry soil will, therefore, contain less moisture in a dry atmosphere than in one carrying large amounts of water-vapor. When the soil is in contact with a saturated air it will take up hygroscopic water to its full capacity and be at the point spoken of as the hygroscopic coefficient. As the soil air is generally considered to be satu- rated or almost saturated with water-vapor,' except in the surface layers or during periods of protracted drought, a soil in normal condition may be considered, for all practical pur- poses, to be at its maximum hygroscopicity. An increase of the temperature of the saturated atmosphere seems to increase hygroscopicity. With a partially saturated air the influence seems to be in the opposite direction.? This, however, is not an important practical point. The hygroscopic coefficient, defined as the maximum hygro- scopic water that a soil will hold, is controlled largely by the texture and organic content of the soil. It may vary from a very low figure in a sandy soil to as high as 15 per cent. for a clay high in organic matter. With a muck or peat, the per- 1 Russell, E. J., and Applyard, A., The Atmosphere of the Soil: Its Composition and Causes of Variation; Jour. Agr. Sci., Vol. VII, Part 1, p. 5, 1915. ?For a full discussion of this point, see Lipman, C. B., and Sharp, L. T., A Contribution to the Subject of the Hygroscopic Moisture of Soils; Jour. Phys. Chem., Vol. 15, No. 8, pp. 709-722, Nov., 1911. THE FORMS OF SOIL-WATER 159 centage would be considerably higher, in some cases reaching 50 or 60 per cent. It must always be kept in mind, however, that the point designated as the hygroscopic coefficient is more or less arbitrary and that there is no sharp line of demarca- tion between the moisture designated as hygroscopic and that which lies near it, but is called capillary. 89. The capillary water..—The moisture above the hygroscopic coefficient but not free to respond to gravity is generally spoken of as the capillary water. The portion of this moisture lying in contact or in the immediate neighbor- hood of the hygroscopic water is probably capable of only sluggish diffusion movement if any.” This part of the capillary moisture is held largely by the colloidal matter and may be considered as transitional between the true hygroscopic and the more active capillary portion. Although so closely related to the hygroscopic water in general properties and character- istics, the soil does not assume it by absorption from vapor- laden air. This separates it at least analytically from the hygroscopic form of moisture. Moreover, it is probably largely in the liquid state, which is hardly true of all of the hygroscopic water. The more active capillary water exists in the large inter- stices and as a film over the particles and the colloidal com- plexes. It is held rather loosely by the soil, yet strongly enough to counteract gravitation. This part of the capillary moisture, being more or less beyond colloidal influence, is free to respond to the forces active in true solutions and, there- fore, may move from place to place as equilibrium stresses may demand. While the inner portion of the capillary water is held by the absorptive power of the colloidal surfaces, the outer and freer portion is maintained by the surface tension +The colloidal conceptions regarding soil-moisture has made it advis- able to give the term capillary a broader significance than its root meaning justifies. * Bouyoucos, G. J.. 4 New Classification of the Soil Moisture; Soil Sci., Vol. XI, No. 1, pp. 33-47, Jan., 1921. 5 160 NATURE AND PROPERTIES OF SOILS of the water film. The distinctive characteristics of these two portions of the capillary water are due to their controls— colloidal in one case, surface tensional in the other.' While the outer portion of the capillary water is undoubt- edly in the form of a more or less continuous film from par- ticle to particle, the bulk of such moisture probably exists normally in the interstices between the soil grains. Such a condition arises because of the pressure developed by the force of surface tension. The pressure due to surface tension, however it may be expressed, varies with the curvature of the film and is proportional to twice the surface tension di- vided by the radius. The less the radius the greater the cur- vature and, therefore, the greater the stress developed by sur- face tension.” The situation so far as the soil is concerned may be ex- plained in an empirical way as follows: Suppose that two par- ticles, each carrying a capillary water film, be brought into such contact that the films coalesce. There are now two distinct surfaces, that at A, A’ (see Fig. 28), with the curva- 1 Bouyoucos classifies these two types of capillary water as free (the more active) and ecapillary-absorbed (the inner group). The distinction is made on the basis of his dilatometer results, the portion which freezes at about O°C being considered as the more active or free. Bouyoucos, G. J., A New Classification of the Soil Moisture; Soil Sci., Vol. XI, No. 1, pp. 33-47, Jan., 1921. 2 Surface tension is the tension of a liquid surface by virtue of which it acts like an elastic enveloping membrane, tending always to contract to the minimum area. While molecules in the interior portion of the liquid are attracted in all directions and are thus at equilibrium, those on the surface are attracted by an overbalancing force toward the interior. In measurement, surface tension is considered as the force with which the surface on one side of a line, one centimeter long, pulls against that on the other side of the line. It is generally expressed in dynes. The pressure due to surface tension varies with the curvature of the film. It is usually expressed as: 2T P— — ron where P is the pressure; T, surface tension; and r, the radius of the drop. As the radius becomes less, the curvature increases and the pres- sure due to surface tension increases. An increase of T will increase the pressure, P, THE FORMS OF SOIL-WATER 161 ture of the original film, and that at B, which is very acute and which naturally must exert a very great outward pull. Under the stress of this pull developed by the surface tension acting in this film of very great curvature, the water is drawn into the space between the particles, where it becomes thicker than the capillary film about the particles. The readjustment continues until the forces developed by the two films become equal. An equilibrium is now established. In the soil the tendency towards adjustment is somewhat similar in so far A ae B Fic. 28.—A conventional diagram showing the coalescence and read- justment of the outer capillary water film of two particles when brought in contact. At the left is shown the condition before the adjustment with a sharp angle at B; on the right, the films are at equilibrium with a thickening at B due to movement from A and A’. as the outer capillary water is concerned. Complete equilib- rium is probably never reached, however, due to constantly disturbing factors. 90. The determination of the amount of capillary water in the soil—The capillary water in a sample of field soil may be determined by making a moisture test in the ordinary way for the total water contained,’ after the gravi- 1A moisture determination on a sample of field soil is generally carried out as follows:—100 grams of the sample, after thorough mixing, is weighed into a suitable weighing dish and air-dried. The sample is then placed in an oven and heated at 100°C or 110°C for four or five hours. It is then cooled in a disiceator and weighed. The loss in weight is water. The moisture is calculated as percentage based on the dry mat- ter of the soil. If the weight of the water lost was 20 grams, the percentage of moisture would be (20 ~ 80) xX 100 or 25 per cent based on dry soil, 162 NATURE AND PROPERTIES OF SOILS tational water has had time to drain away. This represents the hygroscopic plus the capillary water. A determination of the hygroscopic coefficient on another sample yields a figure which, when subtracted from the total water, will give the capillary water present in the soil. The capillary water at various points in a soil column may be obtained by subtracting the hygroscopic coefficient from the various percentages of moisture present, since the hygroscopic moisture is little in- fluenced by height of column or ordinary structural condi- tions. The determination cited above may or may not give the maximum water-holding capacity of a soil. To fill such a need a laboratory method has been devised by Hilgard,t which attempts to show the maximum retentive power of a soil for water. A small perforated brass cup is used, having a diameter of about 5 centimeters and capable of containing a soil column 1 centimeter in height. A short column is used, since it is only under such conditions that a soil may retain against gravity the greatest amount of water. Also the soil is able to expand or contract, as the case may be, on the assumption of water until an equilibrium is reached. A filter-paper disc is often placed in the metal cup, and the soil is poured in, gently jarred down, and stroked off level with the top of the eup. The cup is then set in water and the soil is allowed to take up its maximum moisture. After draining, the weight of the wet soil plus the cup, together with the weights pre- viously obtained, will allow a calculation of the total water retained based on the absolutely dry soil. If the maximum capillary water is desired, the hygroscopic coefficient may be subtracted from the maximum water retained. Since this method is a laboratory procedure and the soil used is not in its normal structural state, the results cannot be accurately applied to field conditions. While the figures 1 Hilgard, E. H., Soils, p. 209, New York, 1911. THE FORMS OF SOIL-WATER 163 obtained may be fairly accurate for a sand, they are certainly much too high for heavy soils. Comparisons with field soils have shown the data obtained by the above method to be from 30 to 1380 per cent. too high.' 91. The capillary capacity of soils—As might nat- urally be expected, the factors that tend to vary the amount of capillary water in a soil are several and their study is rather complex due to the secondary influences that they may ewenerate and to the variable nature of the capillary moisture. These factors may be discussed under four heads: (1) surface tension, (2) texture, (3) structure and (4) organic matter. Any condition that will influence surface tension will ob- viously influence the forces active in the outer portion of the capillary water. A rise in temperature, for example, if the soil is eapillarily saturated, will allow some of the water to become gravitational. A lowering of temperature would cause an opposite change. This theory has been verified by certain experiments by King,’ in which he found, other conditions being constant, a very decided influence on capillary water through change of temperature. Wollny* has shown that a depression of .65 per cent. in sand to as high as 3.7 per cent. in kaolin may occur from a rise in temperature of twenty degrees. While surface tension may be greatly varied by the presence of salts in solution, the soil-water is generally so dilute that the condition is not very important * in determining 1Alway, F. J., and MeDole, G. R., The Relation of Movement of Water in a Soil to its Hygroscopicity and Initial Moistness; Jour. Agr. Res., Vol. X, No. 8, pp. 391-428, 1917. Israelson, O. W., Studies on Capacities of Soils for Irrigation Water; Jour. Agr. Res., Vol. XIII, No. 1, pp. 1-36, 1918. * King, F. H., Fluctuations in the Level and Rate of Movement of Ground Water; U. S. Dept. Agr., Weather Bur., Bul. 5, pp. 59-61, 1892. *Wollny, E., Untersuchungen tiber die Wasserkapacitat der Bodenarten; Forsch. a. d. Gebiete der Agri.-Physik, Band 9, Seite 361-378, 1886. ‘Karraker, P. E., Effect on Soil Moisture of Changes in the Surface Tension of the Soil Solution Brought About By Addition of Soluble Salts; Jour. Agr. Res., Vol. 4, No. 2, pp. 187-192, May, 1915. 164 NATURE AND PROPERTIES OF SOILS capillary capacity except in arid or semi-arid regions. In fact, changes in surface tension through any cause are of little practical importance. The finer the texture of a soil the higher is its capillary eapacity. This is due to the presence of colloidal material and to the greater number of angles in which capillary water may be held. The amount of internal surface exposed by a fine-textured soil is immensety larger than in one of a sandy character. While texture influences both the inner and outer capillary water the structure of the soil has more to do with the active film-like portion. As a clayey soil is granulated the interstitial spaces are enlarged and an increased capillary capacity results. At the same time, compacting a sand will cause a rise in the capillary capacity of, that} soil by increasing not only the actual effective surface, but also the number of angles possible for capillary concentration. Further compact- ing will then cause a decrease. Organic matter, especially when well decayed, is commonly recognized as having great capillary capacity, far excelling the mineral portion of the soil in this respect. Its porosity affords an enormous internal surface, while its colloids exert an affinity for moisture which raises its water capacity to a very high degree. Its tendency to swell on wetting is but a change in condition incident to an approach to its maximum moisture content, and has a very marked influence on the structure of the soil. The water-holding capacity of muck and peat may range as high as 300 or 400 per cent. based on the dry matter present. Assuming a hygroscopic coefficient of 50 per cent., the capillary figure is still very high. Besides this direct effect, organic matter exerts a stimulus toward better granulation, a condition in itself favorable to increased water-holding power. The capillary water in any soil, other conditions being equal, tends to vary with the height of the column. This comes about from the effect of gravity on the outer portion of the capillary THE FORMS OF SOIL-WATER 165 film, tending to give more water at the base of the column. The condition may be explained em- pirically as follows: If a number of par- ticles carrying maximum capillary films are brought together vertically, the weight of a large portion of the conducting film is thrown momentarily on the surfaces at the top. The capillary spaces at this point immediately lose water downward, so that they may assume a greater curvature and thus support this extra weight thrown on them. This curvature must be sufficient to balance the curvature pressure of the par- ticles below plus the weight of the water in the connecting films. The particles be- neath are at the same time undergoing a similar adjustment with a set of particles farther below, losing water in order to allow a change of curvature. The action continues in this manner in an attempt to establish equilibrium, thus giving more water at the bottom of the column. If the amount of capillary water is too great to be supported, enough is lost by gravity to bring about an equilibrium (see Fig. 29). The above illustration, however, does not apply strictly to soil conditions, since only part of the capillary water is in a true film form and free to move with extreme ease. Moreover, rain water is applied from above, where also occurs rapid evaporation. Thus at any particular time the moisture content of a field soil might be higher near the surface than farther down in the soil Fig. 29.—Diagram showing in a con- ventional way the adjustment tendency of the outer capillary water in a long column and the appearance of free water if the weight is too great. 166 NATURE AND PROPERTIES OF SOILS or vice versa as the case may be. As the capillary water in a soil is reduced there is a tendency for the soil column to be more nearly uniform, providing, of course, that the equi- librium forces have had time to act and are not too much influenced by other factors. While representative data regarding the moisture-holding capacity of soils are difficult to give, the following figures from Alway' indicate the general effect of texture and organic matter. The maximum water capacity was determined in the laboratory and the maximum field capacity was obtained by sampling the soils very shortly after irrigation. TABLE XX XI THE MAXIMUM WATER CAPACITY OF VARIOUS SURFACE SOILS AS DETERMINED IN THE LABORATORY AND UNDER FIELD CONDITIONS, RESPECTIVELY 7 i is ey eee eager - Waste Ghskers Sols eres TaHcaNe CAPACITY Lange ange : % % METHOD % MPEUIN GL 29% bytes Nich ace enki - = es: UG 37.0 aHG Pa een tect ye —- ay 12.8 20g Sandy soil, residual.| 1.22 3.3 19.6 34.2 Red loam, residual. . 107 10.0 31.5 49.0 Silt loam, loess..... 1.55 10.1 31.3 56.8 Silt loam, loess..... 4.93 10.2 39.2 60.9 Blackadobe!.2.355.. 2.22 12.9 47.6 60.3 The effect of texture on water capacity is very apparent, a rough correlation existing also between the water retained and the hygroscopic coefficient. The influence of organic matter + Alway, F. J., and MeDole, G. R., The Relation of Movement of Water im a Soil to its Hygroscopicity and Initial Moistness; Jour. Agr. Res., Vol. X, No. 8, pp. 391-428, 1917. *, Note again that moisture percentages are always expressed on dry- soil weight. THE FORMS OF SOIL-WATER 167 is clearly shown by the two loess silt loams. Perhaps most important of all is the marked discrepancy between the actual field capacity and the arbitrary and artificial laboratory method. The normal water-holding capacity of a mineral soil, varying with texture and organic matter, seems to range from INCHES FROM WATER TABLE 0 5 10 15 20 25 30 “WATER Fig. 30.—Diagram showing the distribution of moisture in capillary columns of soil of different textures. The end of each column rests in free water. (Buckingham, E., Bur. Soils, Bul. 38, 1907.) about 10 to 50 per cent. based on dry soil. Muck and peat of course run much higher, 400 per cent. being not uncommon.’ 1 Briggs and McLane have perfected a method of comparing soils on the basis of their capacity to hold water against a definite and constant centrifugal force of one to three thousand times the force of gravity. The soils, in thin layer, are placed in perforated brass cups which fit into a centrifugal machine capable of developing the above force, and are whirled until equilibrium is reached. The resultant moisture per- centage is designated as the moisture equivalent. It really represents the capillary capacity of a soil of minimum column length when subject to a constant and known force or pull. The finer the soil, the greater of course is the moisture equivalent. The authors found that 1 per cent. of clay or organic matter represented a retentive power of about .62 168 NATURE AND PROPERTIES OF SOILS 92. Capillary movement of water.—It has already been shown that different thicknesses of capillary films tend to equalize in the soil due to the pulling forces developed by the angle of curvature between the particles.’ It is evident that differences in curvatures must be the motive force in the capil- lary movement of soil-water. Let it be supposed, for conveni- ence, that three equal spheres when brought in contact contain unequal amounts of water in the angles of curvature (see Fig. 31). In this case the greater pull would exist at A, since the angle here is more acute. Consequently water must move per cent., while 1 per cent. of silt corresponded to a retention of only .13 per cent. of water. Representative data is as follows: ORGANIC MOISTURE SoILs MATTER Sanps| Smut CLay EQUIVALENT % To %o To % Norfolk coarse sand... 9 87.9 7.3 4.8 4.6 Norfolk fine sandy loam. iL 73.4 18.1 8.5 6.8 Wav IGEN Goonenoacs 13) 25.8 64.1 10.1 18.9 Waverly silt loam...... 2.0 14.9 62.9 22.2 24.4 Houston clay loam.... 3.7 30.9 42.5 26.6 32.4 HAO US tomy Claivencrtetsuerateters 1.4 10.0 56.6 33.4 38.2 Briggs, L. J., and McLane, J. W., The Moisture Equivalent of Soils; U. S. Dept. Agr. Bur. Soils, Bul. 45, 1907. 1An ingenious method for measuring quantitatively the capillary pull exerted by a moist soil has been devised by Lynde and Dupré. The apparatus consists of a glass funnel joined to a thick-walled capillary tube by means of a piece of rubber tubing, a water seal being used at this point. The lower end dips into mercury. The soil to be studied is placed in the funnel, and after being saturated is connected by means of a wick of cheesecloth or filter paper to the water column previously established in the capillary tube. If no break occurs between the soil and the capillary water column, the apparatus is ready for use. The excess water having drained away, there is a thinning of the films on the soil surface due to evaporation. Equilibrium adjustments now take place, which result in the drawing upward of the water column. The mercury follows, and the strength of the pull may be measured by the length of the mereury column. The old method of measuring capil- lary power by the water movement through a dry soil is vitiated by two conditions—the length of time necessary, and the fact that the maximum lift cannot be obtained due to excessive friction. This new method uses a wet soil, requires only a short time, and gives a more nearly accurate idea of the power of the capillary pull. It does not, however, THE FORMS OF SOIL-WATER 169 through the connecting film until the pull at A and that at B become the same. Such an adjustment might go on over a large number of films, and if one end of the column was ex- posed to an evaporation of just the correct rate and the other end was in contact with plenty of moisture, large quantities of water would be moved by ecapillarity. This capillary movement may go on in any direction in the soil, since it is largely independent of gravity; yet under natural field conditions the adjustment tends to take place very largely in a vertical direction, due to evaporation and absorption by plants. When a soil is exposed to evaporation, the surface films are thinned and water moves upward to adjust the tension. This explains why such large quantities of soil-water may be lost so rapidly from an exposed soil. Capillary adjustment may go on downward, also, as is the ease after a shower. Here the rapidity of the adjustment is aided by gravity and movement of the water of percolation. The capillary adjustment in a soil tends to take place whether or not the soil column is in contact with free water. If no gravity water is present, the adjustment is merely from a moist soil to a drier one. In studying the rate and height of capillary movement of water in any soil, especially in the yield data regarding rate of movement,—a factor of vital importance to plant growth. Lynde and Dupré, in their results, confirm the statements already made regarding the relation of texture to capillary power: DIAMETER oe LIFT OF WATER IL GRAINS IN Ee Re COLUMN, IN FEET Mil@shipin weMvle es se eos ob aenian o 00 - .25 98 LIN OVSAN Cees eer dee sis enters Seis le 25 - 10 1.78 AV mye fie SATE oie Jo SEF co cle ave w evar 10 - .05 4.05 Slt om mepscorsmasie es aie tae ats .05 - .005 9.99 (CHER nga En cc ee at ey, A ee gee .005- — 26.80 Lynde, C. J., and Dupré, H. A., On a New Method of Measuring the Capillary Lift in Soils; Jour. Amer. Soe. Agron., Vol. 5, No. 2, pp. 107-116, 1913. 170 NATURE AND PROPERTIES OF SOILS laboratory, the maintenance of a supply of free water is usually provided for, since this allows a nearer approach to the maximum eapillary capacity for any point in the column and also gives the most rapid capillary adjustment. To persons familiar with the habits of growing plants, it is evident that eapillary movement must play an important part in their nutrition, since the rootlets are unable to bring their absorptive surfaces in contact with all the interstitial spaces in which the bulk of the available water is held. Con- sequently a consideration of the movement of capillary mois- Fie, 31.—Conventional diagram showing the mechanics of the movement of the film portion of the capillary water. The readjustment takes place in the direction of (A) due to the tension developed by the greater film curvature at that point. ture is necessary, not only as to its mechanics, but also in respect to the factors influencing its rate and height of move- ment. These factors are as follows: (1) surface tension and viscosity ; (2) thickness of capillary film; (3) texture; and (4) structure. Surface tension and viscosity.—As the force developed by surface tension is the activating factor in capillary adjust- ment, any change in the former will influence this movement. Theoretically, a rise in temperature or the presence of soluble salts would decrease the rapidity of the capillary activity of soil-water. In a normal soil, however, the change of surface tension is generally not sufficient to have any very great prac- tical influence. Viscosity, on the other hand, is much more important. If the viscosity of water at 0° C. is taken as 100, THE FORMS OF SOIL-WATER 17] its viscosity at 25° is 50 and at 30°, 45. This explains to a large degree the increased rate of capillary movement due to temperature rise.’ The distance of such adjustment would, however, be lessened somewhat. Salts in solution would tend to check the rate of capillary movement both through in- creased viscosity and the influence on surface tension.2 It would only be in alkali soils, where the concentration of soluble salts is very great, that any considerable retardation would occur. Thickness of capillary film.—It has been repeatedly noticed, in the study of the capillary adjustment between two soils that the lower the percentage of water, the slower is the move- ment. This indicates that the thickness of the outer capillary film, which connects the interstices in which lies the bulk of the movable soil-water, is an important factor in the rate of movement. The above phenomena may be empirically explained as fol- lows: Let it be supposed that a withdrawal of water occurs at A (see Fig. 32), the interstitial space between two of the particles, the water surface being represented by the line aa’. There is an immediate increase in the curvature of this sur- face, and water tends to flow through the capillary film chan- nel (cce’c”) toward this area of greater tension. If water * Bouyoucos has shown that the movement in a soil column of uniform moisture is from the warmer portion toward the colder. The movement from a moist layer to a dryer one goes on more rapidly than when the moist soil is cool and the dry soil warm. Bouyoucos, G. J., Effect of Temperature on Movement of Water Vapor and Capillary Moisture in Soils; Jour. Agr. Res., Vol. V, No. 4, pp. 141-172, Oct., 1915. *Wollny, E., Untersuchungen iiber die Kapillare Leitung des Wassers in Boden. Forsch. a. d. Gebiete d. Agr.-Physik, Band 7, Seite 269-308, 1884, Also, Forsch. a. d. Gebiete d. Agri.-Physik, Band 8, Seite 206-220. 1885. Briggs, L. J., and Lapham, M. H., Capillary Studies; U. S. Dept. Agr. Bur. Soils, Bul. 19, pp, 5-18, 1902. Karraker, P. E., Effect on Soil Moisture of Changes in the Surface Tension of the Soil Solution brought about by the Addition of Soluble Salts; Jour. Agr. Res., Vol. IV, No. 2, pp. 187-192, May, 1915. Davis, R. O. E., The Effect of Soluble Salts on the Physical Proper- ties of Soils; U. S. Dept. Agr. Bur. Soils, Bul. 82, pp. 23-31, 1911. 172 NATURE AND PROPERTIES OF SOILS continues to be withdrawn at A, this adjustment goes on with considerable ease until the film channel (ce’c’”) becomes so thin as to cause its surface now (bb’b’”) to approach very closely to the surface of the soil particle and the inner capil- lary water. The sluggishness of the water movement becomes a factor at this point, impeding the capillary adjustment to- ward A. This point of sluggish capillary movement has been designated by Widtsoe! as the point of lento-capillarity, and Fig. 32.—Conventional diagram for the explanation of the effect of the thickness of water film about the soil particles and their colloidal complexes on the ease of capillary adjustment. is expressed in percentage based on the dry weight of the soil. It lies near the transition zone between the inner and outer capillary water. The amount of capillary water delivered at any one point, therefore, will obviously be influenced by the thickness of the film and may consequently be taken as a measure of rate of adjustment. A short soil column should deliver more water than a longer one, due to the thicker films at the surface of the former. King,” in studying the evaporation from the sur- faces of sand columns of different lengths, their bases being in contact with free water, obtained some significant data. * Widtsoe, J. A., and McLaughlin, W. W., The Movement of Water in Irrigated Soils; Utah Agr. Exp. Sta., Bul. 115, pp. 223-231, 1912. * King, F. H., Principles and Conditions of the Movements of Ground Water; U. S. Geol. Survey, 19th Ann. Rept., Part II, p. 92, 1897- 1898. Also Briggs, L. J., and Lapham, M. H., Capillary Studies; U. 8. Dept. Agr. Bur. Soils, Bul. 19, pp. 24-25, 1902. THE FORMS OF SOIL-WATER 173 He found, for example, that a six-inch column would deliver six times more water to its surface in a given time than a thirty-inch column operating under the same conditions. In air-dry soil it is obvious that, before capillarity may function, a continuous film must be present. Such a condi- tion is impossible unless some of the more active capillary moisture is in the soil. The water content in a soil must often be rather high before capillarity is a noticeable phenomenon. This condition is taken advantage of in the use of soil-mulches, where a loose dry layer of soil on the surface may check evaporation by impeding capillary rise. The presence of oily substances on the soil grains may also be of some importance in this respect. Texture.—In soils of fine texture not only is the amount of film surface exposed greater than in coarse soils but the curvature of the films is also greater, due to the shorter radii. The effective pressure exerted by the films is consequently much higher in fine-grained soil. Both the greater exposure of surface and the increased pressure serve to raise the fric- tion coefficient and retard the rate of flow. The finer the texture of the soil, other factors being equal, the slower is the movement of capillary water. Water should, therefore, rise less rapidly from a water-table through a column of clay than through a sand or a sandy loam. The distance to which water may be drawn by the effective eapillary power of a soil, equilibrium being established, de- pends on the number of interstitial angles. The greater the number of angles, the greater is the total pulling power of the films. As a silt soil contains a larger number of such angles, its capillary pull is greater than that of sand, and con- sequently the ultimate movement would be of greater scope. The finer the texture, then, the slower is the rate of capillary movement but the greater is the distance. The relation of texture to rate and height of capillary move- ment in air-dry soil is shown by the following unpublished 174 NATURE AND PROPERTIES OF SOILS data, obtained in the laboratory of the Department of Soil Technology, Cornell University : TABLE XX XII EFFECT OF MOISTURE ON RATE AND HEIGHT OF CAPILLARY RISE FROM A WATER-TABLE THROUGH AIR-DRY SOIL il 1 2 3 4 5 SOIL Hour Day Days | Days | Days | Days a ies i IncHEs |INcHEs | INCHES INCHES INCHES | INCHES SANG y SOL cee 3.9 5.0 5.9 6.8 6.8 6.9 Clayey soil....... 3) 5.7 8:9). 10:9")) S22 ase Silt. loam. a2 95° 145) 20:6") 24-2) | 26 ele It is seen that the movement in sand is rapid, one-half of the total rise being attained in one hour. The maximum height is reached in about three days. The silt loam in this ease seems to be of just about the proper textural condition for a fairly rapid rise, yet it exerts enough capillary pull to attain a good distance above the water-table. The friction in the clay is greater, however, and this results in a slower rate. Structure has already been shown to affect capillary capac- ity by its influence on the angle interstices and the closeness of the contacts. Evidently, therefore, it may alter both the rate and the height of capillary rise. The loosening of a clay soil or the compacting of a sandy soil will lessen the effective film friction, while at the same time it may strengthen the capillary pull resulting in a faster and a higher capillary flow of water. What may be the best structural condition of any soil in which this result is realized to its highest degree can not be predicted exactly. In general, however, this point is approached when the soil is in the best physical condition for crop growth. Tillage operations, tile drainage, and the addi- tion of lime and organic matter operate toward this result by their granulating tendencies; while rolling, by compacting a THE FORMS OF SOIL-WATER 175 too loose surface, may accomplish the same effect but by an opposite process. At certain seasons of the year ecapillarity should be im- peded near the surface, as it continually carries valuable water upward to be lost by evaporation. This movement may be checked somewhat by producing on the soil surface, by appropriate tillage, a layer of dry, loose soil. This layer, called a soil-mulch, resists wetting because of its dryness, while at the same time it affords but little surface and few angle inter- stices for effective capillary pull. Moisture also moves very slowly from a moist, cool soil to a dry, warm one.’ Thus it is that a farmer, in order to meet immediate or future plant needs, may alter and control capillary movement by careful attention to physical conditions, especially those at the sur- face where evaporation is always active. 93. Gravitational water and its movement.—As soon as the capillary capacity of a soil column is satisfied, further addition of moisture will cause the appearance of free water in the air spaces. By the attraction of gravity, this water moves forward through the soil at a rate varying with con- ditions. In general, the flow is governed by four factors— pressure, temperature, texture, and structure. An under- standing of the operation of these forces is important, since the rapid elimination of free water from the soil is necessary for normal plant growth. It is very evident that any pressure exerted on a water column will alter the rate of flow. Under normal conditions pressure may arise from two sources, atmospheric pressure and the weight of the water column. Changes in barometric pressure are communicated to gravitational water through a movement of the soil-air. As the mercury column rises more air is forced into the soil and the pressure on the soil-water *Bouyoucos, G. J., Effect of Temperature on Movement Water Vapor and Capillary Moisture in Soils; Jour. Agr. Res., Vol. V, No. 4, pp. 141-172, Oct., 1915. 176 NATURE AND PROPERTIES OF SOILS increases. The weight of the free water column may also have some influence. Although King’? and Welitschkowsky? have shown that definite relationships exist between the move- ment of gravity water and both atmospheric pressure and weight of water column, the practical field importance of these factors are rather slight. A rise in temperature of the soil not only varies the relative amounts of capillary and free water present, but at the same time it increases the fluidity and thus facilitates percolation. The expansion of the soil-air also tends to inerease such movement. On the other hand the swelling of hydrogels which may be present tends to impede percolation to such an extent that the movement of free water through a heavy soil is often markedly checked by temperature rise. Of much more practical importance than either pressure or temperature in the flow of gravity water is the texture and the structure of the soil. In working with sands of varying grades, Welitschkowsky,* Wollny,* and others have shown that the flow of water varies with the size of particle, or texture. King °® has demonstrated that in general the rate of flow through such is directly proportional to the square of the diameter of the particles. By the use of the effective mean 1King, F. H., Principles and Conditions of the Movements of Ground Water; U. 8. Geol. Survey, 19th Ann. Dep., Part II, pp. 67-206; 1897- 1898. King, F. H., The Soil, p. 180, New York, 1906. SWelitschkowsky, D. von., Experimentelle untersuchungen uber die Permeabilitit des Bodens fiir Wasser; Archiv. f. Hygiene, Band II, Seite 499-512. 1884. Wollny, E., Untersuchungen iiber die Permeabilitat des Bodens fir Wasser ; Forsch. a. d. Gebiete d. Agy.-Physik, Band 14, Seite 1-28, 1891. ® Welitschkowsky, D. von., Experimentelle untersuchungen liber die Permeabilitat des Bodens fiir Wasser; Archiv. f. Hygiene, Band II, Seite 499-512, 1884. *Wollny, E., Untersuchunger iiber den Einfluss der Struktur des Bodens auf dessen Feuchtigketis—und Temperaturverhaltnisse ; Forsch. a. d. Gebiete d. Agr.-Physik, Band 5, Seite 167, 1882. ’ King, F. H., Principles and Conditions of the Movements of Ground Water; U. S. Geol. Survey, 19th Ann. Rep., Part II, pp. 222-224, 1897- 1898, THE FORMS OF SOIL-WATER 177 diameter of a sand sample he was able to calculate a theo- retical flow which compared very closely to observed percola- tions. In sandy soils low in organic matter this law holds in a very general way, but in clays it fails entirely. For example, if such a law was in force a sand having a diameter of .5 millimeter would exhibit a flow 10,000 times greater than that through a clay loam with a diameter, say, of .005 millimeter; whereas the actual ratio, as observed experimen- tally by King, was less than 200. Such a discrepancy is to be expected as it is impossible accurately to apply mathematics to soils carrying any appreciable amount of colloidal matter. Evidently, therefore, while it can be stated as a general thesis that the flow of gravity water varies with the texture, being much more rapid through a coarse than through a fine soil, no law can be deduced for soils, since structure exerts such a modifying influence. The percolation in a heavy soil takes place largely through lines of seepage, which are really large channels developed by various agencies. If in the drainage of average soil, the farmer depended on the movement of water through the individual pore spaces, the soil would never be in condition for crop growth. These lines of seepage are developed by the ordinary forces that function in the production of soil granulation, as freezing and thawing, wetting and drying, lime, organic matter, roots, and tillage operations. 94. Determination of the quantity of free water that a soil will hold— While there is no particular advantage in finding the quantity of gravitational water that a soil will hold, since a normal soil should never remain saturated for any length of time, it is nevertheless of interest to know by what means such data may be obtained. One method is to saturate a soil column of known weight, and then, by exposing it to percolation, measure the amount of water that is lost. The gravitational water can then be expressed in terms of dry soil. 178 NATURE AND PROPERTIES OF SOILS As valuable a figure may be obtained by ealculation, pro- viding the specific gravity and volume weight of the soil is known together with its percentage of moisture based on dry weight when it is capillarily satisfied. The following formu- lea + may be used: vol. wt. 100 1. Percentage pore space = 100 — [= x T| 2. Percentage free water = Zone Saco % water at (based on dry weight Vol. Wt. maximum of soil) eapillarity Suppose, for example, that a sand with a specific gravity of 2.6 and a volume weight of 1.56 contains 20 per cent. of water when at its maximum retentive power. Its pore space would be 40 per cent. If this pore space were filled with water, the soil would contain 25.6 per cent. based on the dry weight of the soil (per cent. pore space ~ vol. wt.). If the total capac- ity of the soil for water is 25.6 per cent. and the hygroscopic plus the capillary capacity is 20 per cent., the free water must be 5.6 per cent.? 95. Importance of the study of the flow and composi- tion of drainage water—A clear understanding of the factors governing the flow of gravitational water is of special importance in tile drainage operations, particularly regarding the depth of and interval between tile drains. Since percola- tion is so slow in a heavy soil it is evident that the tile must be near the surface in order to secure efficient drainage. In a sand the depth may be increased, because of the slight re- *Percentage of pore space represents the percentage of water by volume that would occupy such a space. Percentage of water by volume divided by volume weight gives percentage of water based on dry weight of soil. Conversely, multiplying percentage of moisture calculated on dry weight of soil by volume weight will give percentage of water by volume. The air space in a soil at any particular moisture content may be ecal- culated as follows: Percentage of air space = % pore space — (%H,0 X Vol. Wt.) ? Below will be found some generalized moisture data on two distinct THE FORMS OF SOIL-WATER 179 sistance offered to water movement. The depths for laying tile in a heavy soil range from one and a half to two and a half feet, while in a sand the tile may often be placed as deep as four feet below the surface. It is evident also that the less deep a tile drain is laid the less distance on either side it will be effective in removing the water; consequently on a clay soil the laterals must be relatively close as compared to the interval generally recommended for a sandy soil. A rational understanding of the movements of gravitation water is clearly necessary in the installation of tile drains not only that the system may be efficient, but also that a minimum effective cost may be realized.* The water lost from the soil by drainage is of especial in- terest in plant production because of the large amounts of nutrient elements carried away each year. Such loss is par- ticularly important in regard to the lime and nitrogen.? The equivalent of approximately 500 pounds of sodium nitrate and 1000 pounds of calcium carbonate have been known to leach from an acre of bare soil every year under humid con- ditions. elasses of soils. As usual, all of the moisture data is expressed as per- centage based on absolutely dry soil. SANDY CLAYEY Soin Som I PRCING COTAVELY (soi 0 sel ceistes, s10.5 aysverat'es 2.67 2.65 Wrolumeswel cht sy). cra sversee ote eters reece 1.60 1.20 OUG a SCO pga ss Se ayotel «occ vanslinte is lors 6-46 3h 40.0% 54.8% lshfaOs Coeuiteenhy gegcooss assoansac 1.0% 10.0% Optimum moisture (average)....... 10.0% 30.0% Maxamnimin tel duicapacity-nmrciaccan ae 17.0% 44.0% Air space at hygro. coefficient....... 38.4% 42.8% Air space at opt. moisture.......... 24.0% 18.7% Air space at max. field capacity..... 12.8% 19% Possible streemwateraceeettacoe nace or 8.0% 1.6% See Kopecky, J., Die physikalischen Eigenschaften des Boden; Internat. Mitt f. Bodenkunde, Bd. IV, Heft 2-3, Seite 138-180. 1914. *For a more complete discussion of tile drains, see Chap. X, para- graph 110. *Lyon, T. L., and Bizzell, J. A., Lysimeter Experiments; Cornell Univ. Agr. Exp. Sta., Memoir 12, June, 1918. 180 NATURE AND PROPERTIES OF SOILS Two methods of procedure are available for the study of drainage problems—the use of an efficient system of tile drains, and the construction of lysimeters. For the first method an area should be chosen where the tile drain receives only the water from the area in question and where the drain- age is efficient. A study of the amounts of flow throughout a term of years will yield much valuable data concerning the factors already discussed. An analysis of the drainage water will throw light on the ordinary losses of plant nutrients from a normal soil under a known cropping system. The advantage of such a method of attack lies not only in the fact that a large area of undisturbed soil is considered, but also in the opportunity to study practical field treatments in relation to the movement and composition of drainage water. The lysimeter method, however, has been the usual mode of approaching such problems. In this method a small block of soil is used, being entirely isolated by appropriate means from the soil surrounding it. Effective and thorough drainage is provided. The advantages of this method are that the varia- tions in a large field are avoided, the work of carrying on the study is not so great as in a large field, and the experiment is more easily controlled. One of the best-known sets of lysi- meters is that at the Rothamsted Experiment Station’ in Eng- land. Here blocks of soil one one-thousandth of an acre in surface area were isolated by means of trenches and tunnels, and, supported in the meantime by perforated iron plates, were permanently separated from the surrounding soil by masonry. The blocks of soil were twenty, forty, and sixty inches in depth, respectively. Facilities for catching the drain- age were provided under each lysimeter. The advantages of such a method of construction les in the fact that the struc- tural condition of the soil is undisturbed and consequently the data are immediately trustworthy. *Lawes, J. B., Gilbert, J. H., and Warington, R., On the Amount and Composition of the Rain and Drainage Waters Collected at Rothai- sted; Jour. Roy. Agr. Soc., Ser. II, Vol. 17, pp. 269-271, 1881. THE FORMS OF SOIL-WATER 181 At Cornell University’ a series of cement tanks sunk in the ground have been constructed. Each tank is about four feet and two inches square and about four feet deep. A slop- ing bottom is provided, with a drainage channel opening into Et: SESS _ 5 alk »> Z = 3in. tile > : A . PO, ABSORBED IN PARTS PER MILLION TIME - CLAYEY |FINE SANDY Soin Soin SIMUMULEES yn oe Pin ec oe ke epee ta 400 2aD ALU MEITETA GES cs) Ss Sa clohs, cea awed aaah 410 250 TEC LO 12 t 2 ee aes a Car ee 415 260 rue TAROT LETS oe SORE eee ASR RE he 435 315 21 LEO LETS Rag gp CD one RR) 440 335 2 5] ANTE epee RS PR ea 445 370 It must not be inferred that, when a solution is brought in contact with a soil, it always becomes weaker because of absorption. Negative absorption may occur in which the sol- vent is taken up more rapidly than the solute. Concentra- tion is thus induced. 141. Selective absorption.?—The fixation phenomena by the soil, whether physical or chemical, is of two types: (1) the absorption of molecules, the compound being taken up un- changed; and (2) the absorption of ions. In the first case, *The law which appears to govern absorption of phosphates and potash by the soil may be expressed mathematically as follows: dy 2. ae K (A—Y) in which K is a constant, A the maximum quantity possible for the soil to absorb and y the quantity actually fixed when v, volume of the solution, has percolated through. A short discussion of the mathematics of this law may be found in the following publication: Schreiner, O., and Failyer, G. H., The Absorption of Phosphates and Potassium by Soils; U. S. Dept. Agr., Bur. Soils, Bul. 32, pp. 23-24, 37-39, 1906. *A very good discussion of selective absorption is found in the following: Parker, E. G., Selective Absorption by Soils; Jour. Agr. Res., Vol. 1, No. 5, pp. 179-188, 1913. 270 NATURE AND PROPERTIES OF SOILS if a residue is left, it is unchanged except in concentration. Such would be the case in the absorption of certain dyes, of gases and of hydroxides of various kinds, where the molecule is fixed intact. This first form of absorption is by no means as important as the selective absorption of ions. Certain compounds, ealled electrolytes,t tend when in solu- tion to ionize or split up into ions. Thus potassium nitrate, a neutral salt, breaks up into K* and NO-, ions, the degree of ionization depending on the concentration of the solution. When such a solution is brought into contact with soil, the latter usually, but not always, exerts a greater affinity for the basic ion, leaving an excess of the acid radical in solution. The water present furnishes small amounts of H* and OH- ions, thereby encouraging the formation of KOH, which is absorbed intact, together with the K+ and OH~ ions. This action, therefore, leaves the Ht and NO-, ions preponderant in the solution, which is of necessity acid in reaction due to the hydrogen ion concentration. This selective absorption may be demonstrated with any neutral salt and any neutral absorbent, the resultant extract always being acid due to the selective absorption of the basic ions. 142. Substitution of bases.°—Associated with the selec- tive absorption of bases from solution there is a liberation of 1 According to the theory of the electrolytic-dissociation or ioniza- tion, many compounds under certain conditions break up into electrically charged portions called ions. Ions may be single atoms or a group of atoms. Many inorganic substances are almost completely ionized. A few organic compounds exhibit marked dissociation but many are not appreciably affected. Water dissociates into H+ and OH- ions to the extent of about .00001 of a per cent. or 1 part in 10,000,000. An acid yields hydrogen ions and other ions carrying the remainder of the molecules. Alkalies give hydroxyl ions and other ions consisting of the remaining portion of the molecules. The acidity or alkalinity of a solution is determined by its hydrogen-ion concentration. ?Van Bemmelen, J. M., Das Absorptionsvermogen der Ackererde; Landw. Vers. Stat., Band 21, Seite 135-191, 1877. Sullivan, E. C., The Interaction beiween Minerals and Water Solu- tions; U. S. Geol. Survey, Bul. 312, 1907. Wiegner, G., Zum Basenaustausch in der Ackererde; Jour. Landw., Band 60, Seite 111-150, 197-222, 1912. THE ABSORPTIVE PROPERTIES OF SOILS 271 other bases from the soil, which appear in the filtrate as ions and in combination with acid radicals. Such phenomena may be considered as mere basic exchange, pushed forward by the mass action of the ion absorbed, and is called substitution of bases. The change may be illustrated as follows: KCl + X, Silicate = X,Cl + K Silicate It is unlikely that this reaction actually takes place to any extent in fertilizer practice.t. It is more probable that the acid produced by the selective absorption liberates the bases from their loose union with the hydrated aluminum silicate complexes. HCl + X, Silicates = X,Cl + H Silicates A dilute solution of potassium chloride filtered through a soil will produce a filtrate containing some calcium, mag- nesium, or chloride or all of these salts and some potassium chloride. The more dilute the solution, the larger will be the proportion retained, but the less the total quantity absorbed. Peters * treated 100 grams of soil with 250 cubic centimeters of a solution of potassium salts, and found that the potassium of separate salts was retained in different proportions, and that the more concentrated solutions lost relatively less than the weaker ones, although more actual potassium was re- moved from the former. TasBLE LIX GRAMS OF K,O GRAMS OF K,O ABSORBED FROM A ABSORBED FROM A SOLUTION 1/10 NorMAL Souvu- | 1/20 NormMat Souvu- TION TION 150) I ae a 0124 .1990 RENO amy aes se 3362 2098 ECO Mo aeeitns 2 oe TAT 3134 1Parker, E. G., Selective Absorption by Soils; Jour. Agr. Res., Vol. 1, No. 5, p. 180, 1913. *Peters, E., Uber die Absorption von Kali durch Ackererde; Landw. Vers. Stat., Band 2, Seite 113-151, 1860. 272 NATURE AND PROPERTIES OF SOILS The same bases are not always absorbed in the same propor- tion by different soils; one soil may have a greater absorp- tive power for potassium, while another may retain relatively more ammonia. They seem to be somewhat interchangeable, as any absorbed base may be released by a number of others in solution. The absorptive power of a soil for certain bases is reflected in the composition of the drainage water from the soil. The latter varies with the soil, and a soluble fertilizer applied to one soil will have a different effect on the composi- tion of drainage water than if applied to another soil. This is well illustrated from lysimeter experiments by Gerlach * at Bromberg. Several soils were used, a portion of each being fertilized and unfertilized respectively. The lysimeters were 1.2 meters deep and contained 4 cubic meters of soil. The drainage water was collected and analyzed for four years. The first year there was no crop, the second year potatoes were erown, the third oats, and the fourth rye. The following re- sults were obtained : TABLE LX AVERAGE COMPETITION OF DRAINAGE WATER IN PARTS PER MIL- LION. BROMBERG. TOTAL On- Sor TREATMENT N NO, | GANIC} K,O CaQ Moor, Soles 26. Mertilized ||) 32-7 |) 30:0 | i274 32e2 405 Untreated | 65.0 | 60.3 | 4.7 | 26.2 507 Sand low in or- ganic matter...| Fertilized| 25.5 | 25.1 A} 25.1 92 Untreated | 20.9} 20.4 uy 8.5 90 Sandy loam high in organic matter. ..... Fertilized| 67.8 | 64.6 | 3.1] 70.2 399 Untreated | 69.5] 66.1 | 3.4] 47.4 414 Gerlach, U., Uber die durch sickerwasser dem Boden Entzogenen Menge Wasser und Nahrstoffe; Il. Landw. Zeitung, 30 Jahrgrange, Heft 95, Seite 871-881, 1910. THE ABSORPTIVE PROPERTIES OF SOILS 273 143. Importance of absorption—Absorption is impor- tant, not only because it allows the soil to retain certain nutri- ents against excessive leaching, but because it facilitates the condensation and concentration of gases within the soil.t Rus- sell and Appleyard? have shown that the inner soil air is held very tightly and must in consequence be under consid- erable pressure. Such gas absorption tends to force reactions which otherwise would be very slow. A part of the catalytic power of the soil may be accounted for in this way. Moreover, the absorption of water by the soil is by no means unimportant. It is because of such phenomena that the moisture of the soil occurs in various forms and possesses distinctly different re- lationships to the plant. The selective absorption of the basic ions by soils of every type is important in a number of ways. In the first place, potassium, calcium, magnesium, and iron function in the soil as bases. Selective absorption tends to conserve these nutri- ents to the exclusion of their acid radicals, which are readily lost in drainage. Phosphorus, however, has a different status, for although it is held as a part of an acid radical (PO,), it is saved from leaching by the insolubility of the compounds which tend to form. In the second place, selective absorption apparently produces residues when fertilizers are added and these residues are almost always acid. Sodium nitrate, am- monium sulphate, potassium chloride, and potassium sulphate will leave an acid residue in the soil solution unless influenced by extraneous factors, such as the addition of lime or the ac- tion of plants. *Patten, H. E., and Gallagher, F. E., Absorption of Vapors and Gases by Soils; U. S. Dept. Agr., Bur. Soils, Bul. 51, 1908. McGeorge, W., Absorption of Fertilizer Salts by Hawaiian Soils ; Haw. Agr. Exp. Sta., Bul. 35, p. 32, 1914. Cook, R. C., Factors Affecting the Absorption and Distribution of Ammonia Applied to Soils; Soil Sci., Vol. II, No. 4, pp. 305-344, 1916. ? Russell, E. J., and Appleyard, A., The Atmosphere of the Soil: Its Composition and the Causes of Variation; Jour. Agr. Sci., Vol. VII, Part 1, pp. 1-48, 1915. 274 NATURE AND PROPERTIES OF SOILS The acidity of soils, which is a function not only of the soil solution but of the solid portions also, is frequently attributed to certain absorptive phenomena, one idea being that, due to physical and chemical absorption of bases, a concentra- tion of the hydrogen ion is produced and actual acidity re- sults. Basie exchange seems to liberate iron and aluminum, the salts of which easily hydrolize and yield acid solutions. If, as some investigators maintain, the toxic principle of the so-called acid soils is active aluminum, manganese or similar elements, absorption may again be the activating phenomenon, since an unsatisfied absorptive capacity, especially for cal- cium and magnesium, seems to favor the presence of such constituents in the soil solution. The absorptive power of the soil is a controlling factor as far as the composition and concentration of the soil solution is concerned. Any study of the dynamic relationships of the water solution that exists in the soil interstices and in the col- loidal complexes which coat the soil particles, must reckon with absorption phenomena and all of the factors which tend to influence them. CHAPTER XIV THE SOIL SOLUTION THE soil is a heterogeneous mixture of solids, gases, and a liquid. The mineral constituents come from the debris of rock, the organic matter is derived from plant and animal tissue, while through and around these complex materials the water and gases of the soil circulate in ever-changing propor- tions. Minute organisms are also present in great numbers, aiding, through their enzymic activities, the intricate trans- formations. As a result of the reactory inter-relations of the soil components, a solution is generated which tends to come into equilibrium with the solids and gases with which it is in contact. As it is from this source that plants obtain their mineral nutrients, the soil solution and its control demand especial attention. The fundamental error of many soil conceptions has been to regard the soil as a static system. Chemical, physical, and biological activities are admitted, but they have been regarded as of little importance in influencing the soil mass as a whole. Such a conception is in error as every constituent of the soil is dynamic. The presence of large amounts of material in a colloidal state makes the constaney of any particular con- dition impossible over any extended period. In studying the soil solution, especially as to its composi- tion and concentration, the phenomenon of absorption can not be ignored. The tendency of certain portions of the soil to go into solution, while other parts are absorbing both the solvent and the solute, must be reckoned with. Moreover, the losses of nutrients to the plant and through leaching are 275 276 NATURE AND PROPERTIES OF SOILS a factor to be considered. Obviously the concentration and composition of the soil solution is first of all a function of the absorptive capacity of the soil complexes, modified by the rate of solution and the magnitude of crop and leaching activities. 144. Absorption and the soil solution.—In a bare moist soil, where there is no evaporation or leaching to disturb equilibrium ‘tendencies, the soil presents a_ three-phase system. The phases are: (1) the solution surfaces, (2) the absorptive or colloidal surfaces, and (3) the SOLUTION SURFACES ABSORPTION ia Sore COMPLEXES SOLUTION Fig. 50.—Diagram showing the equilibrium tendencies that exist between the solution surfaces, the colloidal complexes and the soil solution. soil solution itself. When solution takes place, the con- stituents so affected are acquired in part by the soil mois- ture as a solute and in part by the absorptive complexes. There is a constant attempt at equilibrium, which of course is never attained as long as solution continues. Under field conditions, many other disturbing factors enter. The rate of solution may vary, and the capacity and character of the absorbing colloidal complexes are always changing. Moreover, the amount of water in the soil is never constant, due to drainage and evaporation. The feeding of the plant, as re- 1This term refers to the soil surfaces from which solution takes place. THE SOIL SOLUTION 277 gards both water and nutrients, and losses by leaching, must always be considered. In addition, the effect of tillage as well as the common practices of adding farm manure, plow- ing under of green-crops and applying fertilizers and lime, are constantly effective in obstructing equilibrium adjust- ments.! (See Fig. 50.) The soil solution is, therefore, markedly dynamic in char- acter, constantly changing in composition and concentra- tion. Its important control is absorption, the absorptive sur- faces acting as a depository, in which active reserve nutrients are held. As the solution is depleted in any constituent, quicker adjustment takes place between the solvent and the colloidal complexes than is possible between the solution and the solution surfaces. Rapid adjustments, as far as the sup- ply of nutrients for plants is concerned, is possible only be- cause of the absorptive properties of the colloidal complexes of the soil. 145. Methods of studying the soil solution.—Questions regarding the soil solution are difficult to answer because no adequate procedure has been devised for extracting a repre- sentative sample of the solution as it existed in the soil. More- over, no wholly satisfactory method has been perfected for its measurement in place. Various extractive methods have been tried. Briggs and McLane? attempted to sample the solution by the use of a centrifuge developing a force of two or three thousand times that of gravitation. When the soil contained a rather large quantity of capillary water, a small amount of it could be removed in this way. 1Bouyoucos has shown that even under controlled conditions the equilibrium between finely ground minerals and water is not absolute or real due to the complex hydration and hydrolysis which continually occur. Bouyoucos, G. J., Rate and Extent of Solubility of Mimerals and Rocks under Different Treatments and Conditions; Mich. Agr. Exp. Sta., Tech. Bul. 50, July, 921. ? Briggs, Lyman J., and McLane, John W., The Moisture Equivalent of Soils; U. S. Dept. Agr., Bur. Soils, Bul. 45, pp. 6-8, 1907. 278 NATURE AND PROPERTIES OF SOILS Another device, perfected by Briggs and McCall,: consists of a close-grained, unglazed porcelain tube, closed at one end and provided at the other with a tubulure, by which it can be connected with an exhausted receiver. This tube is mois- tened and buried in the soil. If the moisture content of the soil is sufficient to reduce the pressure of the capillary water surface in the soil to less than half the difference between the pressure inside and outside of the tube, there will be a move- ment of water inward. The water may be collected and ana- lyzed. More recently Van Suchtelen has used another method to obtain the soil solution. He replaces the soil-water by means of paraffin in a liquid state, at the same time subjecting the soil on a filter to suction. The displaced water is considered to represent the soil solution. Later Van Suchtelen and Itano substituted pressure for suction, modifying the apparatus to meet the new procedure. This apparatus has been further perfected by Morgan.* Lipman‘ has proposed a method in which very high pressure, a minimum of 53,000 pounds to the square inch, is utilized in squeezing out the soil-water.°® All such methods are open to the objection that the sample is not representative. The soil solution changes both in con- 1 Briggs, L. J., and McCall, A. G., An Artificial Root for Inducing Capillary Movement of Soil Moisture; Science, N. S., Vol. 20, pp. 566-569, 1904. 2Van Suchtelen, F. H. H., Methode zur Gewinnung der Natiirlichen Bodenlosung; Jour. f. Landw., Band 60, Seite 369-370, 1912. *Morgan, J. F., The Soil Solution Obtained by the Oil Pressure Method; Mich. Agr. Exp. Sta., Tech. Bul. 28, Oct., 1916. 4 Lipman, C. B., A New Method of Extracting the Soil Solution; Univ. Cal. Pub., Agr. Sci., Vol. 3, No. 7, pp. 131-134, 1918. Ramann, E., et al., have proposed a similar method but with less pressure. Internat. Mit. f. Bodenkunde, Bd. 6, Seite 27, 1916. For a good criticism of this method, see Northrup, Zea, Science, N.S., Vol. XLVII, No. 1226, p. 638, June 1918. °Ischerekov in 1907 used ethyl alcohol to displace the water in a soil column utilizing only the force of gravity. Parker claims that this method is of considerable value. He found that data so obtained compared closely with that obtained from the water extract method. Parker, F. W., Methods of Studying the Concentration of the Soil Solution; Soil Sci., Vol. XII, No. 3, pp. 209-232, 1921. THE SOIL SOLUTION 279 centration and composition so readily that the addition of ex- traneous material or the exertion of unnatural pressure defeat the object of the determination. Moreover, the soil solution is probably not homogeneous and unless practically all of it is removed a sample of value cannot be obtained. The signifi- cance of such a sample, if it were attained, is questionable, as it is impossible to know the proportion of the soluble nu- trients that may actually be appropriated by the growing plant. The method of obtaining soil extracts has been used to a greater extent than any other in studying the soil solution. Water is the usual solvent. The Bureau of Soils filter method * is commonly followed. As might be expected, it is purely arbitrary in its procedure, the idea being to make the results comparative rather than strictly quantitative. Soil and water in the proportions of 1 to 5 are mixed, stirred three minutes and allowed to stand twenty minutes. The supernatant liquid is then forced through a Pasteur-Chamberland filter and a clear extract obtained for analysis. The solution obtained is not representative of the soil-water and its solutes. It is only an extract of the soil. The addi- tion of a large amount of water is a disturbing factor. The concentration of the extract is also modified by the absorptive power of the soil, being relatively greater for a sandy than for a clayey soil. Moreover, the differential influence of the solvent comes into play, for as soon as solution begins, the solvent is no longer pure water but a solution of constantly changing efficiency. Nevertheless, the work of Hoagland, Stewart and Burd ? indicates that there is not only a relation- 1Schreiner, O., and Failyer, G. H., Colometric, Turbidity and Titra- tion Methods Used in Soil Investigations; U. 8. Dept. Agr., Bur. Soils, Bul. 31, 1906. ? Hoagland, D. R., The Freezing Point Method as an Index of Varia- tions in the Soil Solution Due to Season and Crop Growth; Jour. Agr. Res., Vol. XII, No. 6, pp. 369-395, 1918. Stewart, G. R., Effect of Season and Crop Growth in Modifying the Soil Solution; Jour. Agr. Res., Vol. XII, No. 6, pp. 311-368, 1918. 280 NATURE AND PROPERTIES OF SOILS ship between the water extract of a soil and its productivity, but a correlation with the strength of the soil solution as well. The extract method is especially valuable in studying the nitrates of the soil solution. As nitrate nitrogen does not suffer as much absorption as do the nutrient bases, that which appears in the extract is a fair measure of the strength of the soil solution insofar as this constituent is concerned. The only method for measuring the concentration of the soil solution 2n situ is that of Bouyoucos.' This is known as the depression of the freezing point method. It is possible, when dealing with a pure solution of a known salt, to calculate its concentration by determining how much the freezing point is lowered or depressed below 0° C. This principle is applied to the soil by using a Beckman thermometer and the proper control apparatus. As the soil solution carries a great num- ber of different ions in unknown proportions, it is impossible to ealeulate even the concentration with accuracy, a factor of somewhat doubtful validity being utilized. The procedure gives nothing regarding the presence of specific ions nor are its results uniform, due to the variable dissociation of the salts present. Nevertheless the method has thrown much lght on the many difficult problems of the soil and its solution. 146. Qualitative composition of the soil solution.—Once the dynamic character of the soil solution is conceded, three points of importance immediately demand attention: (1) the qualitative composition of the soil solution and its concentra- tion 7n toto, (2) the quantitative composition, and (8) the factors most important in influencing both the composition and the concentration of the solution. It must be recognized at the outset that the soil solution Burd, J. S., Water Extractions of Sotls as Criteria of their Crop Producing Power; Jour. Agr. Res., Vol. XII, No. 6, pp. 297-309, 1918. Hoagland, D. R., Martin, J. C., and Stewart, G. R., Relation of the Soil Solution to the Soil Extract; Jour. Agr. Res., Vol. XX, No. 5, pp. 381-395, 1920. + Bouyoucos, G. J., Further Studies on the Freezing Point Lowering of Soils; Mich. Agr. Exp. Sta., Tech. Bul. 31, Nov., 1916. THE SOIL SOLUTION 281 is generally dilute except in arid regions under conditions of alkali. The concentration probably very seldom exceeds 30,- 000 parts per million and is normally very much lower. More- over, the greater proportion of the solute is in an ionic state, molecules appearing only when the concentration is relatively high. It is well to note that the plant absorbs most of its nutrients in the ionic condition. From the knowledge obtained by the analysis of soil ex- tracts, it is safe to assume that all of the common bases and acid radicals normally occur in the soil solution. Thus, K*, Na*, Mg**, Ca**, Fe*t*+, Al** and NH,* ions may be expected as well as such ions as SO%, 8i0%, Cl, POz, NO;, NO; and CO. Since water dissociates slightly, Ht and OH- ions will also be present. The reaction of the solution will depend on its hydrogen-ion concentration and may be alkaline, neutral or acid as the case may be. Most soil solutions seem to be slightly acid,' possibly due to the action of carbon dioxide. Morgan ? found on an examination of the solutions obtained from soils by the oil pressure method that, as the moisture in- creased, the concentration of the solution decreased. These findings are amply corroborated by the work of Bouyoucos ® with the depression of the freezing point method. The latter presents data regarding the actual concentrations at various moisture contents, which seem to indicate the general differ- ences that may be expected between soils of different types. *Gillespie, L. J., The Reaction of Soil and Measurements of Hydro- gen-ion Concentration; Jour. Wash. Acad. Sci. Vol. 6, No. 1, pp. 7-16, 1916. Sharp, L. T., and Hoagland, D. R., Acidity and Adsorption in Soils as Measured by the Hydrogen Electrode; Jour. Agr. Res., Vol. VII, No. 3, pp. 123-145, 1916. Hoagland, D. R., Relation of the Concentration and Reaction of the Nutrient Medium to the Growth and Absorption of the Plant; Jour. Agr. Res., Vol. XVIII, No. 2, pp. 73-117, 1919. *Morgan, J. F., The Soil Solution Obtained by the Oil Pressure Method; Mich. Agr. Exp. Sta., Tech. Bul. 28, 1916. * Bouyoucos, G. J., Further Studies on the Freezing Point Lowering of Soils; Mich, Agr. Exp. Sta., Tech. Bul. 31, pp. 14-15, 1916. 282 NATURE AND PROPERTIES OF SOILS TasLE LXI THE CONCENTRATION OF THE SOLUTION OF VARIOUS SOILS AS DE- TERMINED BY THE DEPRESSION OF THE FREEZING POINT. EX- PRESSED IN PARTS PER MILLION BASED ON DRY SOIL. Som, MOISTURE ee e RA || MoIsTURE Cone ate 2 Pea Me To P. P.M. Superior clay... 18.8 29,268 39.4 415 Miami silt loam. 8.8 19,560 36.0 707 Carrington loam 15.2 16,390 38.5 463 Plainfield sand. 5.0 6,342 24.6 366 PEA ihe: cos arerenes 61.3 23,3383 208.5 2,222 147. Quantitative composition of the soil solution — Data regarding the relative or actual quantities of the nutri- ent elements in the soil solution are not only very meagre but unreliable. Morgan! found, on comparing the solutions ob- tained from different soils by the oil pressure method, that the potassium (K) might vary from 4 to 180 parts per million based on dry soil; the phosphorus (PO,) from .2 to 4.6, and the calcium (Ca) from 6 to 1000 parts per million. King,” in his extensive work with soil extracts, found the nitrate nitro- gen (NO,) extremely variable, ranging from a fraction of a part per million to more than 150 parts per million in the same soil at different times. A greater fluctuation is to be expected, however, in the nitrate nitrogen than with the other elements, since the presence of soluble nitrogen in the soil solution is due very largely to biological activity. The following figures from Morgan, although the different samples should not be com- pared, show what may be expected in general regarding the concentration of particular elements in the soil solution. 1Morgan, J. F., The Soil Solution Obtained by the Oil Pressure Method; Mich. Agr. Exp. Sta., Tech. Bul. 28, 1916. *King, F. H., Investigations in Soil Management; U. 8. Dept. Agr. Bur. Soils, Bul. 26, 1905. THE SOIL SOLUTION 283 TABLE LXII THE AMOUNTS OF POTASSIUM, PHOSPHORUS, AND CALCIUM IN THE SOLUTION OF VARIOUS SOILS AS DETERMINED BY THE OIL PRESSURE METHOD. EXPRESSED IN PARTS PER MILLION BASED ON DRY SOIL. Parts PER MILLION MOISTURE Sorts PER- CENTAGE K PO, Ca |NH,-+NO, +NO, Fine sandy loam. 29 7.18 | 1.54 9.10 91 Medium sandy Oana ess Fs 27.2 9.82 | 141 12.75 | 13.56 Clyde fine sandy hoa hace ks. 41.9 12.44 | 1.85 | 37.12 3.80 Miami silt loam..| 37.8 27.02 | 4.64 | 25.93 1.20 Miami elay...... 24.5 11.08 | 1.13 10.56 1.61 | EOE Ree rane aera 13229) 139 San | 2198) tot) 33.00 Morgan’s data indicate that the least variation may be ex- pected in the phosphorus (PO,) content, which does not differ greatiy in different soil solutions nor does it vary to any great extent in the same soil. Potassium (K) and especially cal- cium (Ca) show considerable fluctuation, as does the nitrate nitrogen (NO,), as has already been emphasized. The figures of Morgan correlate fairly well with the data obtained by the Bureau of Soils? by means of centrifugal extraction. The potassium (K) averaged about 28 parts per million based on the solution, the calcium (Ca) 32, and the phosphorus (PO,) 8 parts per million. 148. Influence of season and crop on the soil sclution.— It has already been emphasized that the concentration and the composition of the soil solution suffer wide fluctuations. The principal causes of such variations are as interesting as Cameron, F. K., The Soil Solution; p. 40, Easton, Pa., 1911. 284 NATURE AND PROPERTIES OF SOILS they are important since they have a bearing not only on the chemical and biological phenomena within the soil but also on its plant relationships. The broadest and most general factors affecting the soil solution are season and crop. Whether the soil is fallow or covered with vegetation, a great seasonal influence is evident on the soil and its solution. Stewart,’ working in California with extracts from thirteen soils held in large containers, found notable fluctuations of nitrates, calcium, potassium, and magnesium both in bare and cropped earth. The phosphates did not show great variation. The soluble nutrients were markedly higher in the bare soils, the differences between the various types being quite noteworthy. The good soils seemed to have the more concentrated soil solution, a conclusion al- ready reached by a number of investigators.2, When crops were growing on these soils, the concentration of soluble nu- trients not only was lower than with the fallowed areas, but it was about the same in every type of soil. The inherent solution capacity of the different soils was roughly indicated by the crop growth. Hoagland’s* study of the concentration 1Stewart, G. R., Effect of Season and Crop Growth in Modifying the Soil Solution; Jour. Agr. Res., Vol. XII, No. 6, pp. 311-368, 1918. *Snyder, H., The Water-Soluble Plant Food of Soils; Science, N. 8., Vol. 19, No. 491, pp. 834-835, 1904. King, F. H., Investigations in Soil Management; Madison, Wis., 1904. King, F. H., Investigations in Soil Management; U. S. Dept. Agr., Bur. Soils, Bul. 26, 1905. Mitscherlich, E. A., Hine Chemische Bodenanalyse fiir Pflanzen- physiologische Forschungen; Landw. Jahrb., Bd. 36, Heft 2, S. 309-369, 1907. Lyon, T. L., and Bizzell, J. A., The Plant as an Indicator of the Relative Density of the Soil Solutions; Proc. Amer. Soc. Agron., Vol. IV, pp. 35-49, 1912. Hall, A. D., Brenchley, W. E., and Underwood, T. M., The Soil Solu- tion and the Mineral Constituents of the Soil; Philosoph. Trans. Roy. Soc., London, Series B, Vol. 204, pp. 179-200, 1913. Pantanelli, E., Ricerche Sulla Concentrazione del Liquide Circolante nei Terreni Libici; Bul. Orto Bot. R., Univ. Napoli, T. 4, pp. 371-383. * Hoagland, D. R., The Freezing Point Method as an Index of Varia- tions in the Soil Solution Due to Season and Crop Growth; Jour. Agr. Res., Vol. XII, No. 6, pp. 369-395, 1918, THE SOIL SOLUTION 285 of the solution in these soils through the growing season by the freezing point method corroborates the conclusions drawn from the water extracts. The investigation also indicates that large amounts of nutrients are made available by cultivation, fallowing, and cropping and that, from the standpoint of the soil solution, the ordinary farm practices are inherently sound. Hoagland’s data regarding some of the soils studied is given in Table LXIII. The moisture content was approximately the same for each soil. TABLE LXIII THE CONCENTRATION OF THE SOIL SOLUTION IN PARTS PER MIL- LION FROM A GOOD AND POOR SOIL EACH FALLOWED OR CROPPED TO BARLEY. FERTILE SOIL Poor Soin DATE ; ; FALLOW - CROPPED FALLOW CROPPED emmys ke Suet 2000 1200 1100 600 Ail O22 seer 1700 500 800 200 Ts To 8 LAN ae 1800 700 1300 400 OCheeor is. d 5s 4300 1900 2900 900 Deer Sno fee 58 3400 1500 1800 1000 J22] 6c bp) ee eee 4200 1900 2700 1800 aes tos 2. 2 a's 6700 3800 6300 3700 Further investigations of Hoagland with Martin * indicate that the effect of cropping on the soil solution persists for a considerable period. A marked relationship was also noted between the soil solution and the physical condition of the soil, due to a change in the colloidal matter with season. An increase in colloidal matter was noted when the soil solution was depleted of its solutes by plant activities. Hoagland, D. R., and Martin, J. C., Effect of Season and Crop Growth on the Physical State of the Soil; Jour. Agr. Res., Vol. XX, No. 5, pp. 397-404, 1920. 286 NATURE AND PROPERTIES OF SOILS 149. Other factors influencing the soil solution.—A num- ber of other conditions, which are really phases of season, influence both the concentration and the composition of the soil solution. Among these are temperature, leaching, and the moisture content of the soil. As the soil warms up in the spring, reactions of all kinds are stimulated and an increase in concentration generally results. If considerable rain-water enters the soil, the soil solution is much diluted. It is also changed in composition, due to the equilibrium adjustments that of necessity occur. The following data from Bouyoucos * show the influence of change in moisture on the concentra- tion of the soil solution: TABLE LXIV CONCENTRATION OF THE SOLUTION OF CERTAIN SOILS AT VARIOUS MOISTURE CONTENTS. LOWERING OF THE FREEZING POINT METHOD. Sorts MOISTURE ae MOgEE ce To P. P. M. % P. P. M. AMG s < Atlee 2.60 3,939 21.98 303 Sandy loam.... 8.30 13,639 21.53 606 POAT anges ev 11.18 13,780 20.97 848 Silt loam. .2e... 17.40 20,1538 34.76 1061 Clayatee tee 18.80 28,940 36.50 1030 If the soil is moistened beyond its water-holding capacity, it is obvious that drainage losses will occur, which will deplete the soil of valuable constituents. Increase of moisture, there- fore, may modify the soil solution temporarily or permanently, according to conditions. Tillage and the addition of various materials also have a 1Bouyoucos, G. J., The Freezing Point Method as a New Means of Measuring the Concentration of the Soil Solution Directly in the Soil; Mich. Agr, Exp. Sta., Tech. Bul. 24, 1915, THE SOIL SOLUTION 287 remarkable influence on the soil solution, especially increasing its concentration during the warmer seasons. Plowing and cultivation by stimulating biological activity may enhance ni- trate production to a marked degree in a short time. Aération will often increase the available mineral elements by the en- couragement of reactions which favor solution. The addition of salts of various kinds has been shown by Bouyoucos? to influence the soil solution profoundiy. The compounds added affected different soils in a diverse manner. When neutral salts were added, the soil solution was increased from 35 to 100 per cent. of the added strength of the salts. In the case of phosphate salts the increase was very much less. 150. The soil solution and productivity——As the crop obtains its nutrients from the soil solution, there must be a direct relationship between the fertility of the soil and the con- centration and composition of the soil solution. The data quoted from Hoagland indicate in a broad way that a fertile soil is capable of maintaining a more concentrated soil solu- tion than is a poorer one. The work of other investigators amply corroborates this assumption.? One rather convincing experiment may be quoted. Hall, Brenchley, and Underwood * analyzed the water ex- tract from certain plats on the Rothamsted Experiment Sta- tion farm, the fertilizer treatment and the yields of which had been recorded for a long term of years. Complete analyses of the soil from the several plats were also made: 1 Bouyoucos, G. J., The Freezing Point Method as a Means of Studying Velocity Reactions Between Soils and Chemical Agents and Behavior of Equilibrium; Mich. Agr. Exp. Sta., Tech. Bul. 37, 1917. Also, Rate and Extent of Solubility of Soils under Different Treat- ments and Conditions; Mich. Agr. Exp. Sta., Tech. Bul. 44, 1919. See also, Spurway, C. H., The Effect of Fertilizer Salt Treatments on the Composition of Soil Extracts; Mich. Agr. Exp. Sta., Tech. Bul. 45, 1919. *See citations page 284. * Hall, A. D., Brenchley, W. E., and Underwood, T. M., The Soil Solution and the Mineral Constituents of the Soil; Phil. Trans. Roy. Soe., London, Series B, Vol. 204, pp. 179-200, 1913. 288 NATURE AND PROPERTIES OF SOILS TABLE LXV YIELDS TO THE ACRE OF CROPS, AND COMPOSITION OF SOIL AND WATER EXTRACT OF SOIL. ROTHAMSTED EXPERIMENT STATION FARM. ENGLAND. COMPLETE ANALY- WATER EXTRACT YIELD TO sIS TREATMENT THE ACRE (POUNDS)| P.O, K,O P.O, K,O % %o Pp. P.M. | P. P.M. Untreated: >.4...c2 1276 .099 183 525 3.40 De ey ate «UN Os a ae 2985 102 257 808 | 30.33 IN == PO tngee aan 3972 3 248 3.900 3.88 N+ K,0:4- P,0,:)) 5087) © 182° | 826 4 025s Farm manure... . 6184 176 167 4.463 | 26.45 151. Summary.—tThe solution as it exists in a normal soil is highly dynamic. Its concentration and composition are fundamentally governed by rate of solution, by absorp- tion, and by the amounts of the various solutes in the solution itself. Many factors are active in preventing a condition of equilibrium between these three phases. Those of especial importance are season and crop. Temperature, moisture con- tent, and leaching are subfactors of season. Tillage of all kinds and the addition of manures, lime, and fertilizers are practical means of modifying the soil solution more to ade- quately meet the needs of the crop. In fact, all of the com- mon practices so successfully used in economic soil manage- ment attain their end through a modification and control of the soil solution. CHAPTER XV THE REMOVAL OF NUTRIENTS FROM THE SOIL BY CROPPING AND LEACHING THE soil solution, because of its dynamic character, offers two sources of loss for nutrient materials, one of which should be economically encouraged, while the other should be reduced by suitable control to as low a point as is consistent with good soil management. These two sources of exhaustion are (1) cropping and (2) leaching or drainage. One is a legitimate expenditure; the other is a waste, which within certain limits in a humid region is unavoidable.* 152. Intake of water by plants—osmosis.—Plants ob- tain their raw materials from the air and the soil, the former furnishing the carbon and the oxygen, most of the water and the nutrients proper coming from the soil. Although many constituents, some necessary and some incidental, pass into the plant from the soil, for convenience of discussion two groups may be established: (1) water, and (2) nutrients prop- er. It must be kept in mind, however, that water, while per- forming certain mechanical functions, has a nutrient relation- ship also. The most important mechanical principle governing the ab- sorption of water by the plant is osmosis.2, The abstract phe- nomenon should be clearly in mind before its plant relation- ships are considered. A bag of collodion (pig’s bladder or parchment paper will do as well) is filled with a strong solu- 1Gases, such as carbon dioxide, nitrogen and possibly ammonia, may be lost from the soil also. ? Water may also be taken up by colloidal absorption which is called imbibition. This is common in seeds. 289 290 NATURE AND PROPERTIES OF SOILS tion of cane-sugar. The walls of such a bag are semi-permea- ble, that is, certain materials will pass through readily while others will pass but slowly. For example, the sugar mole- cules penetrate with difficulty, while the water finds the walls of the bag but a slight obstacle. If this collodion bag with its sugar solution is attached to a capillary tube and immersed in pure water, it at once becomes distended and the liquid will rise in the capillary tube, indi- cating an unequal pressure within the system. The pressure develops because of the separation of the pure water and the sugar solution by a membrane that is penetrated at different rates by the molecules and ions in contact with it. A tendency towards equalization of course occurs and, as the water moves in faster than the sugar moves out, a pressure is developed within the bag which becomes apparent by the rise of the liquid in the capillary tube. Such a phenomenon is called osmosis and the pressure osmotic pressure. Such force prob- ably has much to do with the movement of plant saps and fluids. Under such conditions as those maintained in the ex- periment, the water tends to move from the dilute solution to the more concentrated one. Suppose the collodion bag be considered as typical of the cells, which form the feeding surface of an active rootlet, and the sugar solution the relatively concentrated and partially colloidal cell contents. The water outside the bag will, of course, represent the dilute soil solution which bathes the roots. With such substitutions it can readily be seen why the plant exerts an osmotic ‘‘pull’’ and how the water moves through the cell-wall. Such a transfer will continue until the move- ment of the water in the soil becomes too slow for normal plant activities. Wilting then occurs. (See Fig. 51.) In alkali soils, where the soil solution becomes very concen- trated, the process above described may be reversed. Out- ward osmosis then occurs and plasmolysist may result. +Plasmolysis is a separation of the plasma from the cell-wall due to a REMOVAL OF NUTRIENTS FROM THE SOIL 291 Bouyoucos ' has suggested that the phenomena of wilting may be due, at least partially, to plasmolysis since he has shown by observing the depression of the freezing point that the soil solution becomes very concentrated at low moisture contents. Such a conception of water absorption is simple, yet it often leads to erroneous ideas regarding the intake of nutrients by plants. The amount of any particular nutrient absorbed by the plant is not determined by the quantity of water taken up, since water and nutrients enter more or less independently. The large amount of water imbibed by the plant, later to be lost by transpiration, cannot be accounted for on the basis of a very dilute soil solution and the necessity of rapid trans- piration in order to facilitate the entrance of sufficient nutrient substance. 153. Absorption of nutrients by plants—diffusion—The solution in a normal fertile soil is not only rather dilute in toto but a great proportion of the nutrients therein are in the ionic condition. While both molecules and ions are pre- sented to the absorbing surfaces of the plant, it is only the latter that penetrate to any great extent, although some mate- rials, especially those of an organic nature, do enter in a molecular condition. The presence of water is, of course, nec- essary for both ionic and molecular penetration, but only as a medium for diffusion. Its movement into the plant is, there- fore, of no very great moment in the actual diffusion process, as the phenomenon is called, although the approach of the nutrients to the feeding surfaces is considerably influenced by capillary activity. The tendency of diffusion is to equalize the concentration of a solution as to the ions and molecules of its solute, the molecules and ions of different salts moving more or less inde- loss of water. It is a shrinkage of the protoplasm and when carried beyond a certain point permanently injures the cell. 1Bouyoucos, G. J., The Freezing Point Method as a New Means of Measuring the Concentration of the Soil Solution Directly in the Soil; Mich. Agr. Exp. Sta., Tech. Bul. 24, 1915. 292 NATURE AND PROPERTIES OF SOILS pendently. The absorption of nutrients by plants, in its simplest analysis, is but a working out of this phenomenon. Thus, if the concentrations of K* ions is high in the soil solution and low within the cell, the potassium will move inward in response to diffusion forces, providing, of course, the ions can pass through the cell wall. This penetration is entirely independent of the entrance of water, as far as the ent a SS ne eT oe SD et een y Sh... ES anes soil particles clinging to root-hairs. Above, root-hairs much enlarged. Root-hairs are simple tube-like pro- longations of the border cells. movement of the latter is concerned. Moreover, the equaliza- tion of one ion is more or less unrelated to the concentration equilibrium of any other. The osmosis of the water, on the other hand, is a phenomenon dependent on sum-total concen- tration plus the semi-permeable membrane. 154. Differential diffusion——The intake of nutrients is by no means as simple as the above explanation might lead one to assume, due to the complications interposed by the presence of a semi-permeable membrane. The passage of ions and mole- cules through the cell-wall and the protoplasmic membrane ad REMOVAL OF NUTRIENTS FROM THE SOIL 293 may be a simple mechanical infiltration, although it is prob- ably accompanied by a chemical reaction, or by a change in the colloidal state of the membrane or both. Moreover, differ- ent ions and molecules do not pass through the same cell-wall ' with equal facility. Thus, one kind of ions may pass through very readily while another kind may encounter extreme diffi- culty in responding to diffusion tendencies. Differential diffusion may be ascribed to two conditions: (1) different relationships between the cell-wall and the ions and molecules of the entering material; and (2) differences in the rate at which the entering molecules and ions are utilized in the metabolic activities of the cell in particular and the plant as a whole. The first case has been partially ex- plained. If a compound ionizes into A and B ions and if A ions, due to their relationship to the colloidal cell-wall, enter more easily, a residue of B ions will be left in the soil solution. The second case may be illustrated by assuming the pres- ence of potassium chloride in the soil solution. It ionizes K* and Cl- ions. Now conceive that these ions diffuse through the cell-wall with equal facility in response to equilibrium tendencies. If the potassium ions are used by the cell as rapidly as they enter and are removed from solution, more potassium will be absorbed. This might continue until the potassium ions in the soil solution become much reduced in number. If the chlorine, on the other hand, is but slightly utilized by the plant, little will be drawn from the soil after the initial equalization. Thus, a residue of chlorine might be left from this type of differential absorption. This applica- tion of diffusion principles shows the possibility, or even more, the probability of plants leaving residues in the soil solution. What the residues from different fertilizers may be and what is the practical importance of such differential actions are pertinent questions. *The term cell-wall as used here refers to the cell-wall proper plus the protoplasmic membrane. 294 NATURE AND PROPERTIES OF SOILS 155. Fertilizer residues may be developed in two gen- eral ways: (1) by selective absorption by the soil; and (2) by differential diffusion into the plant. Regarding the first case (see par. 141), it has already been established that soils ordinarily absorb the basic ions more strongly than the acid radicals, thus tending to leave an acid residue in the soil solu- tion. Sodium nitrate, ammonium sulfate, calcium nitrate, potassium chloride and potassium sulfate, therefore, tend to produce an acid residue, when they are first added to a soil. The final result, however, cannot be determined until the action of the crop is known. If the crop especially utilizes the cation or basic radical, it will intensify the selective ab- sorption of the soil and a still more pronounced acid residue will result. This would be the case with ammonium sulfate, potassium sulfate, and potassium chloride. If, however, the anion or acid radical is utilized to the greater extent, the ac- tion of the soil absorption would be nullified and an alkaline residue would tend to develop. This is especially true with sodium nitrate when applied in large amounts over a term of years, the physical condition of the soil becoming impaired due to the presence of sodium carbonate." One other condition is possible. If the plants should use the cation and anion of a fertilizer salt in equal proportions, no residue would result. This seems to happen to an approxi- mate degree with ammonium nitrate, potassium phosphate, potassium nitrate, and ammonium phosphate. Such salts are extremely valuable in long-continued experiments, where the disturbing effects of fertilizer residues are to be avoided. Monoecalecium phosphate, the important constituent of acid phosphate, needs especial consideration. When added to the soil, it immediately reverts to the tricalcium form if active calcium is present.2, Even with the large amount of gypsum 1Hall, A. D., The Effect of the Long Continued Use of Sodium Nitrate on the Constitution of the Soil; Trans. Chem. Soc. (London), Vol. 85, pp. 950-971, 1904. * CaH,(PO,). + 2CaH,(CO,),—=Ca,(PO,). + 4H,0 + 4CO, REMOVAL OF NUTRIENTS FROM THE SOIL 295 carried by acid phosphate, the effect does not seem to be towards acidity even after long periods of application. This discussion, brief as it is, brings out a little studied phase of crop and fertilizer interaction. How the plant util- izes a particular fertilizer after it is once in the soil, what residues are left, and the importance of such residues, are questions of fundamental concern. The possibility of plants influencing the soil and the fertilizers added, as well as the soil and fertilizer influencing the crop, is well worth attention. 156. Do plants directly aid in the preparation of their nutrients?—The conception commonly held regarding the plant is that its direct relation to the soil is more or less passive. Indirectly, of course, it may exert a considerable influence on the availability of the nutrients. In view of the knowledge regarding fertilizer residues and the new concepts as to possible root exudates, the idea that the plant may directly aid in the preparation of its own nutrients is becom- ing more and more plausible. Such influences, if recognized, might occur in three ways: (1) through the action of carbon dioxide, known to be given off in large amounts by roots; (2) through the influence of organic and inorganic acids other than carbonic acid; and (3) by catalytic agents, enzymic or non-enzymic. In a rich, moist soil the number of root-hairs is very large and the relationship between the rootlets and the soil particles very intimate. When in contact with a particle of soil or colloidal complex, the root-hair in many cases almost incloses it, and by means of its mucilaginous wall forms a contact so close as to make the solution held between the particle and the cell-wall distinct from that in the soil proper. Carbon dioxide, excreted under such conditions, may assume a solvent power entirely unique and independent of the amount produced. 1Conner, S. D., Acid Soils and the Effect of Acid Phosphate and Other Fertilizers Upon Them; Jour. Ind. and Eng. Chem., Vol. 8, No. 1, pp. 35-40, Jan. 1916. 296 NATURE AND PROPERTIES OF SOILS The plant might thus facilitate special conditions and aid ma- terially in the preparation of its own nutrients. Sachs,' and later other investigators, grew plants of various kinds in soil and other media in which was placed a slab of polished’ marble or dolomite or calcium phosphate, covered with a layer of washed sand. After the plants had made sufficient growth the slabs were removed, and on the surfaces were found corroded tracings, corresponding to the lines of contact between the rootlets and the minerals. Czapek * repeated the experiments of Sachs, using plates of gypsum mixed with the ground mineral that he wished to test, and this mixture he spread over a glass plate. Cza- pek found that, while plates of calcium carbonate and of calcium phosphate were corroded by the roots, plates of alu- minum phosphate were not. He concludes that if the tracings are due to acids excreted by the roots, these acids must be those that have no solvent action on aluminum phos- phate. This would limit the excreted acids to carbonic, acetic, proprionic, and butyric. By means of micro-chem- ical analyses of the exudations of root-hairs grown in a water-saturated atmosphere, Czapek found potassium, mag- nesium, calcium, phosphorus, and chlorine in the exudate. He concludes that the solvent action of roots is due to acid salts of mineral acids, particularly acid potassium phosphate. He has not proved, however, that the exudations were not from dead root-hairs or from the dead cells of the root cap. In either case they would have some solvent action, but whether sufficient to make them of importance is doubtful. This ob- jection makes the possible exudation of organic and inorganic acids somewhat questionable. Molisch * found that root-hairs secrete a substance having ‘Sachs, J., Aujlésung des Marmors durch Mais-Wurzlen; Bot. Zeitung, 18 Jahrgang, Seite 117-119, 1860. 2Ozapek, J., Zur Lehre von den Wurzelausscheidung ; Jahrb. f. Wiss. Bot., Band 29, Seite 321-390, 1896. *Molisch, H., Uber Wurzelausscheidungen und deren Einwirkung auf REMOVAL OF NUTRIENTS FROM THE SOIL 297 properties corresponding to those of an oxidizing enzyme. His work has been repeated by others, who have failed to ob- tain similar results, but lately Schreiner and Reed! have demonstrated an oxidizing action of roots that is apparently due to a peroxidase. Oxidation alone, however, would hardly suffice to account for the solvent action accompanying the de- velopment of roots, although it is doubtless an important function and useful in other ways. Schreiner and Sullivan? have demonstrated the presence of reducing substances in media in which plants were grow- ing. This work has recently been corroborated by Lyon and Wilson,? working with maize, oats, peas, and vetch. They found that the solutions in which the plants had been growing exhibited both reducing and oxidizing phenomena. Reducing substances were always present, but whether oxidizing mate- rials were so consistently produced could not be definitely decided. The peroxidases were rendered inactive by boiling the solutions. The reducing substances did not always disap- pear with such treatment. This would throw some doubt upon the enzymic character of the reducing materials and suggest that non-enzymic catalytic exudates are a possibility. The interstices between the larger particles of a normal soil are at least partially filled with colloidal material of a more or less gel-like nature. Moreover, the surfaces of some soil grains may be somewhat coated with the same material. Roots of growing plants have been found to cause coagula- tion of at least some colloids, possibly by leaving an acid residue in the nutrient solution by reason of the selective Organische Substanzen; Sitzungsber. Akad. Wiss. Wien-Math. Nat., Band 96, Seite 84-109, 1888. Abstract in Chem. Centrlb. f. Agr. Chem., Band 17, Seite 428, 1888. *Schreiner, Oswald, and Reed, H. S., Studies on the Oxidizing Powers of Roots; Bot. Gazette, Vol. 47, p. 355, 1909. * Schreiner, O., and Sullivan, M. K., Studies in Soil Oxidation; U. 8S. Dept. Agr., Bur. Soils, Bul. 73, 1910. *Lyon, T. L., and Wilson, J. K., Liberation of Organic Matter by Roots of Growing Plants; Cornell Agr. Exp. Sta., Memoir 40, July, 1921. 298 NATURE AND PROPERTIES OF SOILS absorption of bases and rejection of the acid radicals of the dissolved salts. It is conceivable that the root-hairs, by re- moving bases from the solution existing between the cell-wall and the colloidal covering of the soil particle, may cause coagulation of the colloidal matter and thus lberate the nu- trient materials held by absorption. The liberated material, being of a readily soluble nature, would be taken up by the solution between the rootlet and the soil particle, from which the root-hair could readily absorb it. Such an hypothesis would account for the ability of plants to obtain a quantity of nutrients far in excess of that accounted for by the solvent action of pure water, and even beyond what many investi- gators are willing to attribute to the solvent action of water charged with carbon dioxide. 157. The present status of the question.—The available evidence on excretion of acids other than carbonic by the roots of plants does not admit of any very satisfactory conclu- sion as to their relative importance in the acquisition of plant nutrients. There can be no doubt, however, that carbon dioxide resulting from root exudation and from decomposi- tion of organic matter in the soil plays a very prominent part in this operation. The very large quantity of carbon dioxide in the soil, amounting in some eases to nearly 10 per cent. of the soil air, or several hundred times that of the atmospheric air, must aid greatly in dissolving the soil particles. Whatever may be the concentration of the soil-water, it seems probable that the liquid that is found where the root- hair comes in contact with the soil particle, and that is sepa- rated, in part at least, from the remainder of the soil-water, must have a composition different from that found elsewhere in the soil. Many plants grown in solutions of nutritive salts have few or no root-hairs, but absorb through the epidermal tissue of the roots. The special modification by which the root-hairs come in intimate contact with the soil particle and almost surround it, indeates a direct relation between the REMOVAL OF NUTRIENTS FROM THE SOIL 299 soil particles and the plant, as well as between the soil-water and the plant. Such a condition complicates in no small degree the practical questions of soil management and plant nutrition. 158. Why crops vary in their ability to thrive on dif- ferent soils —It is very commonly recognized that crops of different kinds vary in their ability to obtain nourishment from the soil. The difference between the nitrogen, phosphoric acid, potash, and lime taken up by an average corn crop and a wheat crop of average size is striking. The terms ‘‘weak feeders’’ and ‘‘strong feeders,’’ so often heard, indicate the practical field relationships. Aside from the fact that crops do not all need the same quantities of nutrients these differ- ences in ability to grow normally on different soils may be due either to (1) a larger absorbing system or (2) a more active absorptive capacity. Plants with large root systems may be expected to absorb greater amounts, not only of water but of nutrients also.’ Such a development is especially important in time of drought and in addition gives the plant a greater area from which to draw nutrients. Water, as well as nutrients, does not move through any great distance towards the imbibing and ab- sorbing surfaces. Root development, while of some impor- tance in explaining the differences in the feeding capacities of plants, is probably by no means as important as differences in the absorption activity. The absorptive activity of a plant under any given condi- tion of soil, climate, and stage of growth depends on: (1) the concentration and composition of the cell-sap; (2) the char- acter of the cell-wall; (3) the activity of the cell in elabo- rating and removing from solution the materials absorbed; (4) the extent to which exudates—whether these be carbon *Gile, P. L., and Carrero, P. L., Absorption of Nutrients as Affected by the Number of Roots Supplied with the Nutrient; Jour. Agr. Res., Vol. IX, No. 3, pp. 73-95, 1917. 300 NATURE AND PROPERTIES OF SOILS dioxide, organic or mineral acids and their salts or enzymes —act on the colloidal and non-colloidal soil constituents; and (5) synergistic relationships in the soil solution or the cell- wall. The concentration and composition of the cell-sap deter- mines not only the osmotic relationship but has much to do with diffusion tendencies. The ability of the plant to obtain water and nutrients is thus directly affected by such condi- tions. The character of the cell-wall has of course an im- portant influence on such phenomena. If the cell-wall is easily penetrated, it may greatly facilitate the absorbing ¢a- pacity of the plant. If it is slowly penetrated or exerts spe- cial differential influences, it might have a great deal to do with the differences observed between certain plants. The character of the cell-wall has already been shown to be in- volved in the development of certain residues in the soil. The rate at which materials are utilized within the plant is also a factor. If ions or molecules are used rapidly and thus removed from solution, the diffusion of similar ions and molecules is hastened. Such activity would also influence osmotic relationships to a marked extent. This has already been discussed under differential diffusion. It is readily conceivable that exudates, insofar as they are capable of directly affecting the solubility of nutrients, might produce marked differences between plants as far as their absorbing activities are concerned. * Official and Provisional Methods of Analysis; U.S. Dept. Agr., Bur. Chem., Bul. 107 (revised), pp. 14-18, 1908. CHEMICAL ANALYSIS OF SOILS 317 While it is supposed to measure the permanent fertility? of a soil, there is no reason to suppose that there is any rela- tionship between the nutrients extracted by a strong acid in the laboratory and the amounts of the same constituents absorbed by crops over a period of fifty or one hundred years. Moreover, productivity is not necessarily controlled by the amounts of available nutrients in a soil. This further vitiates the data obtained by such an analysis. Snyder? has analyzed a number of Minnesota soils by means of digestion with strong hydrochloric acid, decompos- ing the acid-insoluble residues by fusion and determining their composition. Veitch * has analyzed certain Maryland soils by the hydrochloric acid method and by means of com- plete solution. A few examples are given below to show how soils may vary in the solubility of their constituents in strong hydrochloric acid: TABLE LXX PERCENTAGE OF SOIL CONSTITUENTS INSOLUBLE IN HCl, sp. GR. 1.115 Sorts K,o | cao | Mgo | P,0, | So, Minnesota (Snyder) Maes ERAVEN or cye fers wn 94 20 58 40 74 Elo le F=5 0 eee a 81 61 76 45 90 Experiment Station...| 838 41 36 18 20 Maryland (Veitch) Colmmiiiay - 5. «ors 95 90 34 66 — Ghesapeake: 7 0.626 <2 67 82 29 15 “= Hudson River Shale..| 738 Bit 28 0 — 169. Digestion with dilute acids—A great number of different acids have been used in a dilute condition for ex- * Fertility is used here in the sense of potential productivity, the nutrients in the soil being considered as the controlling factor. *Snyder, Harry, Soils; Minn. Agr. Exp. Sta., Bul. 41, p. 35, 1895. ® Veitch, F. P., The Chemical Composition of Maryland Soils; Mad. Agr. Exp. Sta., Bul. 70, p. 103, 1901. 318 NATURE AND PROPERTIES OF SOILS tracting soils, the idea being in every case to determine the amount of the mineral nutrients immediately available to crops. The scope is thus narrower than in the digestion with strong acids, by which the permanent fertility is sought. Two acids have been commonly utilized in the extraction of soils with dilute solvents: one per cent. citric acid proposed by Dyer,’ and one-fifth normal nitric acid.2— Dyer adopted the one-per-cent. strength as the result of an investigation in which he determined the acidity of the juices in the roots of over one hundred species or varieties of plants representing twenty different natural orders. The implication is that plants pro- duce a solvent action on a soil in proportion to the acidity of their juices, but an examination of Dyer’s figures does not show that the size of the crop ordinarily produced by the plants would in many cases correspond to the acidity of their juices. Thus, of the Crucifere, the horse-radish has several times the acidity of the Swedish turnip or of the field cabbage, although the crop produced by the former is much less than that of the latter two. Dyer’s method gave results on Rothamsted soils that en- abled him to estimate their refative productivity. On other soils and in the hands of other investigators, however, the method is unsatisfactory. In soils rich in calcium and low in iron and aluminum, it may often show the amounts of easily soluble phosphoric acid and potash. In ease of manipulation, the fifth normal nitric acid is preferable to the one-per-cent. citric acid, which is rather tedious to work with. It has been utilized nearly as exten- sively in this country as has the latter in Great Britain. Its use has been confined largely to the determination of the readily available phosphoric acid and potash in the soil, as * Dyer, Bernard, On the Analytical Determination of Probably Avatl- able ‘‘Mineral’’ Plant Food in Soils; Jour. Chem, Soc., Vol. LXV, pp. 115-167, 1894. ? Official and Provisional Methods of Analysis; U.S. Dept. Agr., Bur. Chem., Bul. 107 (revised), p. 18, 1908. CHEMICAL ANALYSIS OF SOILS 319 has the citric acid method. It is obvious that some materials are more readily soluble than others, and for that reason the method will distinguish between phosphorus and potassium in different forms. The calcium phosphates are supposed to be entirely soluble in this strength of acid. According to Fraps,' it dissolves iron and aluminum phosphates to only a slight extent, thus distinguishing between these forms of phos- phorus and ealcium phosphate. Fraps finds also that no potassium is removed from orthoclase and microcline, that less than 10 per cent. is dissolved from glauconite and biotite, and that from 15 to 60 per cent. is dissolved from muscovite, nephelite, leucite, apophyllite, and phillipsite, minerals known to be rather easily available. There are several factors, however, that make the use of one-fifth normal nitric acid an uncertain guide to the avail- able phosphoric acid and potash in the soil. When a soil is treated with the acid, some of it is neutralized by the reac- tions that result and thus its strength is lessened. This may have no relation to the quantities of phosphoric acid or potash dissolved. Some analysts correct for the neutralization and some do not. Again, as with concentrated hydrochloric acid, the degree of solubility of the soil constituents in the nitric acid may not correspond with the ability of the plant to ob- tain these substances. With this, as with the other methods discussed, the objection holds that the results cannot be taken as an infallible guide to the productiveness of a soil, or to its fertilizer needs. The artificial extraction of a soil in the laboratory cannot be expected to simulate the action of a crop even for one year. 170. Extraction with water.—As carbon dioxide is a universal constituent of the water of the soil, and without + Fraps, G. 8., Active Phosphoric Acid and Its Relation to the Needs of the Soil for Phosphoric Acid in Pot Experiments; Tex. Agr. Exp. Sta., Bul. 126, pp. 7-72, 1909. Also, The Active Potash of the Soil and Its Relation to Pot Expert- ments; Tex. Agr. Exp. Sta., Bul. 145, pp. 5-39, 1912. 320 NATURE AND PROPERTIES OF SOILS doubt a potent factor in the decomposition of the mineral matter, it has been proposed to use a solution of carbon diox- ide as a solvent in soil analysis. The amounts of soil con- stituents taken up by this solvent are much less than are taken up by any of the others heretofore mentioned, but all mineral substances used by plants are soluble in it to some extent. The amount of phosphoric acid is so small as to make its detection by the gravimetric method difficult. Like other methods employing very weak solvents, this is open to the objection that much of the material dissolved cannot be re- moved because of the absorptive power of the soil, and as this varies with the character of the soil, adequate comparisons eannot be made. Water charged with carbon dioxide has been very largely replaced by pure water in making such extrac- tions. When soil is digested with distilled water, all the mineral substances used by plants are dissolved from it, but in very small quantities. It has been proposed to employ this extract for soil analysis on the ground that it is a natural solvent and dissolves only those nutrients in a condition to be used by plants. By determining the moisture content of the soil and using a known quantity of water for the extraction, the parts per million of the extracted nutrients may be expressed on the basis of the dry soil or of the solution. The aqueous extract does not by any means contain the entire quantity of nutrients which were in the soil solution and is not an exact measure of the fertility in this form. Absorption holds back an undetermined and variable quantity of the important con- stituents and thus vitiates the method, especally for compar- ing different soils. The method, however, is very valuable for comparing the same soil at different times, especially as re- gards the nitrates. The nitrate radical is not absorbed to any great degree by the soil and presents a very fair measure of the concentration of the soil solution as far as this constituent is concerned. CHEMICAL ANALYSIS OF SOILS 321 The water extract method generally followed in this country is that established by the Bureau of Soils. One hundred grams of soil are mixed with 500 cubic centimeters of water and stirred for three minutes. After standing twenty minutes the supernatant liquid is filtered through a Pasteur-Chamber- land filter under pressure. It is then ready for analysis. Colometric and turbidity methods are usually employed in de- termining the amounts of the constituents removed.t The method is of greatest use in estimating the nitrate content of soils. The quantity of extracted materiai depends on the absorp- tive properties of the soil, on the amount of water used in the extraction, and on the number of extractions. Analyses of the aqueous extract of a clay and of a sandy soil from the Cornell University farm serve to illustrate the greater reten- tive power of the former for nitrates. Sodium nitrate was applied to a clay soil and to a sandy loam soil at the rate of 640 pounds to the acre. Analyses of aqueous extracts some ninety days later showed the following: TABLE LXXI NITRATES INSOIL KIND oF SoIn FERTILIZER (Parts per million) Wlaya eee te Sodium nitrate 7.8 Claire tere sae a hes No fertilizer 1.8 Sandy loam........ Sodium nitrate 150.0 any. MOaMr«is.s 5 ni x No fertilizer 29.7 There was apparently a much greater retention of nitrate by the clay soil, as shown by a comparison of the fertilized and unfertilized plats on both soils. *Schreiner, O., and Failyer, G. H., Colorimetric, Turbidity and Titra- tion Methods Used in Soil Investigations; U. S. Dept. Agr., Bur. Soils, Bul. 31, 1906. 322 NATURE AND PROPERTIES OF SOILS Schulze? extracted a rich soil by slowly leaching one kilo with pure water, one liter of water passing through in twenty- four hours. The extract for each twenty-four hours was analyzed every day for a period of six days. The total amounts dissolved during each period were as follows: TaBLE LXXII | ToTAL MATTER SUCCESSIVE EXTRACTION| DISSOLVED VOLATILE INORGANIC GRAMS GRAMS GRAMS LEuibigst eae ts ee ls ae ’ 00D 340 195 NCCOMCm ey cee occ 120 Oi .063 Geet as ae” 261 On .160 JeMODU ete vse ee Shenae .203 .083 120 MyiiGhie Seok ieines. o00o a 0% .260 082 178 Sixth...........-4) .200 O77 123 It will be noticed that the dissolved matter, both organic and inorganic, fell off markedly after the first extraction. Later extractions were doubtless supplied largely from the substances held by absorption, which gradually diffused into the water extract as the tendency to maintain equilibrium of the solution overcame the absorptive action. With the re- moval of the absorbed substances the equilibrium between the absorption and solution surfaces and the surrounding so- lution is disturbed, diffusion and solution are increased, and more material gradually passes from the soil into the solution. In this way, a more or less uniform and continuous extraction is mantained. In spite of the obvious defects of the water extraction method the work of Hoagland, Burd and Stewart? seems to indicate that such data, if obtained over an extended period, Schulze, F., Uber den Phosphorsaure-Gehalt des Wasser-Auszugs der Ackererde; Landw. Vers. Stat., Band 6, Seite 409-412, 1864. ? Burd, J. S., Water Extractions of Soils as a Criteria of their Crop- CHEMICAL ANALYSIS OF SOILS 323 are a good comparative measure of the concentration and composition of the soil solution (see par. 145). They also con- sider water extractions as criteria of the crop-producing power of a soil so studied. The practical value of such a method as a means of estimating fertility is, however, somewhat ques- tionable, since much time and labor are required to make the necessary extractions and analyses before conclusions at all reliable may be drawn. 171. Fertility evaluation by means of chemical analyses. —The important part that chemistry plays in soil investiga- tion and research should not be overlooked. Nor can a satis- factory presentation of soil phenomena, whether with a tech- nical or an applied bearing, be made without the use of some chemistry. Chemistry, in fact, is the fundamental science that is most utilized in soil study. In spite of these relationships, the value of chemistry in the direct solution of practical fertility problems is neither abso- lute nor final. The objections already raised to the digestion of the soil, either with concentrated or dilute acids, shows the inadequacy of these methods so far as practical problems are concerned. Of all the chemical analyses discussed those that have to do with the determination of organic carbon, total nitrogen, total ealeium and phosphoric acid are of outstanding value. Or- ganic matter is such an important soil constituent that a knowledge of its amount cannot fail to throw much light on the physical and chemical condition of the soil. Much of the soil nitrogen is carried by the organic matter and becomes available in much larger proportion than do the mineral nutrients. An analysis for total nitrogen is, therefore, a Producing Power; Jour. Agr. Res., Vol. XII, No. 6, pp. 297-309, 1918. Stewart, G. R., Effect of Season and Crop Growth in Modifying the Soil Extract; Jour. Agr. Res., Vol. XII, No. 6, pp. 311-368, 1918. Hoagland, D. R., The Freezing Point Method as an Index of Varia- tions in the Soil Solution Due to Season and Crop; Jour. Agr. Res., Vol. XII, No. 6, pp. 369-395, 1918. 324 NATURE AND PROPERTIES OF SOILS fairly reliable guide in some eases to the fertility of the soil under specific consideration. Although the relationship of organic matter and nitrogen to soil fertility is so close that certain generalized tables ‘ may be cited for the interpretation of chemical data, no close cor- relation is possible, especially where soils of markedly different character are compared. So many other factors may enter that practically no opinion can be formed regarding the prod- uctivity of a soil unless other and more detailed data are available. An interesting example of where the nitrogen content fails to indicate the relative fertility of two soils is found in certain unpublished data from the Cornell Agricultural Experiment Station. Two soils are being studied in the lysimeter tanks— Dunkirk silty clay loam and Volusia silt loam. In Table LXXIII is given the nitrogen and calcium content of these soils and the pounds of nitrogen removed to the acre by maize, oats, and barley, respectively, for the years 1915, 1916, and 1917. The treatment and handling of the soils compared has been the same. While the nitrogen, phosphoric acid and potash contents of these soils are about the same, a marked difference is noted in their productivity. This may be due, at least partially, to the calcium content, which is rather high in the Dunkirk, especially in the subsoil. In comparing soils over wide areas 1The following tentative classification of soils on the basis of their percentages of organic matter and nitrogen is offered for generalized field use: PERCENTAGE OF PERCENTAGE OF DESCRIPTION OrGANIC MATTER NITROGEN TGR AED tly sos 5s Ne: ahlle EADS cm 02 30. | | ate Mie davmisy ek eas ioee: ce nena eee encle 3.0- 6.0 -10- .25 Jebel) Coe anidicdoobnoooodopeo6ee 6.0-10.0 © .25- .40 Wainy Wel b6cooccanaaconpooodc above 10.0 above .40 bo on CHEMICAL ANALYSIS OF SOILS 3 TABLE LX XIII THE PERCENTAGES OF NITROGEN AND CALCIUM IN THE DUNKIRK SILTY CLAY LOAM AND THE VOLUSIA SILT LOAM AND THE NITROGEN REMOVED BY CERTAIN CROPS. CORNELL LYSIMETER TANKS. PouNDS oF N REMOVED Cad N PER ACRE SoILs % % MAIZE | OATS | BARLEY 1915 1916 1917 Dunkirk silty clay loam... 53.6 | 62.3| 44.0 TOMI SLPS OO) ae eee eR Pee 040 | 134 Second Loot. 2. . 2k en 280 | .062 AITGEOOG oe alk exe wc a 490 | .064 EM GutbherOGbcl a scl ob os 1.530 | .054 Molusia- sultloami.......¢ +. Deda hei. tl 18:8 1 SLT eeS Ft 070) eS er ea 230 | .145 DECOM LOOT oe cles «cle 165 | .052 IM ATW 076 thin 010) Ree ne Ie .260 | .059 OUEGNE LOOLies 6 teckel 365 | .050 and in a general way there is often some correlation between the amount of calcium present and the productivity. In humid regions soils high in lime are usually fertile. Within certain limits, therefore, calcium becomes significant in fer- tility studies.* Some idea concerning the relative value of the various chem- ical methods, especially those dealing with potash, lime, phos- phorie acid, and magnesia, may perhaps be obtained by com- paring actual data. Burd? has analyzed a number of soils, 1Shedd, O. M., A Proposed Method for the Estimation of Total Calcium in Soils and the Significance of this Element in Soil Fertility; Soil Sci., Vol. X, No. 1, pp. 1-14, 1920. *Burd, J. 8., Chemical Criteria, Crop Production and Physical Classi- fication in Two Soil Classes; Soil Sci., Vol. V, No. 6, pp. 405-419, 1918. 326 NATURE AND PROPERTIES OF SOILS some good, some poor, by several different methods. Repre- sentative figures are given below: TaBLE LX XIV CHEMICAL COMPOSITION OF A GOOD AND A POOR SOIL AS INDICATED BY SEVERAL DIFFERENT METHODS PERCENTAGE OF CONDITIONS K,0 | CaO | MgO | P,O, Bulk analysis Productive silt loam. . 2... .<’. 1-98") 1-485) 2:66 20 Unproductive silt loam....... 185) oOr he a-od 21 Concentrated HCl digestion Produchiversilt loam. ...s56 54 0a) 1431-2246 22 Unproductive silt loam........ 89 | 1.48 | 3.32 20 One per cent. citric acid Productive silt loam.......... 089 | 452°) 22077 |eahom Unprodnuctive sult loam.) 70... 0389 | 422 | 144 | 072 Water extract p.p.-m. | p.p.m.| p.p.m. | p.p.m. Productive silt loamis. 223.2. .. 57 127 40 12 Unproductive silt loam........ 02 45 | ~ 28 5 A comparison of the figures from the good and poor soil Seems to indicate no differences large enough to warrant opin- ions regarding their relative fertility, except in the case of the water extracts. These latter figures, however, are seasonal averages and required as long a time to procure as was neces- sary to grow a crop. Such fertility measurement is not as practicable as actually using the crop as an indicator. 172. Resume.—The conclusion that chemical analyses are of but little direct practical value as a guide to soil prod- uctivity is unavoidable. In spite of the great importance of chemistry in research and teaching, it fails to indicate either the permanent or the immediate fertility of the land. No chemical method is capable of showing substantial and con- stant differences between soils producing within 20 per cent. CHEMICAL ANALYSIS OF SOILS 327 of each other. Even if an analysis should show the nutrients, which would be available over a term of years, it would still be inadequate, since available nutrients are only one of a great number of factors which govern productivity. This produc- tivity equation may be indicated as follows: Productivity = Texture X structure < organic matter < moisture < available nutrients * soil reaction & weather « plant disease X care of farmer, etc., ete. The factors of this equation are variables, their importance in determining productivity depending on many things. An accurate knowledge of the available soil nutrients, even if procurable, would aid but little in solving such an equation. The solution of individual or community fertility problems is best accomplished by the aid of experienced and technically trained men, who understand the scientific principles under- lying the common field procedures and who also are in touch with the experiences of farmers over wide and diverse areas. Such men may advise not only in regard to the crops that should be grown but also as to their rotation, management, and fertilization from seeding until harvest. These men may also institute such codperative experiments and tests as will best throw. light on fertility problems untouched by practical experience. *The samples sent to a chemical laboratory by farmers are gen- erally improperly taken and consequently are not representative. It would be unwise to analyze such soils even if the methods were capable of showing all that could be wished for. CHAPTER XVII ALKALI SOILS 1 Ir HAS already been shown that soils are acted on by a great variety of weathering agents which gradually render soluble a portion of the most susceptible constituents. This soluble material becomes a part of the soil solution and may come in contact with the roots of any crop growing on the land. In humid regions, where a large quantity of water percolates through the soil, this soluble matter has little opportunity to accumulate.” In arid regions, however, where loss by drainage is slight, these salts may often collect in large amounts. Dur- ing periods of dry weather they are carried upward by the capillary rise of the soil-water, while during periods of rain- fall they may move downward again in proportion to the leach- ing action. At one time the lower soil may contain consid- erably more soluble salt than the upper; at another time the condition may be reversed, in which case the solution in con- tact with roots may contain so much soluble matter that vege- tation is injured or destroyed. This excess of soluble salts usually has a marked alkaline reaction, but in any ease it pro- duces what is termed an alkali soil. Large areas of land in every continent carry soluble salts to such an extent that alkali injury is either actual or poten- *For a complete and satisfactory treatise on alkali see Harris, F. S., Soil Alkali, New York, 1920. *Peat soils in humid regions may sometimes contain high concentra- tions of salts, commonly non-toxic, and lower concentrations of ex- tremely toxic salts. Conner, S. D., Excess Soluble Salts in Humid Soils; Jour. Amer. Soc. Agron., Vol. 9, No. 6, pp. 297-301, 1917. 328 ALKALI SOILS 329 tial. It is estimated that 13 per cent. of the irrigated land of the United States contains sufficient soluble salts seriously to interfere with crop growth. This alone amounts to nine mil- lion acres and does not include the millions of acres not under the ditch that are affected to a marked degree by alkali. Sim- ilar figures are available from other continents and, since alkali conditions can be alleviated and controlled to a certain extent, the importance of the subject becomes apparent. Entirely aside from the economic aspects, alkali is of great interest scientifically, offering a research field of such range and complexity as to involve many sciences. A greater por- tion of the practical information regarding alkali and its con- trol has arisen from the purely scientific interest that has been directed towards this peculiar soil condition. 173. Composition of alkali—It has been emphasized pre- viously that the solution of a normal humid-region soil is of such dilution as to be largely ionic in character except in periods of low moisture content. In a soil affected with alkali it is obvious that the molecular state is dominant and that certain salts may exist and function as definite entities. Thus the following bases may be expected to be present— sodium, potassium, magnesium, calcium, and sometimes am- monium. The common acid radicals are chlorides, sulphates, carbonates, bicarbonates, phosphates, and nitrates. The salts that are present and their proportion not only in the soil solu- tion but as a precipitant will vary with conditions. The following table indicates not only the salts that may be present but the composition of the alkali as reported by a number of different investigators. (See table LX XV, p. 330.) 174. White and black alkali—Sulfates and chlorides of the alkalies, when concentrated on the surface of the soil, produce a white incrustation, which is very common in alkali regions during a dry period as a result of the evaporation of moisture. Incrustations of this character are called white alkali. 330 NATURE AND PROPERTIES OF SOILS TABLE LXXV COMPARISON OF ALKALI EXPRESSED IN PERCENTAGE OF THE DIF- FERENT SALTS PRESENT. Wyss biter BILLInes, Mont.’| Yuma, Ariz.? Re lege ,|-———— Sar z Bi eae) |a as : SURFACE 0-72 Bk Eee: Seag CRUST eee CRUST | Gaus KOM ee 16 | — 5.6 | —— | — 4.0 | 22.0 KESO, cee ty gg Se) 6.) ea eee RECO AN eerie ot —}| — Na,SO,.. — | 25.3 | — | 856 | 35.1 | —]} — NaNO...) - 93.1 |. 19.8 | 2.1) — Se Na,CO,. 2 191806) | 13.8 (eo 7 os NaCl wee. 6.6 | 14.7 | —— | === | 8d | ise Na EO) S| 1 hs Ree ee a ee ee ee MeSQ,..... aos ee ee ne nr pe yt MeCho..4.. 12-70) Se ee el een CaGyy... VT Be eee eee ea ae 2 ee NaHCO, — | — | 36.7 6 | 22.0 2 |) 20 WasOne . 2: 21.5 | —— iL) mind ae €A\(0)50) 6.6 | 32.2 @ai(HCO,),)' 4h 21) 6 ba) ee ee Me(HCO,);)| > 22) both 22) eS (N).CO, | 0) A) eee Carbonates of the alkalies, particularly sodium carbonate, dissolve organic matter from the soil, thus giving a dark color to the solution and to the inerustation. For this reason, alkali containing large quantities of these salts is called black alkali. Black or brown alkali may also be produced by calcium chlo- ride or by an excess of sodium nitrate. Black alkali is much more destructive to vegetation than is the white. A quantity of the latter which would not seriously *Headden, W. P., The Fixation of Nitrogen; Colo. Agr. Exp. Sta., Bul. 155, p. 10, 1910. * Hilgard, E. W., Soils, p. 442, New York, 1906. * Dorsey, C. W., Alkali Soils of the United States; U. S. Dept. Agr., Bur. Soils, Bul. 35, 1906. ALKALI SOILS 331 interfere with the growth of most crops might completely pre- vent the development of useful plants if the alkali were black. 175. Origin of alkali—While the presence of alkah and its influence on plants has been known for centuries, it is only within recent years that its probable mode of origin has been understood. The soluble salts have undoubtedly come - from the materials which have formed the soils, the reactions being as complex as the ordinary transformations which take place in soil formation. Some soils have been laid down as deltas in arms of the ocean. If these bodies of water later are cut off from the sea and gradually dry up under arid conditions, an alkali soil will be left. In a similar way saline lakes may disappear and soils heavily charged with alkali will result. The commonest mode of origin for alkali soil is through ordinary weathering under conditions of aridity. Almost any rock will give rise to soils rich in alkali salts if leaching is not a feature in the weathering processes. In western United States the origin of much of the soil affected to the greatest degree with alkali is associated with strata originally carrying much soluble material. When such rock forms soil, the alkali arises not only from the decomposition of the minerals of which the rock is composed, but is greatly reinforced by the soluble salts already present. The Cretaceous and Tertiary beds in Utah, Colorado, and Wyoming are of this character, having been laid down in brackish water. They naturally give rise to soils high in alkali.t One fact that is often overlooked in practice is that the amount of alkali in the surface layers of soil may be greatly in- creased by improper handling. Rapid evaporation after rain or irrigation will carry the soluble salts toward the surface and deposit them near to or in the root zone. Again, over-irriga- 1Stewart, R., and Peterson, W., Origin of Alkali; Jour. Agr. Res., Vol. X, No. 7, pp. 331-353, 1917. See also, Breazeale, J. F., Forma- tion of Black Alkali in Calcareous Soils; Jour. Agr. Res., Vol. X, No. 11, pp. 541-589, 1917. 302 NATURE AND PROPERTIES OF SOILS tion may produce leaching into lower lands, an alkali condition generally resulting if the areas so affected remain water-logged for a long time. Very often alkali is localized in small areas called alkali spots. These vary in size from a few square yards to several acres. In years of good rainfall these areas may be pro- ductive, but in dry years they are often quite sterile. Their origin is generally due to seepage, the ground water being near enough the surface to allow a concentration of salts by eapillarity, especially in dry seasons. A very peculiar type of alkali spot occurs in the Grand Valley of Colorado and elsewhere, the predominant salt being the nitrate, which does not usually occur in large amounts as alkali. Two theories have been advanced to account for the presence of the nitrate salts. One hypothesis’ is that the surrounding shales are comparatively rich in nitrates and that the alkali accumulation is a leaching and seepage process. The other theory is biological in nature.? Such soils are capable of rapid nitrogen fixation by means of their bacterial flora. The idea is advanced that the nitrogen is fixed from the air very rapidly in these spots and later oxidized to the nitrate form. Whatever the origin of the soluble salts the fact re- mains that such spots are quite destructive, spreading very rapidly until whole orchards are wiped out. Water used for irrigation is very often heavily charged with alkali, especially where any amount of the water previously applied to the soil finds its way back into the streams. At Canon City, Colorado, the Arkansas River is very pure. At a point 120 miles below the soluble salts have been known 1Stewart, R., and Peterson, W., The Nitric Nitrogen Content of the Country Rock; Utah Agr. Exp. Sta., Bul. 134, 1914. Also, Further Studies of the Nitric Nitrogen Content of the Country Rock; Utah Agr. Exp. Sta., Bul. 150, 1917. *Headden, W. P., The Fixation of Nitrogen in Colorado Soils; Colo. Agr, Exp. Sta., Bul. 186, 1913. Sackett, W. G., and Isham, R. M., Origin of the Niter Spots in Certain Western Soils; Science, N. 8., Vol. 42, pp. 452-453, 1915. ALKALI SOILS 333 to reach a concentration of 2200 parts per million. The quan- tity of soluble salts that may be present in irrigation water before it is unfit for use depends on certain conditions. This amount will vary with the crop, the rainfall, the soil, the composition of the alkali, and a number of other factors. A aa a aS is eee ‘ ‘SD ‘ = a / ‘ et et 2 DEPT Ed OFRGOVL JIN FEET: Fic. 54.—Diagram showing the amount and composition of alkali salts at various depths in a soil at Tulare, California. (After Hilgard.) GR BERZSERE eee eee a Gar aak PUTOUNTS OF ALKALI IN 100 OF SO/l, Where the alkali is of the sodium sulfate type rather high concentrations are admissible, running as high as 1000 parts per million. Water carrying black alkali must be used with great caution. Table LX XVI indicates the concentration that may be expected in normal irrigation water. The preponderance of sodium chloride is almost always a feature, not only in alkali water but also in soils affected with alkali salts. This may be explained as due to differential ab- 334 NATURE AND PROPERTIES OF SOILS TaBLE LX XVI ANALYSIS OF SOME TYPICAL ALKALINE RIVER WATER OF WESTERN UNITED STATES.1 ToTAL PERCENTAGE OF TOTAL SOLIDS AS STREAM Soups p-p.m.) Cl | SO,|CO;| Na | K | Ca | Mg| SiO, Malad River, Utah ....| 4,395 | 50.0] 2.9) 4.7)37.4 Sevier River at Delta, Wits erreraeerseslevenners W316) P2524 os |ev7 29) Gea eee ero | ieee Rio Grande, Texas ....| 791 | 21.6/30.1 | 11.5/14.8] .8/13.7) 3.0) 3:8 Mill Creek, Montana ..|3,747 | 7.4/17.3 | 35.1/23.5 | 1.4|10.1] 2.2] .7 San Benito, California.| 936 | 13.8/29.0 | 38.3)13.1| 5.4] 6.6] 7.7] 2.6 Buckeye Canal, Arizona] 1,972 | 39.9) 7.3 | 9.6)24.9 26) 16, On earn leech sorption of ions by the soil. Sodium and chlorine ions seem to be about as little absorbed by the soil as any of the com- mon soil constituents. They are thus readily carried through the soil and are free to accumulate in considerable amounts at points where they may become noticeable. Their union of necessity produces large quantities of sodium chloride or com- mon salt.” 176. Effect of alkali on crops.—The presence of rela- tively large amounts of salts dissolved in water and brought into contact with a plant cell has been shown to cause a shrink- age of the protoplasmic lining of the cell. This action, called plasmolysis, increases with the concentration of the solution until the plant finally dies. The phenomenon is due to the osmotic movement of the water, which passes from the cell towards the more concentrated soil solution. The nature of the salt, the species, and even the individuality of the plant, as well as other factors, determine the exact concentration at which the plant succumbs. The carbonates of the alkali bases have, in addition, a corroding effect on the plant tissues, dis- * Harris, F. S., Soil Alkali, p. 232; New York, 1920. ? Dorsey, C. W., Alkali Soils of the United States; U. S. Dept. Agr., Bur. Soils, Bul. 35, 1906. ALKALI SOILS 335 solving the parts of the plant with which they come into con- tact. Such action is not as important as plasmolysis and when it does occur is most noticeable at the root crown. (See Fig. 55.) Indirectly, alkali may influence plants by its effect on soil tilth, soil organisms, and fungous and bacterial growths. Mar- chal,t for example, found that the formation of nodules, con- taining the nitrogen-fixing organisms, did not develop well Fig. 55.—(1) Cross-section diagram of a normal plant cell. (2) Cell after plasmolysis has taken place. on pea roots in nutrient solutions when certain concentra- tions of salts were maintained. Ammonium salts were injuri- ous at a concentration of 500 parts per million. Potassium and sodium salts retarded the nodule development at 5000 and 3333 parts to the million respectively. The quantity of alkali that will cause injury to ammonifying and nitrifying bacteria varies from 250 to 4000 parts per million, depending on con- ditions. 177. Resistance of different plants to alkali—The fac- tors that determine the tolerance of plants toward alkali are: * Marchal, E., Influence des Sels minéraux nutritifs sur la Production des nodosités chez le Pois; Compt. Rend. Acad. Sci. (Paris), Tome 133, No. 24, p. 1032, 1901. 336 NATURE AND PROPERTIES OF SOILS (1) the physiological constitution of the plant, and (2) the rooting habit. The former is little understood, so much de- pending on the character of the alkali solution, the nature of the cell-wall, and the character and activity of the cell con- tents. It has long been known that the toxicity of two salts when together is considerably less than the sum of their detri- mental action when used alone. This ameliorating or antagon- istic action varies for different salts, seeming to be greatest when calcium and magnesium are involved. This is but an example of the complexities which arise when an attempt is made to study the physiological relationships of alkali injury. The rooting habit of plants in their relation to alkali toler- ance is more easily understood. The advantage is always with deep-rooted crops, such as alfalfa and sugar-beets, probably because a portion of the root may be in a less strongly impreg- nated part of the soil. The tolerance of many plants to alkali has been studied in water culture. Such results are not of great practical value, however, as it is only in soil that all of the numerous factors, such as absorption, antagonism, and physical conditions, come into play. Harris and Pittman? found that organic matter in a soil had a marked ameliorating influence on alkali injury, especially from sodium carbonate. High moisture was also an important factor in lowering the toxicity of soluble salts. Guthrie and Helms,’ using a rich garden loam, found the fol- lowing concentrations slightly affecting or entirely preventing germination and growth of certain crops. (Table LX XVII.) Of the cereals, barley and oats are the most tolerant, these being able, in some eases, to produce good crops in soil con- taining two-tenths per cent. of white alkali. Of the forage crops, a number of valuable grasses are able to grow on soil 1Harris, F. S., and Pittman, D. W., Soil Factors Affecting the Toxicity of Alkali; Jour. Agr. Res., Vol. XV, pp. 287-319, 1918. 2 Guthrie, F. B., and Helms, R., Pot EHaperiments to Determine the Limits of Endurance of Different Farm Crops for Certain Injurious Sub- stances; Agr. Gaz., N. S. Wales, Vol. 14, No. 2, pp. 114-120, 1903. ALKALI SOILS 337 TABLE LX XVII EFFECT OF CERTAIN CONCENTRATIONS OF SALTS ON CROPS. EX- PRESSED IN PARTS PER MILLION. NaCl Na,CO, CONDITION WHEAT |BARLEY RyrE | WHEAT |BARLEY RYE Germination affected. | 500 | 1000 | 1000 | 3000 | 2500 | 2500 Germination prevented) 2000 2500 | 4000 5000 6000 5000 Growth affected...... 500 | 1000 | 1500 | 1000 | 1500 | 2500 Growth prevented....| 2000 | 2000 | 2000 | 4000 | 4000 | 4000 containing considerably more than two-tenths per cent of al- kali. Timothy, smooth brome, and alfalfa are the cultivated forage plants most tolerant of alkali, although they do not equal the native grasses in this respect. Cotton also tolerates a considerable amount of alkali. Loughridge,’ after experiments and observation for a num- ber of years, has obtained data regarding the resistance of various crops to the several alkali salts. His results are given in part as follows, expressed in pounds to an acre to a depth of four feet. (See table LX XVIII, page 338.) Although in general the results as to the resistance to alkali of the various crops are so conflicting, the Bureau of Soils,’ in its alkali mapping, has been able to make a rough classifi- eation as follows. (See table LX XIX, page 338.) 178. Conditions that influence the effect of alkali—It has already been mentioned that organic matter and a high moisture content of the soil tended to alleviate alkali toxicity. Should, however, a previously wet soil become dry, the solu- tion, originally very dilute, would become concentrated and 1Loughridge, R. H., Tolerance of Alkali by Various Cultures; Calif. Agr. Exp. Sta., Bul. 133, 1901. See also Kearney, T. H., and Harter, L. L., Comparative Tolerance of Various Plants for the Salts Com- mon in Alkali Soils; U. S. Dept. Agr., Bur. Plant Ind., Bul. 113, 1907. 2 Dorsey, C. W., Alkali Soils of the United States; U. S. Dept. Agr., Bur. Soils, Bul. 35, pp. 23-25, 1906. 338 NATURE AND PROPERTIES OF SOILS TaBLE LXXVIII CROP Na,SO, Na,CO, NaCl pene Grapes.teoccor 40,800 7,990 9,640 45,760 Oranges ia Sone 18,600 3,840 3,360 21,840 IRGAES awe Mee 17,800 1,760 1,360 20,920 Applesicmespre an 14,240 640 1,240 16,120 Peaches........- 9,600 680 1,000 11,280 RUC a 7 MUR ge 9,800 960 1,720 12,480 Barley inches 12,020 12,170 5,100 25,020 Sugar Beet..... 52,640 4,000 5,440 59,840 NOrehuims. 2s. : 61,840 9,840 9,680 81,360 PNG aa eaves a0 5 102,480 2,360 5,760 110,320 Saltiushstee.! 554 125,640 18,560 12,520 156,720 consequently toxic. High moisture should, therefore, be maintained at least as long as the crop is upon the soil. The distribution of the alkali at different depths may have an important bearing as to its effect on plants. Young plants and shallow-rooted crops may be entirely destroyed by the concentration of alkali at the surface, while the same quantity evenly distributed through the soil, or carried by moisture to a lower depth, would have caused no injury. A loam soil, by TABLE LX XIX PERCENTAGE OF | PERCENTAGE OF ToTAL SALTS IN} BLACK ALKALI CROPS Son IN Soin .0O— .20 .0O— .05 |All crops grow .20— .40 .05— .10 |All but most sensitive AO— .60 .10— .20 |Old alfalfa, sugar beet, sorghum, barley .60—1.00 .20— .80 |Only most resistant plants 1.00—38.00 above .30 |No plants ALKALI SOILS 389 reason of its greater water-holding capacity and absorptive power, will contain more alkali without injury to plants than will a sandy soil. Certain of the alkali salts exert a deflocculat- ing action on clay soils and effect an indirect injury in that way. In irrigated regions the injurious effects of alkali are in many cases developed only after irrigation has been practiced for a few years. This is due to what is known as a ‘‘rise of alkali’’ and comes about through the accumulation, near the surface of the soil, of salts that were formerly distributed throughout a depth of perhaps many feet. Before the land was irrigated the rainfall penetrated only a slight depth into the soil, and when evaporation took place salts were drawn to the surface from only a small volume of soil. When, however, irrigation water is turned on the land, the soil becomes wet to a depth of perhaps fifteen or twenty feet. During the por- tion of the year in which the soil is allowed to dry large quan- tities of salts are carried toward the surface by the upward- moving capillary water. Although these salts are in part carried down again by the next irrigation the upward movement constantly exceeds the downward one. This is because the descending water passes largely through the non-capillary interstitial spaces, while the ascending water passes almost entirely through the capillary channels. The smaller spaces, therefore, contain a consider- able quantity of soluble salts after the downward movement ceases and the upward movement begins. In other words, the volume of water carrying the salts downward in the eapil- lary spaces is less than that carrying them upward through these spaces. Surface tension causes the salts to accumulate largely in the capillary spaces, and it is, therefore, the direc- tion of the principal movement through these spaces that de- termines the point of accumulation of the alkali. There are large areas of land in Egypt, in India, and even in France and Italy, as well as in this country, that have suf- 340 NATURE AND PROPERTIES OF SOILS fered in this way, and not infrequently they have reverted to a desert state. 179. Alkali vegetation.—There are a great number of plants that seldom grow on soils other than those affected with alkali. Davy? states that there are 197 species restricted to alkali soils in California. Such plants are generally recog- nized by the farmers in the district as indicators of alkali. Care should be taken, however, in thus classifying alkali land. Such plants should occupy the land to the exclusion of less tolerant species. Some of the plants? whose presence should cause one to surmise alkali conditions are as follows: Greasewood Inkweed Alkali-heath Tussock-grass Salt-grass Bushy samphire Salt-bush Spike-weed Cressa Rabbit bush Sage-brush, which is so often associated in popular literature with alkali, does not grow on land which carries a great amount of soluble salts. In locating land it is, therefore, a good indi- eator of alkali-free conditions, especially if it is growing vig- orously. 180. The handling of alkali lands.*—Ordinarily there are two general ways in which alkali lands may be handled in order to avoid the injurious effects of soluble salts. The first of these is eradication, the second may be designated as con- trol. In the former case an attempt is made actually to elimi- *Davy, J. B., Investigations on the Native Vegetation of Alkali Lands; Calif. Exp. Sta. Rep., 1895-97, pp. 53-75. ? Harris, F. S., Soil Alkali; Chap. VI? New York, 1920. 5 Dorsey, C. W., Reclamation of Alkali Soils; U. 8. Dept. Agr., Bur. Soils, Bul. 34, 1906. Also, Hilgard, E. W., Utilization and Reclamation of Alkali Soils; New York, 1911. Also, Brown, C. F., and Hart, R. A., Reclamation of Seeped and Alkali Lands; Utah Agr. Exp. Sta., Bul. 111, 1910. Also, Dorsey, C. W., Reclamation of Alkali Soils at Billings, Montana; U. S. Dept. Agr., Bur. Soils, Bul. 44, 1907. Also Harris, F.S., Soil Alkali; Chaps. XII, XIII and XIV; New York, 1920. ALKALI SOILS 341 nate by various means some of the alkali. In the latter, meth- ods of soil management are employed which will keep the salts well distributed throughout the soil. In many cases soils would grow excellent crops if the alkali could only be kept well distributed through the soil layers so that no concentra- tion that is toxic could occur, at least within the root zone. In general, steps should be taken toward the control of alkali, whether eradication is attempted or not. Under irrigation, careful attention is always wise. 181. Eradication of alkali—Of methods designed at least partially to free the soil of alkali the commonest are: (1) leaching with under-drainage, (2) correction with gyp- sum, (3) seraping, and (4) flushing. Of the various methods for removing an excess of soluble salts, the use of tile drains is the most thorough and satisfactory. When this method is used in an irrigated region heavy and repeated applications of water must be made, to leach out the alkali from the soil and drain it off through the tile. When used for the ameliora- tion of alkali spots in a semi-arid region, the natural rainfall will often in time effect the removal. In laying tiles it is necessary to have them at such a depth that the soluble salts in the soil beneath them will not readily rise to the surface. This will depend on those properties of the soil governing the capillary movement of water. Three or four feet in depth is usually sufficient, but the capillary move- ment should first be estimated. After the drains have been placed the land is flooded with water to a depth of several inches. The water is allowed to soak into the soil and to pass off through the drains, leaching out part of the alkali in the process. Before the soil has time to become very dry the flooding is repeated, and the operation is kept up until the land is brought into a satis- factory condition. Crops that will stand flooding may be grown during this treatment, and they will serve to keep the soil from puddling, 342 NATURE AND PROPERTIES OF SOILS as it is likely to do if allowed to become dry at the surface. If crops are not grown, the soil should be harrowed between floodings. The operation should not be earried to a point where the soluble salts are reduced below the needs of the crop... The use of gypsum on black alkali land has sometimes been practiced for the purpose of converting the alkali carbon- ates into sulfates, thus ameliorating the injurious properties of the alkali without decreasing the amount. The quantity of gypsum required may be estimated from the amount and composition of the alkali. The soil must be kept moist, in order to bring about the reaction, and the gypsum should be harrowed into the surface, not plowed under. The reaction is as follows: Na.CO, + CaSO, = CaCO, + Na,SO, When soil containing black alkali is to be tile-drained, it is recommended that the land should first be treated with gyp- sum, as the substitution of alkali sulfates or carbonates causes the soil to assume a much less compact condition and thus fa- cilitates drainage. It also prevents the loss of organic matter dissolved by the carbonate of soda and the soluble phosphates, both of which are precipitated by the change. Removal of the alkali incrustation that has accumulated at the surface is sometimes resorted to. Very often the rise of alkali is encouraged by applications of irrigation water, which is allowed to evaporate unretarded. The salts are thus carried upward by the eapillary movement of the soil-water. This *It has been suggested that elemental sulfur could be used to advan- tage in alkali land, especially where carbonates and bicarbonates abound. Sulfur generally oxidizes in the soil quite readily, producing an acid [see par. 221]. Instead of trying to remove all of the alkalinity by leaching, it might be more practicable to add sulfur. Lipman, J. G., Sulfur on Alkali Lands; Soil Sci., Vol. II, No. 3, p. 205, 1916. Hibbard, P. L., Sulfur for Neutralizing Alkali Soil; Soil Sei., Vol, XI, No. 5, pp. 385-387, 1921. ALKALI SOILS 343 method of alkali eradication is never very efficient, and is often dangerous, as it encourages the presence of very large amounts of alkali salts in the surface soil. Often alkali accumulations may be washed from the soil sur- face by turning on a rapidly moving stream of water. The tex- ture of the soil, as well as the slope of the land, must be just right for such a procedure. Generally so much water enters the soil that the land remains heavily impregnated with alkali salts. Both this method and the previous, even if successful, are only temporary. Moreover, lands carrying so much alkali as to admit of either one of these procedures may be so heavily charged as never to yield to any form of either eradication or control. 182. Control of alkali—Where excessive amounts of soluble salts do not exist in a soil the control of the alkali with a view of keeping it well distributed in the soil column is the best practice. The retardation of evaporation is, of course, the main object in this procedure. The intensive use 6f the soil- mulch is, therefore, to be advocated, especially in all irrigation operations where alkali concentrations are likely to occur. Such a method of soil management not only saves moisture, but also prevents the excessive translocation of soluble salts into the root zone. This method of control is the most economical, the cheapest, and the one to be advocated on all occasions, no matter what may have been the previous means of dealing with the alkali situation. Certain soils that are strongly impreg- nated with alkali may be gradually improved by cropping with sugar-beets and other crops that are tolerant of alkali and that remove large quantities of salts. This is more likely to be efficacious where irrigation is not practiced. Certain crops, moreover, while somewhat seriously injured when young, are very resistant once their root systems are developed. A good example is alfalfa, the young plants being very tender while the mature ones are extremely resistant. Temporary eradica- 344 NATURE AND PROPERTIES OF SOILS tion of alkali may allow such a crop to be established. Farm manure has been found especially useful in this respect.t The crop once well established will then maintain itself in spite of the concentrations that may occur later. * Lipman, C. B., and Gericke, W. F., The Inhibition by Stable Manure of the Injurious Effects of Alkali Salts in Soils; Soil Sci., Vol. VII, No. 2, pp. 105-120, 1919. CHAPTER XVIII SOIL ACIDITY A CHEMICAL or physico-chemical viewpoint regarding the soil and its solution is essential in explaining many of the phe- nomena, especially those relating to higher plants and their nutrition. Since plants respond so markedly to their chemical environment, the importance of soil reaction has long at- tracted much attention. Two conditions are popularly recog nized in this respect—soil alkalinity or alkali and soil acidity. The former condition can only occur where soluble salts may concentrate in the soil and is confined largely to arid and semi- arid regions. Soil acidity, on the other hand, is common only in kumid sections. So widespread is it occurrence and so marked is its influence on crop yields that its importance in a practical way surpasses that of soil alkali. 183. General nature of soil acidity..—The nature of soil acidity is so little understood that it is impossible to define or explain it except in the most general terms. So-called soil acidity may be considered for practical purposes as a more or less unfavorable condition for plant growth, arising in the soil through a lack of certain active bases such as calcium and magnesium and which in practice is alleviated by the addition of some form of lime.? Technically three reasons may be suggested as accounting for the harmful effects of soil acidity: (1) unfavorable hydro- *MacIntire, W. H., The Nature of Soil Acidity with Regard to its Quantitative Determination; Jour. Amer. Soc, Agron., Vol. 13, No. 4, pp. 137-161, 1921. *Lime in an agricultural sense refers to all of the compounds of ¢al- cium and magnesium commonly utilized in correcting soil acidity. 345 346 NATURE AND PROPERTIES OF SOILS ven ion concentrations ;+ (2) presence of substances harm- ful to plant growth such as active aluminum, manganese and the like, the presence of which is usually accompanied by a hydrogen ion concentration beyond neutrality; and (3) im- proper nutrition arising from a lack of calcium as a nutrient or as a synergistic agent in facilitating the entrance of other nutrient ions into the plant.” 184. Hydrogen ion concentration—A number of condi- tions are possible if the toxic influence of soil acidity is due to an actual acid. The harmful effect might be due to an ab- normally high hydrogen ion concentration arising from (1) soluble organic or inorganic acids in the soil solution. Again it might be due to (2) insoluble acids or acid salts which, on reaction with water, produce acidity. In this case, the hydro- gen ion concentration of the soil solution at any particular time would not be a measure of the so-called soil acidity.* A harm- ful hydrogen ion influence may also be ascribed (3) to soluble acids, either organic or mineral, absorbed by the soil complexes and which would become active only under certain conditions. An additional feature of the actual acidity theory may lie in (4) the selective absorption of bases by the soil, by which acid- ity might be developed from neutral or even ‘alkaline salts. If the actual acidity explanation is entertained, any one or all of these phases might be considered as contributing to the dele- terious effects so noticeable on certain plants. 185. Active toxic bases.—The explanation of the harm- ful effects of so-called soil acidity as being due to the presence of active toxic bases has of late received much attention. The 1Hydrogen is the one essential constituent of all acids. When dis- solved in water, acids dissociate, the hydrogen ion becoming active. The strength of an acid is determined by its hydrogen ion concentration. *True speaks of this codperative relationship as synergism. By_ it calcium makes other nutrients physiologically available. True, R. H., The Function of Calcium in the Nutrition of Seedlings; Jour. Amer. Soe. Agron., Vol. 13, No. 3, pp. 91-107, 1921. *Rice, F. E., and Osugi, S., The Inversion of Cane Sugar by Soils and Allied Substances and the Nature of Soil Acidity; Soil Sci., Vol. V, No. 5, p. 347, 1918. SOIL ACIDITY 347 presence of active aluminum in so-called acid soils has been known for some time. Abbott, Conner, and Smalley ! showed in 1913 that aluminum salts were the toxic agents in a certain unproductive soil. In 1918, Hartwell and Pember? proved quite definitely that, for certain soils and for certain crops, the aluminum ion was the injurious factor rather than the hydrogen ion that accompanied it. The work of Mirasol * indi- cates that active aluminum is usually present in acid soils.* Although soluble iron is seldom present to an excess, its ferrous salts are known to be toxic to a greater extent than acids of the same concentration.®° While soluble iron may ac- company active aluminum, it is questionable whether it ac- tually figures in acidity effects. The toxic influence of man- g@anese is more probable, since it is more soluble in an acid than a neutral soil. While it is extremely toxic to plants above a certain concentration the recent work of Funchess*® with * Abbott, J. B., Conner, S. D., and Smalley, H. R., Soil Acidity, Nitri- fication and the Toxicity of Soluble Salts of Aluminum; Ind. Agr. Exp. Sta., Bul. 170, 1913. * Hartwell, B. L., and Pember, F. R., The Presence of Aluminum as a Reason for the Difference in the Effect of So-called Acid Soil on Barley and Rye; Soil Sci., Vol. VI, No. 4, pp. 259-277, 1918. *Mirasol, J. J., Aluminum as a Factor in Soil Acidity; Soil Sci., Vol. X, No. 3, pp. 153-192, 1920. *See also, Kratzman, E., Zur Physiologischen Wirkung der Aluminium Salz auf die Pflanze; Chem. Ztg., Jahrgang 38, S. 1040, 1914. Ruprecht, R. W., Toxic Effect of Iron and Aluminum Salts on Clover Seedlings; Mass. Agr. Exp. Sta., Bul. 161, 1915. Miyake, K., The Toxic Action of Soluble Aluminum Salts upon the Growth of the Rice Plant; Jour. Biol. Chem., Vol. 25, No. 1, pp. 23-28, 1916. Conner, 8S. D., Liming in Its Relation to Injurious Inorganic Com- pounds in the Soil; Jour. Amer. Soc. Agron., Vol. 13, No. 3, pp. 113- 124, 1921. *Conner, S. D., Liming in Its Relation to Injurious Inorganic Com- ee in the Soil; Jour. Amer. Soc. Agron., Vol. 13, No. 3, p. 114, 1921. *Funchess, M. J., Acid Soils and the Tozicity of Manganese; Soil Sci., Vol. VIII, No. 1, p. 69, 1919. See also, Kelly, W. P., The Influence of Manganese on the Growth of Pineapples; Haw. Agr. Exp. Sta., Bul. 23, 1909. Skinner, J. J., and Reid, F. R., The Action of Manganese Under Acid and Neutral Soil Conditions; U. S. Dept. Agr., Bul. 441, 1916, 348 NATURE AND PROPERTIES OF SOILS Alabama soils indicates that it is probably of minor importance as compared with aluminum. A toxic effect from magnesium is possible, especially if there is not enough ealcium to prevent it from exerting a poisonous influence. The presence of alumi- num or iron in an active form is generally accompanied by a high hydrogen ion concentration due to hydrolysis,! which takes place readily in many soils. 186. Lack of nutrients——Less is known regarding this condition than of the two previously discussed. The lack of sufficient nutritive calcium in an acid soil has often been sug- gested.? In addition, it may be possible that some plants re- quire more calcium and other bases for their metabolic proec- esses when growing on a so-called acid soil, due to the gen- eration of particular conditions within the cells. Plants like alfalfa absorb large amounts of calcium and may find an acid soil especially unfavorable on this account. True * has shown that the presence of calcium in consider- able amount is necessary when certain plants are growing in nutrient solution, that other nutrient ions may penetrate the plant cells. Potassium, for example, was but slightly absorbed even when present in large amounts, unless a certain concen- Hydrolysis is a double decomposition in which one of the inter- acting substances is water. The water produces H+ and OH- ions, the former uniting with the non-metallic portion of the substance and the hydroxyl with the remainder. Active basic radicals give, with feeble acids in water, salts which are alkaline. Active acids and active bases give neutral salts. Active acids and less active bases yield salts which are acid in reaction. A feeble base and a feeble acid may produce a salt which is either acid or alkaline. Ammonium sulfide (NH,).S in solution is alkaline, since the ammonium hydroxide which tends to form is more dissociated than the hydrogen sulfide which also is present. Aluminum silicates in water hydrolize readily and since aluminum hydroxide is less dissociated than silicic acid, the hydrogen ions predominate over the hydroxyl ions and an acid reaction results. See Truog, E., Soil Acidity: Its Relation to the Growth of Plants; Soil Sci., Vol. V, No. 3, pp. 169-195, 1918. Also, Soil Acidity: Its Relation to the Acidity of the Plant Juices; Soil Sci., Vol. VII, No. 6, pp. 469-474, 1919. ° True, R. H., The Function of Caleiwm in the Nutrition of Seed- lings; Jour. Amer, Soc. Agron., Vol. 13, No. 3, pp. 91-107, 1921. SOIL ACIDITY 349 tration of calcium ions was provided. This relationship, spoken of as synergism, may be seriously interfered with by so-called soil acidity. 187. The present status of the question.—Each of the general hypotheses which have been advanced to explain the detrimental influence of soil acidity has considerable plausible evidence in its support. Cane-sugar, which is inverted only in the presence of an acid, was found by Rice and Osugi' to be inverted in soils, even when the water extracts from these same soils were neutral or even alkaline. This seemed to indi- eate that the acidity was actual and was inherent with the soil mass rather than with the soil solution. This would also sug- gest the presence of insoluble or absorbed acids that might be liberated by hydrolysis, thus producing a harmful hydrogen ion concentration. Other equally valuable data are available on this phase of soil acidity. The work of Hartwell and Pem- ber ? and of Mirasol,* however, is even more conclusive in re- gard to aluminum as a toxic agent, especially as they studied the problem from the plant standpoint. Conner,* investigating the comparative influence of sulfuric acid and aluminum sulfate on plants, has obtained some in- teresting data corroborating the work of Hartwell and Pember. By comparing a given hydrogen ion concentration with the same hydrogen ion concentrations plus equivalent amounts of aluminum ions, he was able to demonstrate the greater toxicity of aluminum to barley and rye in water culture. Since soluble aluminum so often accompanies an unfavorable hydrogen ion Rice, F. E., and Osugi, 8., The Inversion of Cane Sugar by Soils and Allied Substances and the Nature of Sotl Acidity; Soil Sci. Vol. V, No. 5, pp. 333-358, 1918. ? Hartwell, B. L., and Pember, F. R., The Presence of Aluminum as a Reason for the Difference in the Effect of So-called Acid Soil on Barley and Rye; Soil Sci., Vol. VI, No. 4, pp. 259-277, 1918. *Mirasol, J. J., Aluminum as a Factor in Soil Acidity; Soil Sci., Vol. X, No. 3, pp. 153-193, 1920. “Conner, S. D., Liming in Its Relation to Injurious Inorganic Com- pounds in the Soil; Jour. Amer. Soe. Agron., Vol. 13, No. 3, pp. 113- 124, 1921. 390 NATURE AND PROPERTIES OF SOILS concentration, the importance of aluminum in acidity cannot be avoided. TaBLE LXXX RELATIVE WEIGHTS OF BARLEY AND RYE GROWN IN WATER CUL- TURE. THE HYDROGEN ION CONCENTRATION IS EX- PRESSED IN PH.1 H Ion RELATIVE WEIGHTS TREATMENT CoNCENTRA- TION PH BARLEY | RYE Cee ny sree Men ae el 6.3 100 100 ESO POE Ie, ier i 93 95 CSOD euseatae mr 4.2 68 65 ESO ics dotted ee 3.9 73 65 ASO pee! me: see 3.9 55 47 The only conclusion possible at the present time is that there are probably several kinds of acidity and many degrees of the same acidity as far as toxic influences are concerned. Moreover, dissimilar plants seem to be affected differently by the same acidity, while the same plants respond diversely at different times. Hoagland? and others* have demonstrated that some plants grow better in a slightly acid medium, which 1The hydrogen ion concentration of an acid in solution is a measure of the dissociation of that acid and of its strength. The specific acidity of pure water is taken as 1, the number of grams of H+ ions to a liter being .0000001 or 10-7. The exponent of the power is taken as an expres- sion of the acidity. Pure water has a PH value of 7, which is approxi- mate neutrality. An acid solution containing 4000 times more H+ ions would have a PH value of 3.4. * Hoagland, D. R., Relation of the Concentration and Reaction of the Nutrient Medium to the Growth and Adsorption of the Plant; Jour. Agr. Res., Vol. XVIII, No. 2, pp. 73-117, 1919. *Gillespie, L. J., The Reaction of the Soil and Measurements of Hydrogen ion Concentration; Jour. Wash. Acad. Sci., Vol. 6, No. 1, pp. 7-16, 1916. Sharp, L. T., and Hoagland, D. R., Acidity and Adsorption in Soils as Measured by the Hydrogen Electrode; Jour. Agr. Res., Vol. VII, No. 3, pp. 123-145, 1916. Gillespie, L. J., and Hurst, L. A., Hydrogen-ion Concentration—Soil Type—Common Potato Scab; Soil Sci., Vol. VI, No. 3, pp. 219-236, 1918. SOIL ACIDITY 351 seems to indicate that the hydrogen ion concentration less than a Ph value of 7, so often reported in so-called acid soils, is concomitant with a toxic constituent or with malnutrition and is not in itself the harmful agent.t This argument, how- ever, does not admit that the hydrogen ion is not in many cases the true explanation of the toxicity of certain acid soils, nor does it suggest that lack of nutrients may not be a serious consideration. In light of the explanations offered above, it is evident that the term soil acidity is inadequate to express the inorganic toxicity that accompanies a hydrogen ion concentration below Ph 7, as the condition referred to is, in many cases, not due to the hydrogen ion in detrimental concentration.” Since the term is of long standing and since so-ealled acid soils almost invariably yield an acid reaction with litmus paper, the phrase will continue in use in spite of its misleading inference. 188. Why soil acidity develops.*—No matter what hypoth- 1 Joffe found that while alfalfa plants experienced difficulty in becom- ing established in soils having high hydrogen ion concentrations due to the addition of sulfuric acid, once the seedlings became established they showed normal color and vigor and made excellent growth on soils having a Ph value as low as 3.8. Joffe, J.S., The Influence of Soil Reaction on the Growth of Alfalfa; Soil Sci., Vol. X, No. 4, pp. 301-307, 1920. 2 Researches on Danish soils extending from 1916 to 1920 show that the Ph value on different soils may vary from 3.4 to 8.0. A rather constant relationship was observed between the type of vegetation and the hydrogen ion concentration, many species being found only on soils within a certain range of Ph values. In water culture studies so-called acid-soil plants grew best at a Ph of about 4. Alkaline-soil plants seemed to give the strongest growth at a Ph of 6 to 7. Olsen, C., The Concentration of the Hydrogen Ions in the Soil; Science (N. S.), Vol. LIV, No. 1405, pp. 539-541, Dec. 2, 1921. *White, J. W., Studies in Acid Soils; Ann. Rep. Penn. State Col., 1912-1913, pp. 55-104, Skinner, J. J., and Beattie, J. H., Influence of Fertilizers and Soil Amendments on Soil Acidity; Jour. Amer. Soe. Agron., Vol. 9, No. 1, pp. 25-35, 1917. Conner, S. D., Soil Acidity as Affected by Moisture Conditions of the Soil; Jour. Agr. Res., Vol. XV, No. 6, pp. 321-329, 1918, Martin, W. H., The Relation of Sulfur to Soil Acidity and to the Control of Potato Scab; Soil Sci., Vol. IX, No. 6, pp. 393-408, 1920. 302 NATURE AND PROPERTIES OF SOILS esis may be considered as best explaining soil acidity, sci- entific and practical men are agreed that the addition of cer- tain compounds of calcium and magnesium tend to alleviate the detrimental condition. Conversely, almost every one is willing to admit that the most reasonable cause of its develop- ment is the loss or inactivity of certain bases. A lack of cal- cium seems especially prone to allow an increased hydrogen ion concentration to develop and may at the same time en- courage the activity of certain toxic bases or produce malnu- trition. The tendency of all soils in a humid region is, there- fore, towards acidity, their condition depending on the activ- ity of certain factors which seem to produce such a condition. The four important factors generally specified as encour- aging acidity are: (1) leaching losses, (2) cropping losses, (3) absorption phenomena within the soil, and (4) fertilizer residues. The loss of nutrient bases from the soil has already been emphasized (par. 163) and the importance of such removal is evident from the standpoint of plant nutrition. Over a period of ten years, the removal of nutrients from the Cornell lysi- meter soils,t by drainage and rotation cropping together, amounted to 3702, 1741, and 942 pounds to the acre, respec- tively, for lime (CaO), potash (K,O), and magnesia (MgO). The loss of such amounts of bases cannot but permit the rapid development of soil acidity. No matter how well supplied the soil may be with favorable bases, it will in time become acid. Absorption, in its influence on soil acidity, produces its effect by rendering certain bases inactive rather than by removing them from the soil. When the activity of such bases as calcium is reduced by absorptive influences, not only does the hydrogen ion concentration of the soil solution tend to in- crease, but the hydrolysis of compounds carrying aluminum and similar bases seems to be encouraged. The acidity as de- * Unpublished data, Cornell Agr. Exp. Sta., Ithaca, N. Y. SOIL ACIDITY 353 veloped may have a nutritive relationship as well as a toxic effect. When fertilizer salts are added to the soil, the basic ions are usually absorbed to a greater degree than the acid radi- cals. This tends to develop actual acidity in the soil solution, which may in itself be toxic or may facilitate the development of detrimental ions. If the erop utilizes the basic ion of the fertilizer added to a greater extent than the acid radical, it will aid in the development of acidity. If the plant, on the other hand, absorbs the acid radical, it will tend to counter- act the selective absorption by the soil. The combined influ- ences of soil and crop on ammonium sulfate tend to develop acidity, while the effect on sodium nitrate is toward alkalinity. A salt such as potassium nitrate should leave no residue. The decomposition of organic matter, especially when green- manures are plowed under, is often considered as increasing the acidity of the soil. Such may be the case at the beginning of the decomposition process, but the data‘ available on the subject seem to indicate that organic matter, if it exerts any influence on acidity, tends to reduce rather than accentuate it. This result may occur through the liberation of bases from the organic matter as decomposition proceeds. 189. Relative tolerance of acidity by plants.—Since so many intermediate influences are possible in acid soils, and since plants respond so differently to these influences, it is im- possible to forecast the relative resistance of different crops on the same soil. The response of the same crop on differen* acid soils is likewise difficult to foretell. It is known that certain crops are often more tolerant to soil acidity than others. Of the common weeds sheep sorrel, 1White, J. W., Soil Acidity as Influenced by Green Manures; Jour. Agr. Res., Vol. XIII, No. 3, pp. 171-197, 1918. Stephenson, R. E., The Effect of Organic Matter on Soil Reaction; Soil Sci., Vol. VI, No. 6, pp. 413-439, 1918. Howard, L. P., The Reaction of the Soil as Influenced by the De- composition of Green Manures; Soil Sci., Vol, IX, No. 1, pp. 27-38, 1920. 304 NATURE AND PROPERTIES OF SOILS paint-brush, daisy, and plantain seem especially resistant. This does not mean, however, that they grow better on an extremely acid soil than on one that is slightly acid or neutral. Some of the common crops that are tolerant of acidity are strawberry, blackberry, watermelon, red-top, Rhode Island bent-grass, cowpea, soybean, rye, millet, and buckwheat. Such crops as alfalfa, red clover, timothy, maize, oats, barley, cab- bage and sugar-beet seem to be susceptible in various degree to acid conditions. Reasons for the above differences are not as yet known, since plants apparently alike in every other respect differ in their reaction to the same acid condition. The following pairs of plants may be listed as examples: watermelon and musk- melon, blackberries and raspberries, apple and quince, turnip and beet, beans and alfalfa, red-top and timothy, rye and barley. The first of each pair mentioned will grow well on acid soils, while the second crop in each case is very detri- mentally affected.t 190. Tests for soil acidity..—The great importance of soil acidity to plant growth has directed much attention towards methods for determining the acidity of the soil. 1 Hartwell, B. L., Need for Lime as Indicated by Relative Toxicity of Acid Soil Conditions to Different Crops; Jour. Amer. Soc. Agron., Vol. 13, No. 3, pp. 108-112, 1921. 2Some of the important methods are compared and discussed in the following articles: Sechollenberger, C. J., Relation Between the Indications of Several Lime-requirement Methods and the Soil Content of Bases; Soil Sci., Vol. III, No. 3, pp. 279-288, 1917. Christensen, H. R., Haperiments in Methods for Determining the Reaction of Soils; Soil Sci., Vol. IV, No. 2, pp. 115-178, 1917. Stephenson, R. E., Soil Acidity Methods; Soil Sci. Vol. VI, No. 1, pp. 33-52, 1918. Blair, A. W., and Prince, A. L., The Lime Requirement of Soils According to the Veitch Method, Compared with the Hydrogen-Ion Concentration of the Soil Extract; Soil Sci., Vol. IX, No. 4, pp. 253- 259, 1920. Hartwell, B. L., Pember, F. R., and Howard, L. P., Lime Require- ment as Determined by the Plant and by the Chemist; Soil Sci., Vol. VII, Ne. 4, pp. 279-282, 1919. SOIL ACIDITY 399 Such methods may be divided, for convenience of discussion, under two heads: quantitative determinations and qualita- tive tests. In the first case the methods devised purport to give the lime requirement of the-soil. The second group of methods attempts to determine whether the soil is acid and may in addition give some general idea as to the degree of acidity. 191. Lime-requirement determinations.—A great num- ber of methods has been advanced for the determination of the lime requirement of soils. The methods may for convenience be grouped under three heads: (1) those using a neutral salt," (2) those utilizing a basie substance,* and (3) miscellaneous procedures. In the first group, some neutral salt such as potassium ni- trate is added to the soil and the amount of actual acidity developed is determined under suitable control. The actual acidity produced by selective absorption and basic exchange is thus taken as a measurement of the soil acidity and is gen- erally figured to pounds of lime to the acre. In the second group some basic substance, preferably that which is used in practice to correct acidity, is added to the soil. The amount of the basic substance necessary to render the soil alkaline or neutral is determined in pounds to the *The Hopkins methods utilize potassium nitrate or sodium chloride. Caleium acetate is used in the Jones method. Hopkins, C. G., Knox, W. H., and Pettit, J. H., Ad Quantitative Method for Determining the Acidity of Soils; U. S. Dept. Agr., Bur. Chem., Bul. 73, pp. 114-121, 1903. Jones, C. H., Method for Determining the Lime Requirement of Soils; Jour. Assoc. Off. Agr. Chemists, Vol. I, No. 1, pp. 43-44, 1915. *The Veitch method utilizes calcium hydroxide, the Tacke method calcium carbonate and the method proposed by Hutchinson and Mac- Lennan calcium bicarbonate. Veitch, F. P., Comparison of the Methods for the Estimation of Soil Acidity; Jour. Amer. Chem. Soc., Vol. 26, pp. 637-662, 1904. Tacke, Br., Uber die Bestimmung der freien Huwmussduren; Chem. Ztg., Bd. 21, Heft. 20, S. 174-175, 1897. Hutchinson, H. B., and MacLennan, K., The Determination of the Lime Requirement of the Soil; Chem. News, Vol. 110, p. 61, 1914. 396 NATURE AND PROPERTIES OF SOILS acre. Calcium hydroxide and calcium carbonate are often used. Many investigators consider that the hydrogen ion concen- tration of the soil solution is a fair measure of the lime re- quirement of a soil.1 They thus assume that the concentra- tion of the hydrogen ion is a comparative indication of the amount of lime necessary to alleviate the detrimental influ- ences due to acidity. Bouyoucos? claims that the depression of the freezing point (see par. 145) may be used to measure soil acidity. He found that the depression of the freezing point was less for a neutral soil than for one either acid or alkaline. 192. The Veitch method.—In order to show something of the procedure necessary in determining the lime require- ment of the soil, the Veitch method, which utilizes calcium hydroxide, will be briefly described. Eleven and and one-fifth grams of soil are placed in a suitable Erlenmeyer flask and treated with a standard lime-water solution. The amount of soil taken and the strength of the calcium hydroxide solution are such that each cubic centimeter of the latter absorbed by the soil indicates the need of 300 pounds of calcium oxide to the acre. A number of samples are run at the same time, receiving progressively larger amounts of lime-water. The tGainey, P. L., Soil Reaction and Growth of Azotobacter; Jour. Agr. Res., Vol. XIV, No. 7, pp. 265-271, 1918. Gillespie, L. J., and Hurst, L. A., Hydrogen Ion Concentration— Soil. Type—Common Potato Scab; Soil Sci., Vol. VI, No. 3, pp. 219- 236, 1918. Plummer, J. K., Studies in Soil Reaction as Indicated by the Hydro- gen Electrode; Jour. Agr. Res., Vol. XII, No. 1, pp. 19-31, 1918. Joffe, J. H., Hydrogen Ion Concentration Measurements in Sotls in Connection with Their Lime Requirements; Soil Sci., Vol. IX, No. 4, pp. 261-266, 1920. Blair, A. W., and Prince, A. L., The Lime Requirement of Soils According to the Veitch Method Compared with the Hydrogen Ion Con- centration of the Soil Eatract; Soil Se:., Vol. IX, No. 4, pp. 253-259, 1920. *Bouyoucos, G. J., The Freezing Point Method as a New Means of Determining the Nature of Acidity and Lime Requirements of Sotls; Mich, Agr. Exp. Sta., Tech. Bul. 27, 1916, SOIL ACIDITY 307 samples are brought to dryness over a steam bath and then taken up with about 100 cubic centimeters of water. The samples, after shaking, are allowed to settle, and the super- natant liquid is treated with phenolphthalein. By the use of a number of samples with varying amounts of lime-water, the amount of the reagent necessary to neutralize the soil can be approximately determined. The objections that can be urged against the Veitch method may serve to indicate the difficulties that are in general en- countered in using most of the methods for determining the lime requirement of soils. The method is, in the first place, very artificial, there being no assurance that the amount of calcium absorbed is the same as that necessary to neutralize the soil under field conditions. In the second place, it is subject to considerable error. Even with the most careful manipulation, the method is hardly accurate within 300 pounds of calcium oxide to the acre. If the results from such a method are to be applied directly to practical liming it must be assumed that the amount of lime necessary to neutralize an acid soil is the same as that capable of alleviating the acidity for a particular crop. In lght of the variable influences of acidity on plants, this is an un- scientific assumption to say the least. Acidity itself is too intangible a condition. Moreover, it is in many cases not only inadvisable but also unprofitable to satisfy the full lime re- quirement of a soil. Some crops are unharmed or may even be benefited by moderate acidity. The selection of a lawn grass, for example, which is tolerant to acidity may allow the suppression of certain troublesome weeds that would spring up if the soil was limed. Since the results from lime-requirement methods must be so radically modified to suit field conditions, they seem but little better in a practical way than qualitative tests, which distinguish only in a general manner between different de- grees of acidity. The rapidity and simplicity of qualitative 358 NATURE AND PROPERTIES OF SOILS tests give them an advantage over the somewhat questionable lime-requirement determinations. As the amount of lime applied is at best only an estimate, a simple test, rationally correlated with the many other factors that must be consid- ered, may prove as satisfactory as a more complicated pro- cedure. 193. Qualitative tests for acidity—litmus paper.—Per- haps the oldest test for acidity is the use of litmus paper. This may be used alone or in connection with some sensitiz- ing agent. Potassium nitrate, a neutral salt, is often utilized in this capacity. As has already been explained (par. 141), the addition of such a salt, especially to a soil lacking in ac- tive bases, results in a marked selective absorption and the development of a hydrogen ion concentration. In using litmus paper and potassium nitrate it is assumed that the selective absorption and basic exchange is an approximate measure of the so-called soil acidity. The procedure is as follows: A small amount of the soil to be tested is placed in a small dish or other container and moistened with a neutral potassium nitrate solution. A thick batter is produced by mixing. The soil is then smoothed down and one end of a strip of neutral litmus paper is care- fully applied. The reddening of the paper is an indication of acidity, while the rate of the reaction is a rough measure of the degree. The portion of the paper not in contact with the soil may be used for comparison when the change is slight. The unused end may even be moistened with distilled water to make the comparison more accurate. 194. The zinc-sulfide test—Another qualitative test based on the same general principles has more recently been * Barlow, J. T., Soil Acidity and the Litmus Paper Method for Its Detection; Jour. Amer. Soc. Agron., Vol. 8, No. 1, pp. 23-30, 1916. Karraker, P. E., The Value of Blue Litmus Paper from Different Sources as a Test for Soil Acidity; Jour. Amer. Soc. Agron., Vol. 10, No. 4, pp. 180-182, 1918. SOIL ACIDITY 359 developed. This is the zine-sulfide method.t The soil sample, usually 10 grams, is placed in an Erlenmeyer flask and treated with an excess of neutral calcium chloride and zine sulfide. About 75 cubie centimeters of water are added. The mixture is boiled for one minute to control frothing and to develop uniform ebullition. A strip of moistened lead acetate paper is now laid over the mouth of the flask and allowed to remain there exactly three minutes, the boiling being continued at a uniform rate. The reactions involved in the test are as fol- lows: Soil + xCaCl, (neutral) ss Ca, Soil + xHCl 2HCl + ZnS = ZnCl, + HS H.S (Expelled by boiling) + Pb(C,H,0,), = PbS (black) + 2C,H,0, The selective absorption and basic exchange of the soil de- velops actual acidity, which produces hydrogen sulfide from the zine sulfide. The gas is driven off against the lead acetate paper, producing a black color. The principle involved is the same as that already explained for the litmus test, a different means being employed for measuring the actual acidity de- veloped. 195. Comparison and criticism of qualitative tests —A comparison and criticism of these two methods will amply show the advantages and disadvantages of qualitative tests ” *Truog, E., New Method for the Determination of Soil Acidity; Science, N. 8., Vol. 40, pp. 246-248, 1914. Truog, E., Testing Soils for Acidity; Wis. Agr. Exp. Sta., Bul. 312, 1920. * There are a number of other qualitative tests for acidity, of which the following may be mentioned: Ammonia test.—In this test the soil is placed in a bottle and treated with a strong solution of ammonia. After shaking, the soil is allowed to settle, the depth of the color developing in the supernatant liquid being considered as indicating the degree of acidity. This color depends on the amount and character of the soil organic matter rather than on the acidity. Acid test for carbonates.—In this test a sample of the soil is treated with a few drops of dilute hydrochloric acid. Effervescence indicates the 360 NATURE AND PROPERTIES OF SOILS in general. The litmus paper test is simple and rapid. It ean be used with equal facility in the laboratory and field. While its readings may not correlate very definitely with the actual amount of lime that should be applied, it gives a basis for an estimate that in practice should include a number of factors besides so-called soil acidity. One objection to the method lies in the difficulty of obtaining sensitive litmus paper. Again the intensity of the color change is not great and in the hands of an inexperienced person may seem insignificant. In spite of its limitations, it is one of the best practical qualita- tive tests for soil acidity now available. The zinc-sulfide test is much more striking than the litmus test and thus is more easily interpreted. On account of the marked change of color there is always a temptation to read into this test a quantitative value which it does not possess to any greater degree than does the litmus paper method. The zine sulfide test is not as rapid as the litmus test, nor is it a satisfactory field method. Moreover, it is more complex and requires a much more extensive technique. Again it does not distinguish between a neutral and an alkaline soil. Lit- mus paper, on the other hand, indicates alkalinity and acidity with equal facility. The zine-sulfide test is not a method suited for those inexperienced in laboratory procedure. The deductions from the two tests, however, should be approxi- mately the same. 196. Resume.—Soil acidity is a more or less unfavorable biological condition, which develops in soils due to the lack or presence of sufficient favorable bases in the carbonate or bicarbonate forms. A soil, however, may be alkaline and yet fail to effervesce. Potassium sulfo-cyanate test——A new test has recently been proposed in which a sample of soil held in a test-tube is treated with an alcoholic solution of potassium sulfo-cyanate (KSCN). If the supernatant liquid turns red, soluble iron is present, the degree of color indicating the amount. It is assumed that the soluble iron is a comparative measure of the active aluminum in the soil and that aluminum is the toxic constituent. Comber, N. M., A Qualitative Test for Sour Soils; Jour. Agr. Sci., Vol. 10, part 4, pp. 420-424, 1920. SOIL ACIDITY 361 inactivity of certain bases, especially those which tend to- wards soil alkalinity. These necessary bases may be rendered inactive by absorption phenomena or may be actually lost through leaching and cropping. The specific and usually de- leterious influence of so-called soil acidity may be due to an excessive hydrogen ion concentration or to toxic bases such as aluminum, iron, and manganese, which become active when ionic calcium and similar bases are lacking, thus encouraging a hydrogen ion accumulation. It is not improbable that in some cases the detrimental influence may be improper nutri- tion, either due to a lack of calcium as a nutrient or as a syner- gistic agent necessary for the absorption of other nutrients by plants. These detrimental conditions are alleviated in practice by the application of some form of lime. A number of different methods has been devised to ascer- tain quantitatively the lime requirements of soils. They are all more or less inaccurate. Moreover, the lime requirement of a soil and the lime necessary for best plant growth on that soil are not of necessity the same. Plants respond very differ- ently to the diverse conditions that may develop in the various acid soils and it is seldom necessary or practicable entirely to neutralize a very acid soil in order to correct its deleterious condition. While lime-requirement methods are valuable in research, qualitative tests are sufficient in practice. The amount of lime that should be applied is determined not only by the degree and nature of the acidity but also by the char- acter of the crops, the length of rotation, the system of fer- tilization, and similar factors. At best the amount of lime that should be applied to the acre is but an estimate based on many conditions, of which acidity is one. A qualitative test seems as satisfactory a basis for such an estimate as a more carefully controlled quantitative determination. CHAPTER XIX LIMING THE SOIL? WuueE soil acidity is a condition but imperfectly under- stood, most investigators are agreed that it is due to a lack or inactivity of certain bases, especially those that tend to reduce the hydrogen ion concentration of the soil solution and to give the soil an alkaline reaction. The correction of acidity obviously les in the addition of compounds which carry the necessary bases in such forms that the acidity may be partially or wholly alleviated. The base most commonly used to correct acidity is calcium, although magnesium is often applied, especially in connec- +The following publications may be of interest: Hopkins, C. G., Ground Limestone for Acid Soils; Ill. Agr. Exp. Sta., Cire. 110, 1907. Ellett, W. B., Lime for Virginia Farms; Va. Agr. Exp. Sta., Bul. 187, 1910. Brown, P. E., Bacteriological Studies of Field Soils: The Effects of Lime; Ia. Agr. Exp. Sta., Res. Bul. 5, 1912. Whitson, A. R., and Weir, W. W., Soil Acidity and Liming ; Wis. Agr. Exp. Sta., Bul. 230, 1913. Frear, W., Sour Soils and Liming; Penn. Dept. Agr., Bul. 261, 1915. Miller, M. F., and Krusekopf, H. H., Agricultural Lime; Mo. Agr. Exp. Sta., Bul. 146, 1917. Mooers, C. A., Ground Limestone and Prosperity; Tenn. Agr. Exp. Sta., Bul. 119, 1917. Shorey, E. C., The Principles of the Liming of Soils; U. S. Dept. Agr., Farmers’ Bul. 921, 1918. McCool, M. M., and Millar, C. E., Some General Information on Lime and Its Uses and Functions in Soils; Mich. Agr. Exp. Sta., Special Bul. 91, 1918. ‘Agee, Alva., The Right Use of Lime in Soil Improvement; New York, 1919, Hudelson, R. R., Keeping Soils Productive; Mo. Agr. Exp. Sta., Cire. 102, 1921. 362 LIMING THE SOIL 363 tion with calcium. Calcium is employed because it is not only effective with all types of acidity but because it is ecompara- tively cheap and plentiful. Potassium in active form is too expensive, sodium is likely to generate harmful compounds in the soil, while magnesium in large amounts is sometimes harmful. Calcium compounds may be applied in excess and yet no harmful effects on plant growth are ordinarily lkely to result." 197. Forms of lime.—The term lime correctly used re- fers only to calcium oxide (CaO). In a popular and agri- cultural sense the scope of the word has been broadened to include all of the commercial compounds of calcium and mag- nesium commonly applied to the soil to correct the so-called acidity. The term in its agricultural sense refers to the fol- lowing compounds either alone or in mixture: calcium oxide (CaO), magnesium oxide (MgO), calcium hydroxide (Ca- (OH),), magnesium hydroxide (Mg(OH),), calcium ear- bonate (CaCO,), and magnesium carbonate (MgCO,). Such compounds as gypsum (CaSO,.2H,O), mono-calcium phos- phate (CaH,(PO,),), and calcium silicate (Ca,Si0,), insofar as they are carriers of calcium, also might be spoken of as lime. As might be expected, liming materials do not appear on the market as single compounds of magnesium or calcium, nor are they by any means pure. The better grades of the oxides and hydroxides are generally used in the trades, the more im- pure materials having an outlet as agricultural ime. The car- bonated forms of lime have a number of different sources and vary to a marked degree in purity. Lime, in whatever form it may appear on the market, almost always carries magnesium as well as calcium, the latter usually predominating. Three general groups of lime, as it is commercially handled, Floyd, B. F., Some Cases of Injury to Citrus Trees Apparently Induced by Ground Limestone; Fla. Agr. Exp. Sta., Bul. 137, 1917. Wyatt, F. A., Influence of Calcium and Magnesium Compounds on Plant Growth; Jour. Agr. Res., Vol. VI, No. 16, pp. 589-619, 1916. 0604 NATURE AND PROPERTIES OF SOILS may be recognized: (1) burned lime,’ (2) water-slaked or simply slaked lime,? and (3) carbonated lime.* The devices for producing burned lime are various, rang- ing from the farmer’s lime heap to the immense eylindrical kilns of commerce. In any case the general result is the same. The limestone with which the kiln is charged is decomposed by the heat, carbon dioxide and other gases are discharged, and calcium and magnesium oxides are left behind.* The purity of burned lime, as it is sold for agricultural purposes, is quite variable, ranging from 60 to 98 per cent. of calcium and mag- nesium oxides. As high as 40 per cent. of burned lime may be magnesium oxide, if the original stone was dolomitic. The impurities of burned lime consist of the original impurities of the limestone, such as chert, clay, iron compounds, and the like, as well as unburned fragments of the stone. These ma- terials are often partially screened out before the product ap- pears on the market. Slaked lime is produced by adding water to the burned product, a hydroxide resulting from the direct union of the oxides of calcium and magnesium with water.° Often some of the calcium and magnesium oxides remain unslaked. Four lime compounds may, therefore, appear in freshly slaked lime, besides the original impurities of the burned materials. Com- 1Often spoken of as burnt lime, oxide of lime and quick lime. It may be purchased either in the lump form or in a finely ground condi- tion. It is highly caustic and reacts readily with water. * Incorrectly designated in trade as hydrated lime or lime hydrate. It is strongly alkaline and quite caustic but not to the degree exhibited by calcium and magnesium oxides. Calcium hydroxide and magnesium hydroxide are soluble in cold water to the extent of about 17 parts and .09 parts in 10,000, respectively. ®The carbonated forms of lime are often incorrectly spoken of as lime carbonate and carbonate of lime. Calcium and magnesium carbo- nates are soluble in pure cold water to the extent of only about .13 and 1.06 parts in 10,000, respectively. The reaction to litmus is slightly alkaline. *CaCO, + Heat — CaO + CO, MgCO, + Heat = MgO + CO,, > CaO + H,O= Ca (OH).. MgO + H,O = Mg(OH),. LIMING THE SOIL 365 mercial slaked lime ranges in composition from 60 to 75 per cent. of lime expressed as calcium plus magnesium oxides. Both the burned and slaked forms of lime tend to absorb ecar- bon dioxide from the air, producing calcium and magnesium earbonate. This is called air-slaking." A number of lime compounds are sold under the head of earbonated lime. Of these pulverized or ground limestone is the most common. There is also bog lime or marl, oyster shelis and artificial carbonates. The latter are by-products from certain industries. All of these are quite variable in their content of calcium and magnesium carbonates. Pul- verized limestone may vary in purity from 75 to 98 per cent., 90 per cent. being a fair average. Highly magnesian stone is generally avoided, although stone carrying from 15 to 20 per cent. of magnesium carbonate is often used. The magnesium carbonate, however, usually makes up less than 5 per cent. of the lime present. The figures * quoted in table LX XI (see page 366) show the average composition of liming materials offered for sale in Pennsylvania from 1916 to 1920 inclusive. 198. Determining the need for lime——The lack of lime in the soils of humid regions is so universal that liming will generally increase crop growth. For example, 72 per cent. of the soils of Pennsylvania * are sour, while 75 per cent. of the cultivated lands of Indiana‘ show acidity by the ordinary tests. While it is safe to assume that the productivity of three-fourths of the soils in the eastern part of the United States would be raised by liming, it is a question in many cases whether such treatment would pay. 1Ca(OH),. + CO,= CaCO, + H.O. Mg(OH), + CO, = MgO, + H.O. bea eee J. W., Lime Report; Penn. Dept. Agr., Vol. 4, No. 2, Feb. ® White, J. W., Lime Requirements of Pennsylvania Soils; Penn. Agr. Exp. Sta., Bul. 164, 1920. *Wiancko, A. T., Conner, S. D., and Jones, 8. C., The Value of Lime on Indiana Soils; Ind. Agr. Exp. Sta., Bul. 213, 1918. 366 NATURE AND PROPERTIES OF SOILS TasLeE LXXI NUMBER 5) | INSOLU- Form or LIME OF Res MgO BLE SAMPLES ° |Marrrr Burned lime (low mg.)... 59 70.01 209 (| alg Burned lime (high mg.).. + 02.23 | 33.07 | 2.81 Slaked lime (low mg.)....| 242 64.26 3.10 |) ole Slaked lime (high mg.)..| 107 48.87 | 28.07 | 1.58 Pulverized limestone...... 161 47.83 3.19 | 6.82 Pulverized oyster shell.... + 47,60 .O9: |: .Oehe Artificial carbonate....... 72 50.70 2.52 ; 1.29 Ma) DS Soi cae Ane eA RL 22 46.75 1.00 | 5.90 The first point to be determined in deciding whether or not lime should be applied is in regard to the .acidity and its degree. The litmus or zine sulfide test will supply this in- formation, although a quantitative determination may be made. The general degree of acidity, unless it is very high, is not sufficient, however, in deciding whether it would be wise to lime the soil. The nature of the crops is a factor, as well as the type of the rotation, the fertilizer to be used, and to what extent farm manure and green-crops are utilized. Often special considerations are involved, such as scab on potatoes, which is encouraged by liming. All of the factors mentioned, as well as the experiences of the community with lime, should be considered in deciding whether liming would pay. If the increased crops that will probably result from an application of lime will not pay a good interest on the investment, then liming is not to be advised. An application sufficient to make possible the production of good crops of clover or alfalfa is probably all that can be used profitably. 1The tests are discussed in Chapter XVIII. LIMING THE SOIL 367 199. Form of lime to apply.—The experimental data regarding the relative effectiveness of the different forms of lime are not only meagre but also somewhat contradictory. In practice it is best to assume that the effectiveness of the lime depends on the amount of magnesium and calcium ear- ried and is influenced to a much less degree by the particular combinations in which these bases may occur. For example, one and a half tons of medium to finely ground limestone carrying 50 per cent. of calcium oxide should be as effective as one ton of burned lime analyzing 75 per cent. caleium oxide. While there is a difference in the rapidity with which the various forms react, there seems to be but little difference be- tween them over the period of a rotation when they are ap- pled in chemical equivalent amounts. Accepting this relationship as a practical working basis, four factors must be considered in deciding what form of agricultural lime to apply. These factors are as follows: (1) chemical equivalents, determined by chemical combina- tion and purity; (2) cost a ton, freight on board; (3) freight; and (4) cost of haul and application to the land. It is evident that, if the various forms of lime are equally effective in chemical equivalent quantities, once these amounts are determined the question becomes a problem in arithmetic.? The importance of the factors above listed can best be shown by working out an actual case.” 1CaO x 1.32 = Ca(OH), MgO x 1.44—=Mg(OH), CaO x 1.78 = CaCO, MgO x 2.09 — MgCo, Ca(OH), X .76—Ca0d Mg(OH), x .69=MgO Ca(OH), Xx 1.35 = CaCO, Mg(OH), X 1.44— MgCo, CaCO, x .56—CaO MgCO, X .48= MgO CaCO, X .74=Ca(OH), MgCO, x .69 —=Meg(OH), CaO X .70—=MgO MgO x 1.39 =CaO * Calcium oxide and calcium hydroxide have an advantage over ground limestone in percentages of calcium carried and possibly in initial ac- tivity. They are, however, more disagreeable to handle and do not mix with the soil so well since they tend to lump on becoming moist. Partially or wholly carbonated lumps are often found in the soil years after the caustic lime has been applied. 368 NATURE AND PROPERTIES OF SOILS Suppose that slaked lime carrying 70 per cent. of calcium oxide (CaQ) sells in carload lots at $8.00 a ton and that pul- verized limestone of a fair degree of fineness costs in bulk $4.50 and analyzes 50 per cent. of calcium oxide. Assume the freight as $3.00 a ton and the cost of hauling to the farm and applying to the land as $1.00 more. The application of 1 ton of the agricultural slaked lime would cost $8.00 + $3.00 + $1.00 = $12.00. It would be necessary to apply 1.4 tons of the limestone to every ton of slaked lime. This would amount to $6.30 + $4.20 + $1.40 = $11.90. The difference in this case is very slight be- tween the two forms. lLessening the freight or shortening the haul would give the advantage to the limestone, while in- creasing these would favor the use of slaked lime. It is obvious from such calculations that a flat reeommenda- tion cannot be made in a county or community regarding the lime to use. Each individual case should be calculated, con- sidering the cost items already mentioned. 200. Amount of lime to apply.—The possibility of an application of lime paying and the form to purchase can usu- ally be determined with considerable assurance. Such is not the case, unfortunately, regarding the amount of a given kind of lime to apply to the acre. So many factors, of which soil reaction is only one, are active in determining crop growth that acre applications are at best estimates and often admit- tedly guesses. Not only the degree of acidity but the texture and the structure of the soil, the crops grown in rotation, the length of the rotation, the fertilizers used, the amount of farm manure added in a given period, and similar conditions must be considered. In ordinary practice, it is seldom economical to apply much more than a ton of limestone or its equivalent to the acre, unless the soil is very acid and the promise for increased crop yield exceptionally good. In many eases, it seems unnecessary entirely to correct the acidity of a soil in order to promote normal crop growth. The following figures, LIMING THE SOIL 369 while merely tentative, serve in a general way as guides in practical liming operations for a four- or five-year rotation with average soils. The general degree of acidity may be estimated from a qualitative test. TABLE LX XXIT SUGGESTED AMOUNTS OF AVERAGE PULVERIZED LIMESTONE THAT SHOULD BE APPLIED TO THE ACRE UNDER VARIOUS CONDITIONS.! LIMESTONE—POUNDS TO THE ACRE ACIDITY Sanpy Loam CLAY LoAmM WiGGer ALG! Wo olka. becce ee 1200+1500 1800-2500 SILT O10 oo eae et ee Coe 1800+-2500 2500-3000 201. Changes of lime in the soil When calcium oxide or calcium hydroxide are added to the soil, they undergo a very rapid transformation, especially if the soil is moist. The oxide takes up water and becomes the hydroxide, while the latter almost as quickly changes to the carbonate. The reac- tions are as follows: CaO + H,O = Ca(OH), Ca(OH), + CO, = CaCO, + H,0 It is generally supposed that when once the carbonate is formed in the soil or added as pulverized limestone, it is more *The equivalent amounts of burned or slaked lime may readily be calculated from the chemical equivalents already quoted. Caleulate for example the amount of slaked lime, carrying 65 per cent. of CaO and 5 per cent. of MgO, necessary to equal an application of 2000 pounds of adequately pulverized limestone containing 48 per cent. of CaO and 2 per cent. of MgO. The 5 per cent. of MgO in the slaked lime and the 2 per cent. of MgO in the limestone are equivalent in neutralizing capacity to 6.9 and 2.8 per cent. of CaO, respectively. The slaked lime and the limestone, therefore, carry the equivalent of 71.9 and 50.8 per cent. of 200 50 CaO, respectively. 2S 1413 pounds, the amount of slaked lime necessary to equal 2000 pounds of the limestone. 370 NATURE AND PROPERTIES OF SOILS or less stable, except for slow solubility. In most cases, how- ever, the carbonate, especially magnesium carbonate, is rap- idly decomposed and earbon dioxide is given off, the bases presumably entering the unsaturated aluminum silicates which are likely to be present in acid soils." The actual loss of lime in drainage water occurs through the influence of carbon dioxide which changes the insoluble carbonate to the soluble bicarbonate. The bicarbonate is washed out as such or ionizes, the calcium and the magnesium being lost in the ionic state. The presence of nitrates in the soil, either from biological activity or from fertilizers, also greatly facilitates the loss of lime from the soil in drainage. Such influence is to be especially expected during the summer and fall. In spite of the direct effect of carbon dioxide and nitrates on the loss of lime, the controlling factor seems to be the amount of water passing through the soil rather than its concentration. The following unpublished data from the Cornell University lysimeters show the losses of lime that may be expected under different conditions.2 These figures are averages of ten years’ work with Dunkirk silty clay loam. TABLE LX XXIII AVERAGE ANNUAL LOSS OF NITROGEN AND LIME BY LEACHING. CORNELL LYSIMETERS. AVERAGE OF 10 YEARS. PouNDS TO THE ACRE PER YEAR ConDITION LIME EX- LIME EX- NITROGEN | PRESSED AS | PRESSED AS CaO CaCO, Iara: SOM ckew etree 69.0 597.0 993.6 otatlonavetic cos eee hee 345.9 617.1 Grasse eee Ae 2.5 363.8 648.9 1MacIntire, et al., The Non-existence of Magnesium Carbonate in Humid Soils; Tenn. Agr. Exp. Sta., Bul. 107, 1914. ?Complete data on these lysimeters will be found in par. 163. LIMING THE SOIL 371 202. Effect of lime on the soil—In heavy soils there is always a tendency for the fine particles to become too closely associated. Such a condition interferes with air and water movement. The granular structure that should prevail is somewhat encouraged by the addition of lime, especially the caustic forms. In practice, however, the amounts of lime ap- plied are generally too small to have much importance in this respect. Chemically, lime brings about many complex changes in the soil. Basic exchange is forced and certain mineral nu- trients tend to become more available. The hydrogen ion concentration is lowered and deleterious bases, such as alumi- num and manganese, are forced back into less active combi- nations. Oxidation processes seem also to be stimulated, thus favoring the elimination of organic toxins, which often de- velop when improper decay takes place. The charge that quicklime in normal amounts produces a rapid and detri- mental oxidation of the soil organic matter is probably an over-statement.t While lime of all kinds promotes the oxida- tion of organic matter, calcium oxide, when added in rational amounts, is probably no more active over the term of the rota- tion than calcium carbonate. Most of the favorable soil organisms and some of the un- favorable ones, such as those that produce potato-scab, are benefited by judicious liming. The bacteria that fix nitrogen from the air, either alone or in the nodules of some legumes, are especially stimulated by the application of lime. The change of ammoniacal nitrogen to the nitrate form, which is a biological phenomenon, requires active basic material. Other- wise this necessary transformation will not proceed. The decomposition of both carbohydrate compounds (fermenta- tion) and of nitrogenous materials (putrefaction) depends on lime, that the decay products may be favorable. 1MacIntire, W. H., The Carbonation of Burned Lime in Soils; Soil Sei., Vol. VII, No. 5, pp. 325-446, May, 1919. 372 NATURE AND PROPERTIES OF SOILS Of the general and specific influences of lime just men- tioned the correction of acidity is the one commonly ascribed to it in the popular mind. The mere correction of the soil reaction, however, is probably no more important than a number of other direct and indirect influences of lime. It is evident that the benefits that may result from liming a soil will accrue from a combination of influences rather than from one effect alone. 203. Crop response to liming.—Much experimental work has been done in various parts of the world in determining the relative response of different crops to liming and the rea- son for certain well-known differences. As might be expected, the results, while in close agreement as to some crops, show striking disagreements as to others. This is to be expected, since the varying conditions of the experiments would have a marked influence on the response of the plants under con- sideration. Of legume crops, alfalfa and red and white clovers respond most markedly to lime. The response of soybeans, garden peas and field peas, while less, is still quite noticeable. Alsike clover is more tolerant to acidity than red clover and, as the soil of a region declines in active bases, it is common to find it gradually replacing the latter. Japanese clover, cowpeas, vetch, and field beans do not seem to be greatly benefited by lime. Of the non-legumes that are favorably influenced by lime, blue-grass, maize, timothy, oats, barley, wheat, and sorghum may be mentioned. Rye is less benefited by liming than is barley. Red-top, cotton, strawberries, and potatoes do not seem to be particularly stimulated by liming. Certain plants, such as blueberries, watermelons, and rhododendron are ac- tually injured by the use of lime. There are a number of reasons why plants may be benefited by lime, these reasons being numerous and complex enough to account for the differences in response among common LIMING THE SOIL 373 crops. The possible influences of lime on plants may be listed as follows: (1) direct nutritive action; (2) synergistic rela- tionships either in the soil solution or in the cell-wall; (3) re- moval or neutralization of toxins of either an organic or inor- ganic nature; (4) effect on plant diseases; (5) liberation of mineral nutrients; and (6) encouragement of the biological preparation of nutrient materials. In some eases the calcium may function as a direct nutrient ; in others the intake of nutrients may be facilitated by the presence of calcium and magnesium; while in still other cases the elimination or alleviation of a toxic condition may be the important result. It is easy to conceive that any two or all three of these relationships might be fulfilled simultaneously by hme. The stimulating influence of lime might also make the plant a more active agent and thus encourage it to aid to a greater extent in the preparation of its own nutrients. Certain diseases may be retarded or even entirely suppressed by lime, as is the ‘‘finger-and-toe’’ disease of the Crucifere. The lberation of mineral nutrients, such as potash and phosphoric acid, by the addition of lime, is somewhat uncer- tain although it evidently does occur in many eases.1. The process is probably a more or less complicated physical or chemical change. The stimulation to plants by such an ac- tion is difficult to establish, since so many disturbing factors are active in obscuring the results. Lime is undoubtedly very important in the use of acid phosphate, the active compound of which is mono-caleium phosphate (CaH,(PO,).). In the presence of active calcium, the reversion compound is (Ca,(PO,).,),? rather than the very insoluble iron and alumi- num phosphates (FePO, and AIPO,). The formation of nitrates proceeds rather slowly in most *Plummer, J. K., The Effects of Liming on the Availability of Soil Potassium, Phosphorus and Sulfur; Jour. Amer. Soc. Agron., Vol. 13, No. 4, pp. 162-171, 1921. *CaH,(PO,), + 2CaH,(CO,),—= Ca,(PO,), + 4H,O + 4CO, 374 NATURE AND PROPERTIES OF SOILS acid soils, since there is but little active basic material to stimulate the nitrifying organisms directly or to neutralize the nitrous acid that is formed.t The addition of lime is the most economical method of supplying this base. This response of the nitrifying bacteria to lime is a matter of great moment to crops that need large amounts of nitrate nitrogen and may account in some cases for the early response of certain crops to liming. The tolerance of some plants to acid soils might be accounted for on the supposition that they need but small amounts of nitrogen or are able to absorb their nitrogen in forms other than the nitrate. 204. Method and time of applying the lime.—Although lime is lost rapidly from most soils, appearing in the drain- age water in large amounts, it does not seem to correct to any great extent the acidity of the soil layers through which it is earried.2 Lime applied at the soil surface will tend to disap- pear, but will have little effect on the soil below. The action of lime seems to be a contact phenomenon and the more thor- oughly it is mixed with the soil, the greater will be the num- ber of active focii and the more rapid and effective will be the results of the treatment. Lime is best applied to plowed land and worked into the soil as the seed-bed is prepared. It should be thoroughly mixed with the surface three to five inches of soil. Top-dressing of lime is seldom recommended except on permanent meadows and pastures. The time of year at which lime is applied is immaterial, the system of farming, the type of rotation, and such considerations being the deciding factors. The soil should not be too moist when the application is made, as the 12NH, + 30, = 2HNO, + 2H,0. 2HNO, + CaCO, = Ca(NO,), + H.O + CO,. Ca(NO;), + O; = Ca(NO;).. ? Wilson, B. D., The Translocation of Calcium in a Soil; Cornell Agr. Exp. Sta., Memoir 17, 1918. Stewart, R., and Wyatt, F. A., Limestone Action on Acid Soils; Ill. Agr. Exp. Sta., Bul. 212, 1919. LIMING THE SOIL 375 lime, especially the slaked and ground burned forms, tends to ball badly and thus thorough distribution is prevented. A lime distributer should be used, especially if the amount to be applied is at all large. A manure-spreader can be util- ized and even an end-gate seeder may be pressed into service. Small amounts of lime may be distributed by means of the fertilizer attachment on a grain drill. As with the applica- tion of any material, the evenness of distribution is as im- portant as the form and amount of lime used and should by no means be neglected. A discussion of the application of lime is never complete without some consideration being given to the place in the rotation at which the liming is best done. In a rotation of maize, oats, wheat, and two years of clover and timothy, the lime is often applied when the wheat is seeded in the fall. It can then be spread on the plowed ground and worked in as the seed-bed is prepared. Its effect is thus especially favor- able on the new seeding. Thorne! has shown, however, in certain Ohio experiments, that maize is affected more favor- ably than any of the crops above mentioned and as the money value of this increase is practically as much as that from the hay, he favors applying the lime to the maize. With pota- toes in the rotation, the lime should follow the potato crop, especially if scab is prevalent. In practice the place of lime in the rotation is usually determined by expediency, since the vital consideration is, after all, the application of lime regu- larly and in conjunction with a rational rotation of some kind. 205. The calcium and magnesium ratio.—A physiological balance seems to be necessary in a nutrient solution in con- tact with a normally growing plant. This balance varies with the plant and with numerous other conditions. The reason for such antagonistic action between the ions of certain ele- ments is difficult to explain and many theories have been ad- +Thorne, C. E., The Maintenance of Fertility. Liming the Land; Ohio Agr. Exp. Sta., Bul. 279, 1914. 376 NATURE AND PROPERTIES OF SOILS vanced. Loew,’ in 1901, worked out the optimum ratio for a number of different plants growing in water culture. He found that both calcium and magnesium alone were toxic and it was only when the ratio of these ions fell within certain limits that the toxicity disappeared. This ratio varied be- tween 1 of CaO to 1 of MgO and 7 of CaO to 1 of MgO. The question was immediately raised as to the advisability of using limestone or even burned and slaked lime, the mag- nesium eontent of which approached in any degree the cal- cium present. Recent field and laboratory tests have shown, however, that magnesium salts may be applied in ordinary amounts alone or with calcium compounds with impunity.’ The absorptive capacity of the soil seems to take care in a very effective way of any toxicity that might result from a soil solution physiologically unbalanced. 206. The fineness of limestone—The hardness of the stone, its purity, and its fineness are items of extreme im- portance to the manufacturer of pulverized lime. The softer the limestone, the easier the grinding and the finer the product with a given expenditure of power. The higher the percent- age of calcium and magnesium, the greater is the effectiveness of a given quantity. The farmer, other conditions being more or less equal, is especially interested in the fineness of the product. It is a well-known fact that the finer the division of any material, the more rapid the solution. This, however, 1 Loew, O., The Physiological Role of the Mineral Nutrients of Plants ; U.S. Dept. Agr., Bur. Plant Ind., Bul. 1, p. 53, 1901. *Gile, P. L., and Ageton, C. U., The Significance of the Lime-Mag- nesia Ratio in Soil Analyses; Jour. Ind. and Eng. Chem., Vol. 5, pp. 33-35, 1913. Thomas, W., and Frear, W., The Lime-Magnesia Ratio in Soil Amend- ments; Jour. Ind. and Eng. Chem., Vol. 7, No. 12, pp. 1042-1044, Dee. 1915. Lipman, C. B., A Critique of the Hypothesis of the Lime-Magnesia Ratio; Plant World, Vol. 19, No. 4, pp. 83-105, Apr. 1916. Wyatt, F. A., Influence of Calcium and Magnesium Compounds on Plant Growth; Jour. Agr. Res., Vol. VI, No. 16, pp. 589-619; 1916. Stewart, R., and Wyatt, F. A., Limestone Action on Acid Soils; Il. Agr. Exp. Sta., Bul, 212, 1919. LIMING THE SOIL 377 is not the only importance of fineness. Lime produces its in- fluence largely through contact, and the finer the lime is ground, the more thorough is the mixing with the soil and the greater the number of operating focil. White! presents the following significant data as a result of certain laboratory and greenhouse studies at State College, Pennsylvania. TaBLE LXX XIV A COMPARISON OF VARIOUS GRADES? OF LIMESTONE WHEN APPLIED AT THE SAME RATES. 100 MESH] 689 | 90-40 | 8-12 COR DUETS dienes MesuH | Meso | MEsH Solubility in carbonated water.| 100 | 57 | 45 | 28 Value in correcting acidity....| 100 57 27 18 Mormation of nitrates. «2.2... 100 94 56 12 lamtemrowtlsncs ies oo. s kee 100 69 22 5) These figures show that the finer grades of limestone are much more rapidly effective. Further data by the same au- 1White, J. W., The Value of Limestone of Different Degrees of Fine- ness; Penn. Agr. Exp. Sta., Bul. 149, 1917. Also, Thomas, W., and Frear, W., The Importance of Fineness of Sub-division to the Utility of Crushed Limestone as a Soil Amendment; Jour. Ind. and Eng. Chem., Vol. 7, No. 12, pp. 1041-1042, 1915. Broughton, L. B., et al, Tests of the Availability of Different Grades of Ground Limestone; Md. Agr. Exp. Sta., Bul. 193, 1916. Kopeloff, N., The Influence of Fineness of Division of Pulverized Limestone on Crop Yield as Well as the Chemical and Bacteriological Factors in Soil Fertility; Soil Sci., Vol. IV, No. 1, pp. 19-67, 1917. Frear, W., The Fineness of Lime and Limestone Application as Re- lated to Crop Production; Jour. Amer. Soc. Agron., Vol. 13, No. 4, pp. 171-174, 1921. ?Lime is graded by sieves carrying a certain number of meshes to the linear inch. An 80-mesh sieve has 80 openings to the linear inch or 6400 to the square inch. Screens rated as carrying the same number of meshes often do not give the same grade of material, due to a difference in the size of wire used. Material of 60 to 80 mesh refers to those sizes that will pass through a 60-mesh but will be held by an 80-mesh screen. A standardization of sieves and methods of expressing such analyses is much needed. 378 NATURE AND PROPERTIES OF SOILS thor indicate that while the coarser lime is less rapid in its action, it remains in the soil longer and its influence should be effective for a greater period of years. TABLE LXXXV DECOMPOSITION OF LIMESTONE DURING THE THREE YEARS AFTER APPLICATION. PERCENTAGE OF DECOMPOSITION MESH HiegH CALCIUM HicgH MAGNESIUM STONE STONE 100 mesh and smaller... 92.4 91.2 60) .to0..80) meshs. 2k. 342 81.5 (Wee 20: to, 40° mesh 5 425... 46.7 34.9 Sito 2 emesh va. eae 14.9 5.9 The conclusion is likely to be drawn that limestone should be ground as finely as possible. Such an assumption is at fault in several ways. In the first place, very fine lime is difficult to handle and unpleasant to distribute. Again, the cost of grinding increases very rapidly with the fineness, being entirely too expensive compared with the results attained. Moreover, finely ground material does not possess the lasting qualities of the coarser lime. Because of the cost of grinding the stone to a very fine condition and the rapidity with which such material disappears from the soil, a medium ground lime seems to be a more desirable commercial product. Such material has enough of the finer particles to give quick re- sults and yet enough of the coarser fragments to make it last over the period of the rotation. A pulverized limestone, all of which will pass a 10-mesh sieve, 70 per cent. of which will pass a 50-mesh sieve and 50 per cent. of which will pass a 100-mesh sieve, should give excellent results and yet be cheap enough to make its use worth while. The following figures show in an approximate way the LIMING THE SOIL 379 mechanical composition of limestone on sale in Pennsylvania for 1920': TaBLe LXXXVI MECHANICAL COMPOSITION OF SOME LIMESTONE OFFERED FOR SALE IN PENNSYLVANIA IN 1920. AMOUNT PASSING SIEVE, MESH LIMESTONE 10 Bele ly 00 Re orders os cit Sec aul 100 98 92 EN OE eee al ee 100 99 88 Sy ck oh eect ERNE EAE ed OV a ee 100 89 73 NT ree oe ie foee b ohOk Sok 100 70 58 Fy yo sate eee ee 100 57 50 (0): a napa ere) cai aay se 100 44 34 207. Gypsum and other soil amendments.—Gypsum, in which form calcium sulfate (CaSO,.2H,O) is usually applied to soil, has been used for years and was popular long before commercial fertilizers were available to any extent. The use of gypsum was probably familiar to the Romans. It fre- quently goes by the name land plaster. It is widely distribu- ted in nature and easily ground. Its beneficial effect has been noted, particularly with clover and alfalfa, crops which re- spond especially to potash. Its popularity has waned in recent years, however, since its effectiveness on soils where it has long been used has apparently decreased. This possibly has been due in part to the acid residue that ultimately must re- sult from the use of such material and to the failure to lib- erate potassium—a property with which it has very gen- erally been credited and which, when applied to some soils, it may possess. The experimental work in this respect is somewhat conflicting, possibly due to the fact that the con- pen elnee, J. W., Lime Report; Penn. Dept. Agr., Vol. 4, No. 2, 1921. 380 NATURE AND PROPERTIES OF SOILS ditions of contact between the soil and the gypsum were ab- normal. MeMillar’ found that the potash of certain Minne- sota soils treated with one per cent. of gypsum was appre- ciably influenced three months after the application. When gypsum has proven beneficial to crop growth, the effect may have been due to the nutrient influence of the sulfur it con- tains or to the potash liberated from its soil combinations. The use of gypsum as a soil amendment is now seldom recom- mended, especially if the other forms of lime are available. Sodium chloride has a marked effect on the productivity of some soils, especially when certain crops such as asparagus are grown. Wherein its effectiveness lies is not well under- stood. Increased fertility arising from the addition of sodium and chlorine, which are plant constituents, is probably not the reason of its influence, as these substances are usually available in soils far beyond any possible plant requirement. When common salt shows a beneficial influence, it is probably due to its tendency to liberate certain mineral nutrients such as potassium, calcium, and magnesium. Since it tends to leave an acid residue in the soil and since some form of lime will generally give better and more permanent results, the use of common salt is not recommended except in certain cases. : The use of di-calcium silicate (Ca,Si0,) in an experimental way as a liming material has recently received some attention. Cowles,” in 1917, presented data from which he concluded that 1MeMillar, P. R., Influence of Gypsum upon the Solubility of Potash in Soils; Jour. Agr. Res., Vol. XIV, No. 1, pp. 61-66, 1918. Morse, F. W., and Curry, B. E., The Availability of Soil Potash in Clay and Clay Loam Soils; N. H. Agr. Exp. Sta., Bul. 142, 1909. Bradley, C. E., The Reaction of Lime and Gypsum on Some Oregon Soils; Jour. Ind. and Eng. Chem., Vol. 2, No. 12, pp. 529-530, 1910. Briggs, L. J., and Breazeale, J. F., Availability of Potash in Certain Orthoclase-bearing Soils as Affected by Lime or Gypsum; Jour. Agr. Res., Vol. VIII, No. 1, pp. 21-28, 1917. Cowles, A. H., Calcium Silicates as Fertilizers. Metal. Chem. Eng., Vol. 17, pp. 664-665, 1917. LIMING THE SOIL 381 this compound was of greater value than either ground lime- stone or slaked lime as an amendment. He also concluded that silicon was an essential element in plant nutrition. Hart- well and Pember,' in 1920, found di-calcium silicate approxi- mately equal to limestone insofar as the correction of acidity was concerned. Lettuce was used as an indicator. They found no indication that the silicon was of any value, but, as their experiments were with soil, this, of course, does not op- pose the idea that silicon is an essential element in the growth of plants. Hartwell and Pember concluded that the beneficial influ- ence of phosphorus and calcium compounds added to the soil might, in many cases, be due to the precipitation of active aluminum quite as much as to the supplying of nutrients or the correction of actual acidity. Such a conception of the influence of liming materials may ultimately mean an in- crease in the number and nature of the compounds that may be used as soil amendments. 208. Importance of lime in soil improvement.?—The in- fluence of successively liming a soil over a period of years may tend to raise or lower the fertility of the soil, according to the system of soil management that accompanies the appli- eations of the lime. The use of lime alone will undoubtedly increase crop yield for a time. Basic exchange will be en- 1 Hartwell, B. L., and Pember, F. R., The Effect of Dicalcium Silicate on an Acid Soil; Soil Sci., Vol. X, No. 1, pp. 57-60, July, 1920. 7A number of general references on the importance of lime were given at the beginning of the chapter. See also, Wiancko, A. T., et al., The Value of Lime on Indiana Soils; Ind. Agr. Exp. Sta., Bul. 213, 1918. Stewart, R., and Wyatt, F. A., Limestone Action on Acid Soils; Il. Agr. Exp. Sta., Bul. 212, 1919. Lipman, J. G., and Blair, A. W., The Lime Factor in Permanent Soil Improvement ; Soil Sci., Vol. IX, No. 2, pp. 83-114, Feb. 1920. Hartwell, B. L., and Damon, 8. C., Six Years’ Experience in Improving a Light Unproductive Soil; Jour. Amer. Soc. Agron., Vol. 13, No, 1, pp. 37-41, Jan. 1921. 382 NATURE AND PROPERTIES OF SOILS couraged, soil bacteria will be stimulated, and more nutrients will become available for crop use. Such stimulation, how- ever, will soon wane, and if nothing is returned to the land, productivity must ultimately drop back to even a lower level than before the lime was applied. Lime is, to a great extent, a soil amendment and as it in- creases crop growth, the draft on the soil becomes larger. Greater effort is necessary, therefore, in order to maintain the fertility of the land when lime is used than when such ap- plications are not made. Farm manure, crop residues and green-manures should be utilized to the fullest extent and when these are insufficient to keep up the potash and phos- phorie acid of the soil, commercial fertilizing materials must be resorted to. Lime improperly used exhausts the soil, but when properly and rationally applied it becomes one of the important factors in the mamtenance of a more or less con- tinuous productivity. It is interesting in this connection to consider certain fig- ures from the Ohio Experiment Station.t Maize, oats, wheat and clover and timothy were grown in a five-year rotation on both limed and unlimed plats fertilized in various ways. The results of table LXX XVII (page 383) are averages for a period of twelve years. It is immediately evident that the effectiveness of the lime was increased by the use of both fertilizers and farm manure. Conversely, the returns from the fertilizers and the manure were markedly influenced by the lime. The lime increased the effectiveness of the acid phosphate 20 per cent. The in- creases with the acid phosphate plus potassium chloride and with the complete fertilizers were 22 and 10 per cent., re- spectively. Lime increased the returns of farm manure only 4 per cent., indicating that manure itself may function as a Thorne, C. E., The Maintenance of Soil Fertility. Liming the Land; Ohio Agr. Exp. Sta., Bul. 279, 1914. LIMING THE SOIL 383 TABLE LXX XVII RELATIVE ROTATION VALUES OF CROP INCREASES DUE TO LIMING AND FERTILIZING A STANDARD ROTATION OVER A TWELVE- YEAR PERIOD. OHIO EXPERIMENT STATION. THE ACID PHOSPHATE TREATMENT IS TAKEN AS 100 FOR THE LIME GAIN AND ALSO FOR THE UNLIMED FERTILIZER GAIN. GAIN FROM F'ER- GAIN Mr teh Esa FERTILIZERS TO THE ROTATION FROM CS LIME UNLIMED| LimEp CIA PHOS BATES ss 5, Sse Sale ral ccs 2 100 100 120 Acid phosphate plus potassium IMCL OMe ee aoe c t ccexs bos tacee mois 114 142 173 Acid phosphate, potassium chloride and sodium nitrate..... 119 Die 255 Memanmers IGMONSs @h)..as0 ss obese ees 113 287 300 soil amendment. These figures serve in a definite way to em- phasize the correlation between liming and the other factors that must be considered in soil improvement and fertility maintenance, CHAPTER XxX SOIL ORGANISMS, CARBON, SULFUR, AND MINERAL CYCLES A vast number of organisms, both vegetable and animal, live in the upper layers of the soil and determine to a very large degree its dynamic character.?, By far the greater por- tion of these organisms belong to plant life, producing those changes, both organic and inorganic, which control, in large degree, the productivity of the soil. While most of the or- ganisms are so minute as to be seen, if visible at all, only by the aid of a microscope, a small proportion attain the size of the larger rodents. For convenience of discussion the life of the soil may be classified into macro-organisms and mucro- organisms. 209. Macro-organisms—animal forms.—Of the macro- organisms in the soil, the animal types are chiefly (1) rodents, (2) worms, and (38) insects; and the plant forms (1) the large fungi and algx, and (2) roots. The burrowing habits of rodents—of which the ground squirrel, the mole, the gopher, and the prairie dog are familiar examples—result in the pulverization of considerable quanti- *General references: Lipman, J: G., Bacteria in Relation to Country Life; New York, 1908. Conn, H. W., Agricultural Bacteriology; Philadelphia, 1918. Marshall, C. E., Microbiology; Philadelphia, 1917. ?It has been estimated that every acre of soil contains at least 2000 pounds of living material exclusive of roots. If these organisms were confined to a surface foot of soil, weighing, when moist, 4,000,000 pounds to the acre foot, they would make up .05 per cent. by weight of the nor- mal field soil. 384 SOIL ORGANISMS 385 ties of soil. While the effect is rather beneficial and is analo- gous to tillage, the activities of these animals are generally unfavorable to agricultural operations and such soil inhabi- tants have been more or less exterminated in arable land. The common earthworm is the most conspicuous example of the benefits that may accrue from the presence of animals. Darwin, as the result of careful measurements, states that the quantity of soil passed through these creatures may under favorable conditions in a humid climate, amount to ten tons of dry earth to the acre annually. The earthworm obtains its nourishment from the organic matter of the soil, but takes into its alimentary canal the inorganic matter as well, ex- pelling the latter in the form of casts after it has passed en- tirely through the body. The ejected material is to some ex- tent disintegrated, and is in a flocculated condition. The holes left in the soil serve to increase aération and drainage. The activities of the worms bring about a notable transportation of lower soil to the surface, which aids still more in effecting aeration. Darwin’s studies led him to state that from one- tenth to two-tenths of an inch of soil is yearly brought to the surface of land in which earthworms exist in numbers normal to fertile soil. Earthworms naturally seek a heavy compact soil, and it is in soil of this character that they are most needed because of the stirring and aération that they accomplish. Sandy soil and that of arid regions, in which are found few or no earth- worms, are not usually in need of their activities. There is a less definite, and probably a less effective, action of a similar kind produced by insects. Ants, beetles, and the myriads of other burrowing insects and their larve effect a considerable movement of soil particles, with a consequent aeration of the soil. At the same time they incorporate into the soil a considerable quantity of organic matter. 210. Macro-organisms—plant forms.—The larger fungi are chiefly concerned in bringing about the first stages in the 386 NATURE AND PROPERTIES OF SOILS decomposition of woody matter, which is disintegrated by the penetrating mycelia of the fungi. These break down the structure, and thus greatly facilitate the work of the decay bacteria. Action of this kind is largely confined to the forest and is not of great importance in cultivated soil. Another function of the large fungi is exercised in the intimate, and possibly symbiotic, relation of the fungal hyphe to the roots of many forest trees, in soil where nitrification proceeds very slowly, if at all, for nitrates are apparently not abundant in forests. Alge, except in special cases, do not exist in the soil to any large extent. Certain Colorado soils,t however, seem to contain appreciable numbers of this form. While the pres- ence of both the larger fungi and the alge is interesting, their importance in soil fertility is probably rather slight. The roots of plants are important in the soil both by con- tributing organic matter and by leaving, on their decay, open- ings which render the soil more permeable to air and water. The dense mass of roctlets, with their minute hairs, that is left in the soil after every harvest, furnishes a well-distributed supply of organic matter, which is not confined to the furrow slice, as is artificially incorporated manure. The action of roots on the soil is not by any means entirely physical. Dur- ing the life of the plant the elimination of tissue and the presence of exudates make the rootlets rather important chem- ical agents.2, The chemical and biological importance of de- caying organic matter has already been adequately empha- sized.* 211. Micro-organisms—protozoa.—The micro-organisms of the soil belong to the following groups: (1) protozoa, (2) fungi and alge, (8) actinomyces, and (4) bacteria. 1Robbins, W. W., Alge in Some Colorado Soils; Colo. Agr. Exp. Sta., Bul. 184, 1912. *See paragraphs 156 and 157. *See paragraphs 64 and 132. SOIL ORGANISMS 387 While nematodes, rotifers, and similar organisms are some- times found in soil, the protozoa are the only important micro- scopic animal group usually present. The importance of protozoa in soils was especially emphasized in 1909 by Russell and Hutchinson,’ who maintained that the protozoan flora so interfered with the ammonia-producing bacteria as materially to lower the productivity of the soil. Partial sterilization seemed to alleviate this condition, possibly by killing the harmful protozoa. The findings of Russell and Hutchinson have resulted in much research as to the importance of proto- zoa in a normal soil. While Waksman ? found that the presence of protozoa was concomitant with low bacterial numbers, he does not consider all protozoa harmful to biological activities. Fellers and All- son,* in an examination of New Jersey soils, found protozoa in every sample, the number of species ranging from two to twenty-eight. Soils rich in organic matter or containing large amounts of water carried the greater number. Besides the 104 species of protozoa identified in New Jersey soils, ten genera of alge and six of diatomes were isolated. Nematodes were common. The number of protozoa ranged from a very few to as high as 4500 to a gram of soil. When occurring in such numbers, these animals must be of considerable impor- * Russell, E. G., and Hutchinson, H. B., The Effect of Partial Sterili- zation of Soil on the Production of Plant Food; Jour. Agri. Sci., Vol. III, pp. 111-144, 1909. Also, The Effect of Partial Sterilization of Soil on the Production of Plant Food. II. The Limitation of Bace- terial Numbers on Soils and Its Consequences; Jour. Agr. Sci., Vol. V, part 2, pp. 152-221, 1913. ?Waksman, S. A., Protozoa as Affecting Bacterial Activities in the Soil; Soil Sci., Vol. II, No. 4, pp. 363-376, 1916. Also, Sherman, J. M., Studies on Soil Protozoa and Their Relation to the Bacteria; I. Jour. Bact., Vol. 1, No. 1, pp. 35-66, 1916. II. Jour. Bact., Vol. 1, No. 2, pp. 165-184, 1916. Kopeloff, N., and Coleman, D. A., A Review of Investigations in Soil Protozoa and Soil Sterilization; Soil Sci., Vol. III, No. 3, pp. 197-269, 1917. *Fellers, C. R., and Ailison, F. E., The Protozoan Fauna of the Soil of New Jersey; Soil Sci., Vol. IX, No. 1, pp. 1-24, 1920. 388 NATURE AND PROPERTIES OF SOILS tance in soils, although it is doubtful whether they are detri- mental except under special conditions." 212. Micro-organisms—fungi and alge.—Of the higher fungi, molds are the only group that apparently attain any particular importance in soils, although yeasts have been found to occur and may in special cases exist in considerable num- bers. It is only recently, however, that fungi have received much attention, although their presence has been noted many times. Such common genera as Fusarium, Mucor, Aspergillas, and Pencillium are usually present in normal soils. In gen- eral, a large amount of organic matter is conducive to the activity of such fungi. Molds occur in soils in both the active and the spore stage and probably pass their various life cycles entirely in the soil. Waksman,” in a detailed study of soil fungi, found that most of the organisms were capable of producing considerable ammonia from nitrogenous organic matter. A large propor- tion of the fungi isolated were also able to decompose cellulose rather rapidly. Different soils seemed to have a distinct and characteristic fungal flora. Over one hundred distinct species of fungi were isolated by Waksman belonging to thirty-one genera. Some pathogenic species, such as different Fusaria and Alternaria, were found. The numbers ranged from 80,- 000 to a gram of soil under forest conditions to 14,000,000 to a gram in a meadow soil. The numbers were usually larger 1Koch, G. P., Studies on the Activity of Soil Protozoa; Soil Sci., Vol. II, No. 2, pp. 163-181, 1916. 2Waksman, S. A., Soil Fungi and Their Activities; Soil Sci., Vol. II, No. 2, pp. 103-155, 1916. Also, McLean, H. C., and Wilson, G. W., Ammonification Studies with Soil Fungi; N. J. Agr. Exp. Sta., Bul. 270, 1914. Kopeloff, N., The Effect of Soil Reaction on Ammonification by Certain Soil Fungi; Soil Sci., Vol. V, No. 1, pp. 541-574, 1916. Coleman, D. A., Environmental Factors Influencing the Activity of Soil Fungi; Soil Sci., Vol. V, No. 2, pp. 1-66, 1916. Brown, P. E., The Importance of Mold Action in Soil; Science, N. S., Vol. XLVI, No. 1182, pp. 171-175, 1917. Conn, H. J., The Microscopic Study of Bacteria and Fungi in Soil; N. Y. State Agr. Exp. Sta., Tech. Bul. 64, 1918. SOIL ORGANISMS 389 in the surface soil. While the microscopic alge are probably present in soils, it has never been shown that they are of practical importance. 213. Actinomyces.—The actinomyces are a_ filamentous form of organisms, widely distributed in nature and are prob- ably more nearly related to the bacteria than to the molds, although they produce spores and develop into branching forms of considerable complexity. Their production of aérial hyphe is quite unlike the habits of bacteria. These thread organisms exist in the soil in both the vegetative and the resting stage and often make up quite a large proportion of the soil flora. They are extremely difficult to study, since they produce hard compact growths. It is questionable also, whether the growths produced artificially are exactly like those occurring in the soil. Hiltner and Stormer! found that 20 per cent. of the soil organisms developing on gelatin plates inoculated from the soil were actinomyces. Conn? reports a range from 11 to 75 per cent. under similar cultural conditions. The average was 38 per cent. Conn estimates that 20 per cent. of the average flora consists of actinomyces. The organisms were generally greater in meadow soil than in cultivated land, indicating the relationship of these thread forms to cellulose decomposi- tion. McBeth* found actinomyces of wide distribution in soils and he concludes that they are undoubtedly an impor- tant factor in the decomposition of the cellulose of the soil organic matter. 1 Hiltner, L., and Stormer, K., Studien tiber die Bakterienflora des Ackerbodens; Kaiserliches Gesundheitsamt, Biol. Abt. Land-u. Forstw., Bd. 3, 8. 445-545, 1903. Conn, H. J., A Possible Function of Actinomycetes in Soil; Jour. Bact., Vol. 1, No. 2, pp. 197-207, 1916. °’McBeth, I. G., Studies on the Decomposition of Cellulose in Soils; Soil Sci., Vol. I, No. 5, pp. 437-487, 1916. Also, Waksman, S. A., and Curtis, R. E., The Actinomyces of the Soil; Soil Sci., Vol. 1, No. 2, pp. 99-134, 1916. Waksman, S. A., Cultural Studies of Species of Actinomyces; Soil Sci., Vol. VIII, No. 2, pp. 71-207, 1919. 390 NATURE AND PROPERTIES OF SOIL 214. Bacteria.—Of the several forms of micro-organisms in the soil, bacteria are probably the most important. In fact, the abundant and continued growth of higher plants on the soil is absolutely dependent on the presence of bacteria. Through their action chemical changes are brought about which result in the solution of both organic and inorganic material necessary for the life of higher plants, and which, in part at least, would not otherwise be available. Bacteria are single cell organisms and are probably the simplest forms of life with which we have to deal. They are generally much smaller than yeasts, multiplying by elongat- ing and dividing into half. They are, therefore, often called fission fungi. Molds multiply by budding. The activities of both groups are similar, in that they produce their effects very largely by the production of enzymes.!| The importance of enzymic influences must constantly be borne in mind in all biological transformations in the soil. Bacteria are very small, the larger individuals seldom ex- ceeding one or two microns (.001 to .002 m.m.) in diameter. In the soil there is good reason to suppose that there are many groups which are too small to be seen under the micro- scope. Such organisms may, therefore, function as a part of the colloidal matter of the soil. Many of the soil bacteria are equipped with extremely delicate vibrating hairs called flagella, which enable the organisms to swim through the 1 Bacteria, as well as most fungi, bring about their important trans- formations largely by means of enzymes. These enzymes are catalytic agents and are generally considered as colloidal in nature. A number of transformations may be accelerated by enzymes, the exact reaction de- pending on the nature of the enzyme itself. The change in the soil of ammonia (NH,) to the nitrate form (NO,) is an example of oxidation and is spoken of as nitrification. The reversal of this action is desig- nated as reduction and is probably not entirely enzymic. A splitting action is very common. The breaking up of glucose into alcohol and carbon dioxide is an example of this (C.H,,O, = 2C,H,OH + 2C0O,). A fourth reaction that may be hastened by enzymic influence is hydrol- ysis. Cane-sugar may thus quickly produce glucose and fructose (Ci3H22011 4° H,0 = C.H,20, hr C,Hi20.). SOIL ORGANISMS 391 soil-water. The shape of bacteria is varied in that they may be nearly round, rod-like, or spirals. In the soil the rod- shaped organisms seem to predominate. As already stated, the primary method of multiplication of bacteria is by simple division, the process being very rapid under favorable conditions. The phenomena frequently takes place in thirty minutes. This almost unlimited capacity to increase in numbers is extremely important in the soil since it allows certain groups quickly to assume their normal func- tions under favorable conditions, even though their numbers were originally small.t Bacteria may thus be considered as a force of tremendous magnitude in the soil, held more or less in check by conditions, but ever ready to exert an influ- ence of profound importance on crop growth. In the soil bacteria probably exist as mats or clumps, called colonies, on and around the soil particles wherever food conditions are favorable. Natural and artificial forces tend to break up these colonies and, as many groups are flagellated, bacteria becomes well distributed through the soil. In gen- eral the greatest numbers are found in the surface layers of the soil, since conditions of temperature, aération, and food are here more favorable. Many of the soil bacteria are able to produce spores, thus presenting both a resting and a vege- tative stage. The production of spores is often extremely important as it allows the organisms to survive unfavorable conditions of many kinds. The number of bacteria present in soil is quite variable as many conditions markedly affect their growth. The meth- ods” of determining the numbers are extremely inaccurate, 1If a single bacterium and every subsequent organism produced sub- divided every hour, the offspring from the original cell would be about 17,000,000 in twenty-four hours. In six days the organisms would greatly surpass the earth in volume. Under actual conditions such multiplication would never occur, due to lack of food and other limitations. The counting of soil bacteria is generally carried out somewhat as follows: A small sample of soil (usually .5 gram) is placed in a sterile Erlenmeyer flask and treated with 100 cc. of sterile water. The sample 392 NATURE AND PROPERTIES OF SOILS since many organisms cannot grow in the artificial media commonly used. Moreover, it is almost impossible to break up the clump of colonies in such a way as to determine the number of individuals present. It is fairly certain, however, that the numbers of bacteria in soil are very large, possibly ranging from 500,000 to 100,000,000 to a gram of dry soil. Good soils seem, in general, to carry the greatest numbers. The bacterial flora, as well as the other soil organisms, fluctu- ate markedly with season, the numbers usually being great- est in the summer months. 215. Conditions affecting bacterial growth.1—Many con- is then well shaken in order to produce a suspension containing the bacteria originally present in the soil. Dilutions of 1 to 20,000, 1 to 100,000 and 1 to 200,000 based on the original soil sample are made. Gelatin or agar plates are then inoculated, three from each dilution. After adequate incubation the colonies on the plates are counted, each colony supposedly representing one original organism. The numbers of bacteria that were present in the original soil are then calculated. The agar or gelatin of the plates generally receive a sterile extract from the soil together with certain added materials, organic or inorganic, in order that the growth of the bacteria may be hastened. Such a count does not represent by any means all of the bacteria of the soil, as some groups will not develop at all, while others require special media. Slowly growing groups of organisms, that would prob- ably appear if time were given, escape the count, since the plates are so quickly covered by more abundant growths. The suspension from the soil, used to inoculate the plates, does not contain all of the organisms as single individuals, since it is impossible completely to break down the clump formation. This tends to make the counts too low. Special media and technique are of course necessary in studying fungi, alge and actinomyées. 1Rahn, Otto, The Bacterial Activity in Soil as a Function of Grain-size and Moisture Content; Mich. Agr. Exp. Sta., Tech. Bul. 16, 1912. Plummer, J. K., Some Effects of Oxygen and Carbon Dioxide on Nitri- fication and Ammonification in Soils; Cornell Agr. Exp, Sta., Bul. 384, 1916. Greaves, J. E., and Carter, E. G., Influence of Barnyard Manure and Water Upon the Bacterial Activities of the Soil; Jour. Agr. Res., Vol. VI, No. 23, pp. 889-926, 1916. Brown, P. E. The Influence of Some Common Humus-forming Mate- rials of Narrow and of Wide Nitrogen-carbon Ratio on Bacterial Num- bers; Soil Sci., Vol. 1, No. 1, pp. 49-75, 1916. Waksman, S. A., Bacterial Numbers in Soils, at Different Depths and in Different Seasons of the Year; Soil Sci., Vol. I, No. 4, pp. 363-380, 1916. Gainey, P. L., The Effect of Time and Depth of Cultivating a Wheat SOIL ORGANISMS 393 ditions of the soil affect the growth of bacteria. Among the most important of these are the supply of oxygen and mois- ture, the temperature, the presence of organic matter, and the acidity or the basicity of the soil. All soil bacteria require for their growth a certain amount of oxygen. Some bacteria, however, can continue their activ- ities with much less oxygen than can others. Those requir- ing an abundant supply of oxygen have been called aérobic bacteria, while those preferring little air are designated as anaérobic bacteria. This is an important distinction, because those bacteria that are of greatest benefit to the soil are, in the main aérobes, and those that are injurious in their action are chiefly anaérobes. However, it seems likely that an aérobie bacterium may gradually accommodate itself within certain limits to an environment containing less oxygen, and an anaérobic bacterium may accommodate itself to the pres- ence of a larger amount of oxygen. It is quite possible that the aérobic and anaérobie organisms function in the soil at the same time, since a portion even of a well aérated soil is always highly charged with carbon dioxide. It is not improb- able, also, that there exists a more or less beneficial inter- relation between the two general groups. Bacteria require moisture for their growth, optimum water for higher plants seemingly being the best moisture for the development and activity of favorable soil organisms of all kinds. With a decrease of moisture the soil becomes well aérated, while an excessive water supply tends to encourage anaerobic conditions. Moisture, when aération and tempera- ture are favorable, seems to be the main control of biological changes within the soil. Soil bacteria, like other plants, continue life and growth Seed-Bea upon Bacterial Activity in the Soil; Soil Sci., Vol. II, No. 2, pp. 193-204, 1916. Greaves, J. E., and Carter, E. G., Influence of Moisture on the Bac- terial Activities of the Soil; Soil Sci., Vol. X, No. 5, pp. 361-387, 1920. 394 NATURE AND PROPERTIES OF SOILS under a considerable range of temperature. Freezing, while rendering bacteria dormant, does not kill them, and growth begins slightly above that point.’ It has been shown that some nitrification occurs at temperatures as low as from 37° to 39° F. It is not, however, until the temperature is con- siderably higher that bacterial functions are pronounced. From 70° to 110° F. their activity is greatest, and it dimin- ishes perceptibly below or above those points. The thermal death point of most forms of bacteria is between 110° and Fig. 56.—Some important decay organisms found in soils. (a), Acti- nomyces threads; (b), a colony of Actinomyces; (e) and (d), Pro- teus vulgaris; (e), B, fluorescens; (f), B. subtilis. 160° F., but the spore forms even resist boiling. Only in some desert soils does the natural temperature reach a point sufficiently high actually to destroy bacteria, and there only near the surface. In fact, it is very seldom that soil tempera- tures, other conditions being favorable, become sufficiently high to curtail bacterial activity. The presence of a certain amount of organic matter is es- sential to the growth of most, but not all, forms of soil bac- *In the seasonal study of bacteria it has been repeatedly noticed that the counts increased during the winter, especially after a freeze followed by a thaw. It was considered for a time that a special winter flora was present, and was able to multiply in the soil-water which failed te freeze. It is now considered that this increase is only apparent, the freezing having disrupted the bacterial clumps, thus increasing the number of colonies appearing on the plates during incubation. SOIL ORGANISMS 395 teria. The organic matter of the soil, consisting as it does of the remains of a large variety of substances, furnishes a suit- able food supply for a very great number of forms of organ- isms. The action of one set of bacteria on the cellular matter of plants embodied in the soil produces compounds suited to other forms, and so from one stage of decomposition to another this constantly changing material affords sustenance to bac- terial flora, the extent and variety of which it is difficult to conceive. A soil low in organic matter usually has a lower bacterial content than one containing a large amount, and, under favorable conditions, the beneficial action, to a certain point at least, increases with the content of organic substance; but, as the products of bacterial life are generally injurious to the organisms producing them, such factors as the rate of aération and the basicity of the soil must determine the effectiveness of the organic matter. The so-called acidity of the soil is probably as important a factor in bacterial activity as it is to higher plants.. In general, favorable soil organisms of all kinds seem to func- tion better in a soil carrying sufficient active base to generate conditions favorable for higher plants. An exception some- times occurs, however, notably in the case of the ‘‘finger-and- toe’’ disease of certain Crucifere, which is retarded by liming. The activities of many soil bacteria result in the formation of acids which are injurious to the bacteria themselves, and unless there is present some base with which these can com- bine, bacterial development is inhibited by such products. This is one of the reasons why lime is so often of great benefit when applied to soils, and especially to those on which alfalfa and red clover are growing. For the same reason the presence of lime hastens the decay of organic matter in certain soils, and the conversion of nitrogenous material into compounds available to the plants. As showing the value of lime in the process of nitrate formation it has been 396 NATURE AND PROPERTIES OF SOILS pointed out that in the presenee of an adequate supply of ealeium the availability of ammonium salts is almost as high as that of nitrate salts, but where the supply of calcium is insufficient the value of ammonium salts is relatively low. 216. Organisms injurious to higher plants.—While the macro-organisms may, under certain conditions, be detri- mental to the growth of higher plants, it is the smaller in- habitants of the soil that attract especial attention in this re- spect. While protozoa may, under special circumstances, be extremely detrimental, injurious organisms are confined mostly to fungi and bacteria. They may be entirely parasitic in their habits or only partially so, while they may injure higher plants by attacking the roots or even the tops. Those that infest parts of the plant other than the roots are not strictly soil organisms, as they pass only a part of their eyele in the soil. Some of the more common diseases pro- duced ‘by soil organisms are: wilt of cotton, cowpeas, water- melon, flax, tobacco, tomatoes, and other plants; damping-off of a large number of plants; root-rot; and galls. Injurious fungi or bacteria may live for long periods in the soil, if the conditions necessary for their growth are main- tained. Some of them will die within a few years if their host plants are not grown on the soil, but others are able‘to main- tain existence on almost any organic substance. Once a soil is infected it is likely to remain so for a long time, or indeed indefinitely. Infection easily occurs. Organisms from infected fields may be carried on implements, plants, or rub- bish of any kind, in soil used for inoculation of leguminous crops, or even in stable manure containing infected plants or in the feces resulting from the feeding of such plants. Flooding of land by which soil is washed from one field to another may be a means of infection. Prevention is the best defense from diseases produced by such soil organisms. Once a disease has procured a foothold, it is often. impossible to eradicate all its organisms. Rota- SOIL ORGANISMS 397 tion of erops is effective for some diseases, but entire absence of the host crop is often necessary. The use of lime is bene- ficial in the case of certain diseases. Chemicals of various kinds have been tried with little success. Steam sterilization is a practical method of treating greenhouse soils for a num- ber of diseases. The breeding of plants immune to the dis- ease affecting its particular species has been successfully car- ried out in the case of the cowpea and cotton, and can doubt- less be accomplished with others. In regions in which farming is confined largely to one crop or to a limited number of cereals, it is the common ex- perience that yields decrease greatly in the course of a score of years after the virgin soil is broken. The cause for this is attributed by Bolley,! in large measure, to a diseased con- dition of the plants, due to the growth of various fungi that inhabit the soil and attack the crops grown on it. He reports that he experimented with pure cultures taken from wheat grains, straw, and roots, and has demonstrated that certain strains or species of Fusarium, Helminthosporium, Alter- naria, Macrosporium, Colletotrichum, and Cephalothecium are directly capable of attacking and destroying growing plants of wheat, oats, barley, brome-grass, and quack-grass, and that within limits the disease may be transferred from one type of crop to another. 217. The beneficial influences of soil organisms.— While the macro-organisms of the soil are usually beneficial to higher plants, the more important relationships are occupied by the micro-organisms. The micro-organisms of the soil take an active part in removing dead plants and animals from the surface of the land, and in bringing about the other oper- ations that are necessary for the production of higher plants. The first step in preparation for plant growth is to remove the remains of plants and animals that would otherwise accumu- late to the exclusion of higher plants. These are decomposed 1 Bolley, H. L., Wheat; N. Dak. Agr. Exp. Sta., Bul. 107, 1913. 398 NATURE AND PROPERTIES OF SOILS through the action of organisms of various kinds, the inter- mediate and final products of decomposition assisting plant production by contributing nitrogen, and certain mineral compounds that are a directly available source of plant nutri- ents, and also by the effect of certain of the decomposition products on the mineral substances of the soil, by which they are rendered soluble and hence available to plants. Through these operations the supply of carbon and nitro- gen required for the production of organic matter is kept in circulation. The complex organic compounds in the bodies of dead plants or animals, in which condition higher plants cannot use them, are, under the action of micro-organisms, converted by a number of stages into the simple compounds used by plants. In the course of this process, a part of the nitrogen is sometimes lost into the air by conversion into free nitrogen, but fortunately this may be recovered and even more nitrogen taken from the air by certain other organisms of the soil. Higher fungi and actinomyces are particularly active in the early stages of decomposition of both nitrogenous and non-nitrogenous organic matter. Molds are capable of am- monifying proteins, and even re-forming complex protein bodies from the nitrogen of ammonium salts. Certain of the molds and of the alge are apparently able to fix atmospheric nitrogen, and contribute in addition a supply of carbohy- drates required for the use of the nitrogen-fixing bacteria. While the higher fungi are important in such transforma- tions, their activities in almost every stage are excelled by those of the bacteria. Because of this, the vital biological transformations within the soil are generally ascribed to bac- terial action, the bacteria receiving the greatest attention of the numberless organisms making up both the soil flora and fauna. 218. Biological cycles——Because of a lack of knowledge regarding the flora and fauna of the soil, it is obviously im- SOIL ORGANISMS 399 possible to discuss in detail the transformations caused by individual species of organisms or even by groups of related species. From the standpoint of soil fertility such an at- tempt is unnecessary, as a practical understanding of the changes through which a given soil constituent passes as it is prepared for plant nutrition, is much more important than the possession of specific knowledge regarding the organ- isms concerned. As a consequence it has become customary to discuss the biological transformations of the more impor- tant soil constituents, including as much regarding the speci- fice organisms and groups of organisms involved as is con- sistent with a clear fertility viewpoint.t Four cycles are gen- erally recognized, as follows: (1) the carbon eycle, (2) the sulfur cycle, (3) the mineral cycle, and (4) the nitrogen eycle. 219. The carbon cycle.—Since all organic compounds carry carbon, nitrogenous as well non-nitrogenous materials are involved in the carbon cycle. Nevertheless attention will be directed for the time being only toward the carbon and the changes that it undergoes from the time it enters the soil until it is removed either by aération, leaching, or by plant absorption. Most of the carbon compounds enter the soil as plant tissue, although animal remains contribute appreciable amounts. These carbonaceous materials are immediately attacked in the soil by a host of different organisms capable of producing fermentation. While such bacteria as Bacillus subtilis, Ba- cillus mycoides, and the like have a great deal to do with the decay processes, they are by no means the only agents. Most of the microscopic fungi, as well as the larger fungi and alge, * There are two general ways of studying the soil flora. A classification of the organisms may be attempted. This requires the isolation and study of individuals and has so far met with but little success. The second approach is a biochemical one, in which the transformations oc- curring in the soil are studied first, the specific organisms involved being a secondary consideration. The determination of the capacity of the soil to produce ammonia is an example of this method of study. 400 NATURE AND PROPERTIES OF SOILS aid in the initial transformation, being particularly effective in decomposing cellulose. The actinomyces, present in such large numbers, seem to be especially fitted for the breaking down of such resistant material. The result of these complex decomposition processes is the formation of a partially decayed group of carbon-bearing material, some being quite simple while others are extremely complicated. The change is accompanied through its entire course by the formation of carbon dioxide and water, the end- products of carbohydrate decay. The same heterogeneous group of soil organisms, which initiate the simplification of carbonaceous materials, seem to continue the process until only the end products and the more resistant portions of the original tissue remain. The transformations above discussed are not the only sources of carbon dioxide within the soil. Some carbon diox- ide is brought down in rain-water, while still more is given off by the roots of living plants (see par. 156). Moreover some earbon dioxide is obtained from the inorganic matter of the soil, especially if the land has recently received an applica- tion of limestone. The reactions within the soil seem to de- compose such carbonates rather readily, carbon dioxide being given off (see par. 201). 226. The loss of carbon from the soil.—Carbon diox- ide, the importance of which has already been fully discussed (par. 132), may suffer transformation in a number of ways in the soil. It may be lost (1) to the atmospheric air; (2) it may react with the mineral constituents of the soil and be held at least temporarily by the soil mass; or (8) it may be removed by leaching. Since the soil-water is always more or less charged with carbon dioxide and since ‘it carries car- bonate and bicarbonate salts, considerable carbon is continu- ally being removed in this way. In this regard the figures from the Cornel lysimeter tanks! are especially interesting. 1 Unpublished data. Cornell Agr. Exp. Sta., Ithaca, N. Y. SOIL ORGANISMS 401 The data are expressed in pounds to the acre and are averages of ten years’ experimentation. The carbon was lost as the bicarbonate, only traces of carbonates being present. (See table LXX XVIII, page 402). i COs, GY je TQ T-ASE = GREEN FARM “S.. MANURE MANURE - ~ SNe, SOIL DECAY REACTIONS \4 PARTIALLY DECOMPOSED MATERIALS CARBO N= ae pain) a BIOLOGICAL ACTIVITIES REACTIONS Fig. 57.—Diagram showing the transformations of carbon, commonly spoken of as the ‘‘carbon cycle.’’ LEAC : LOSSES It is apparent that a drainage loss of about 1200 pounds of bicarbonate (HCO,) may be expected each year to the acre, without considering the carbon dioxide which is respired to the atmosphere. This latter loss probably at least equals, if it does not greatly exceed, the loss of carbon in the bicar- bonate form. Together they cause a disappearance of several hundred pounds of carbon a year under the conditions main- 402 NATURE AND PROPERTIES OF SOILS TABLE LXXXVIII LOSS OF CARBON FROM THE SOIL IN DRAINAGE, EXPRESSED IN POUNDS TO THE ACRE PER YEAR. CORNELL LYSIMETERS. HCO, CARBON ERE USE (POUNDS) (POUNDS ) Bane, :SOtlc i Geta eet sete eel tees 1391 273 TROCA TOMS ache eee wetter 1350 265 (GRASS he Aoi pa se ene eine in eGo ae 11938 234 tained in the Cornell lysimeters. The application of two tons of green-manure to the acre would be necessary to replace even the drainage loss cited above. Small amounts of carbon may be removed by means other than drainage or diffusion into the atmospheric air. Nu- merous investigators! have shown that plants are capable of assimilating various organic materials. Recently it has been demonstrated that higher plants may utilize a consid- erable variety of carbohydrate compounds.” Such materials, when thus assimilated, no doubt supply the plant with en- ergy and thus are foods rather than nutrients. The ready response of certain crops, such as maize, to applications of farm manure lends plausibility to the theory that considerable carbon may be removed from the soil by plants and that the carbon dioxide of the air is not the only immediate source of the element carbon. 221. The sulfur cycle—Sulfur is an essential plant nu- 1 Hutchinson, H. B., and Miller, N. H. J., The Direct Assimilation a Inorganic and Organic Forms of Nitrogen by Higher Plants; Centrlb. f Bakt., II, Band 30,8. 513-547, 1911. 3 Mazé, je , Influence, sur le ‘développement de la plante, des substances minérales qui s’accumulent dans ses organes comme résidus d’assimila- tion; Compt. Rend. Sci., Paris, Tome 152, pp. 783-785, 1911. Ravin, P., Nutrition carbonée des plantes a l’aide des acides organique libres et combinés ; Ann. Sci. Nat. Bot., Ser. 9, No. 18, pp. 289-446; 1913. Knudson, L., Influence of Certain Carbohydrates on Green Plants; Cornell Agr. Exp. Sta., Memoir 9, July 1916. SOIL ORGANISMS 403 trient, being utilized by such crops as alfalfa, turnips, and cabbage in much larger amounts than is phosphorus. Ames, J. W., and Boltz, G. E., Effect of Sulfofication and Nitrifica- tion on Potassium and Other Soil Constituents; Soil Sci., Vol. VII, No. 3, pp. 183-195, 1919. See also, Tottingham, W. E., and Hart, E. B., Sulfur and Sulfur Composts in Relation to Plant Nutrition; Soil Scei., Vol. XI, No. 1, pp. 49-65, 1921. SOIL ORGANISMS 407 is yet to be determined, and will depend on a number of fac- tors. The soil must, of course, be deficient in the constituent composted with sulfur. Otherwise, an application of sulfur alone would give just as good results. Again the cost of composting must be reckoned with. It yet remains to be proven by crop growth whether the efficiency of sulfur is any greater when it is composted with such materials as raw rock phosphate and farm manure and applied to the soil, than when these materials are added separately. 225. The mineral cycle.——The strictly mineral constitu- ents of the soil seem to undergo as complex and intricate transformations as do the elements that are considered as more closely related to the soil organic matter, such as ecar- bon, nitrogen and sulfur. While a part of the mineral cycle is purely chemical or physico-chemical, the biological phase is by no means unimportant. In fact, were it not for the in- fluence of organisms within the soil, little or no mineral mat- ter, such as phosphoric acid and potash, would ever become available to higher plants. When plant or animal tissue enters the soil, it undergoes decay in the manner already described, the ash constituents being liberated and either utilized directly by higher plants again or converted into a part of the soil mass. The main source of the mineral nutrients for any plant is of course the inorganic portion of the soil rather than the organic part. It is thus necessary to investigate what influence, if any, soil organisms have on such material. The action of organisms on the inorganic portions of the soil is of two kinds: (1) direct, and (2) indirect. In the former the soil organisms themselves attack the mineral mat- ter, rendering part of it available. Some of this soluble ma- terial is absorbed by the organisms, becoming a part of the eell contents. When the fungus or bacterium dies, this ma- terial through decay again becomes available and may be used by higher plants. While most soil organisms probably 408 NATURE AND PROPERTIES OF SOILS function to a certain extent in this direction, some are es- pecially active. It is known that B. mycoides, B. mesentert- cus and B. megatherium are capable of assimilating phos- phorus in considerable quantities, while such organisms as Beggiotoa and Ophidomonas store up sulfur in large amounts. In the same way iron, potassium, calcium, and like elements may be utilized. While such biological action is at the time a direct competition with higher plants, more mineral ma- terial is ultimately available in the soil through such activ- ities. While the direct effects of organisms on soil minerals is no doubt very important, the direct influences seem to be more vital in a practical way. While this indirect influence may be in part enzymic, it is probably largely due to the production of carbon dioxide, which accompanies all types of life processes. The sulfurous acid and nitrous acid of the sulfur and the nitrogen cycles, respectively, are also active to a certain extent. The preceding discussion of the sulfur compost indicates how vigorous the biological oxidation with- in the sulfur cycle may become under certain conditions. In the soil, however, carbon dioxide is probably by far the most important.1. Since the significance of carbon dioxide has already been adequately discussed (pars. 17, 58 and 132), it is sufficient at this point to state that this gas, because of its large amounts and its intimate relationship to the mineral material, is probably the most effective solvent agent in the soil. 1 Typical reactions involving tri-calcium phosphate, orthoclase and cal- cium carbonate are as follows: Ca,(PO,), + 2CO, + 2H,0 — Ca,H,(PO,), + Ca(HCO,).. 2K AISi,O, + CO, + 2H,0 — H,A1S8i,0, + K,CO, + 4Si0,. CaCO, + H,O + CO, = Ca(HCO;)>. CHAPTER XXI SOIL ORGANISMS—THE NITROGEN CYCLE OF THE various nutrient materials applied to the soil for the use of plants nitrogen has the highest commercial value and is absorbed in very large quantities. Moreover, nitro- gen is lost from the soil in considerable amounts in drainage water and possibly to some extent in gaseous form. The ereat importance of this element and of its compounds in agriculture and the possibility of it becoming a limiting factor in crop production has lead to much study regarding its re- actions and movements in the soil. The original source of the world’s supply of combined nitro- gen has been the atmosphere and, as the free gas is exceed- ingly inert,’ the natural forces which facilitate its combina- tion must be extremely powerful. The movement of nitrogen from air to soil, from soil to plant, from plant back to soil or to animal, and from animal to soil, with a return to air at various stages, involves many forces, many factors, many or- ganisms, and many reactions. These complicated changes are spoken of as the nitrogen cycle. 226. The nitrogen cycle—In tracing the various trans- formations through which the nitrogen passes, the conspicu- ous feature is the great complexity of the cycle. Apparently the nitrogen cycle is much more extended and intricate than either the carbon or sulfur cycles. This complexity, however, * Because nitrogen is such an inert gas, it must not be inferred that it forms inactive compounds with other materials. In combination it is extremely active, seemingly being the basis of all plant and animal life processes, 409 410 NATURE AND PROPERTIES OF SOILS is more apparent than real. The transformation of nitro- gen has received so much attention and study that more is known regarding the changes involved. The other cycles are probably just as extended and complicated, the lack of knowledge forcing a simpler presentation. From the standpoint of soil fertility the compounds that are produced in the nitrogen cycle and the relation of these materials to plant growth are of major consideration. While the organisms involved in the transformation should receive as much attention as is practicable, the approach should be by means of biological-chemistry rather than through bacteri- ology. It must not be inferred that the carbon, sulfur and nitro- gen cycles are distinct or that transformations may proceed in one with no activity in the others. As a matter of fact, the cycles are interlocked in a hopelessly intricate manner. The decomposition of proteid matter involves all of the cycles already mentioned. The carbon, sulfur, and nitrogen un- dergo distinctly different transformations, but the changes are so closely related as to make definite lines of distinction very difficult. Proteid matter may produce urea, carbon dioxide, water, and sulfates. Certain of these products often strongly influence the solubility of the soil minerals. Thus, the four cycles already mentioned would be involved in the decomposition of one original compound. 227. Decay and putrefaction.—The decomposition of most nitrogenous matter is very rapid in a normal soil, the putrefactive influences producing partially decayed sub- stances of great variety.2, Some of these materials are very complicated, while others are capable of being absorbed di- *Decomposition and decay are general terms, referring to all types of biological degradation. Fermentation refers to the decomposition of carbohydrates, while putrefaction has to do with nitrogenous materials. The two latter terms are generally very loosely used. * Lathrop, E. C., Protein Decomposition in Soils; Soil Sci., Vol. I, No. 6, pp. 509-532, 1916. SOIL ORGANISMS 411 rectly by plants without further change. Carbon dioxide and water are formed continuously as the process advances. The sulfur of the proteid compounds produces hydrogen sulfide or free sulfur and later sulfates. Hutchinson and Miller,! as well as other investigators, have studied the question of the assimilation of nitrogenous organic compounds by higher plants. The general conclu- sions indicate that such a source of nitrogen is quite impor- tant and sometimes allows the plant to benefit markedly from the assimilation of such materials. Maize, for example, seems to be particularly stimulated by farm manure, which earries large amounts of organic nitrogenous compounds such as urea. Acetamide, urea, barbituric acid, creatinine, alloxan, peptone, and a number of other organic compounds have been shown to be available to certain higher plants. Decay and putrefaction are carried on by a large number of organisms, the higher fungi as well as such bacteria as B. subtilis, B. mycoides, and similar micro-organisms engag- ing in the decomposition processes. Some of the charac- teristic, although not constant, products formed in the pu- trefaction of albumin and proteins are albumoses, peptones, and amino acids, followed by the formation of cadaverine, putrescine, skatol, and indol. Where an abundant supply of oxygen is present, or where a sufficient supply of carbo- hydrates exists, the latter substances are not formed. There are many other products of putrefaction, including a num- ber of gases, as carbon dioxide, hydrogen sulfide, marsh gas, phosphine, hydrogen, nitrogen, and the like. Present-day knowledge of the subject does not make it pos- sible to present a list of the organisms concerned in each step, or to name all the intermediate products formed. For the student of the soil the first consideration is a knowledge of * Hutchinson, H. B., and Miller, N. H. J., The Direct Assimilation of Inorganic and Organic Forms of Nitrogen by Higher Plants; Centrlb. f. Bakt., IT, Band 30, Seite 513-547, 1911. 412 NATURE AND PROPERTIES OF SOILS the circumstances under which the nitrogen is made avail- able to plants, and the conditions that are likely to encourage its loss from the soil. 228. Ammonification may be considered as the second step in the simplification which nitrogenous compounds undergo in the soil. As the name implies, it is the stage of the decay process in which ammonia is one of the important products. Like other processes of decomposition, there are many species of organisms capable of producing ammonia, the higher fungi Fig. 58.—Some soil organisms important in the nitrogen cycle. (a) Azotobacter agilis; (b) nitrate bacteria. Urea bacteria, (c) Uro- bacillus miguelii and (d) Urobacillus leubii. and alge as well as bacteria participating in the changed in the character of the nitrogen compounds. Different soil organisms display diverse abilities in con- verting the nitrogen of the same organic material into am- monia, some acting more rapidly or more thoroughly than others. In tests by certain investigators in which the same bacteria were allowed to act on different substances, the order of their efficiency was reversed with a change of substance. This characteristic preference of a class of organisms for the decomposition of certain substances is made evident by the experiments of Sackett,! who found that in some soils dried Sackett, W. G., The Ammonifying Efficiency of Certain Colorado Soils; Colo. Agr. Exp. Sta., Bul. 184, 1912. SOIL ORGANISMS 413 blood was ammonified more rapidly than was cottonseed meal, while in other soils the reverse was true. While the soil fungi have been but little studied, the litera- ture available seems to indicate that they take an important part in all soil processes, except possibly the fixation of at- mospheric nitrogen and the formation of nitrates. Most soil fungi produce ammonia readily. Waksman' found such forms as Mucor racemosus, Pencillium lilacinum, and Rhiz- opus sp. II compared favorably in capacity to produce am- monia with Bacillus mycoides when grown in artificial cul- ture, blood and cottonseed meal being the sources of nitrogen. Kopeloft ? found that certain fungi seemed to prefer an acid medium for their ammonifying activities. This suggests that a natural provision is thus made for ammonification, no mat- ter what the soil reaction may be. Among the bacteria producing ammonification are B. my- coides, B. subtilis, B. mesentericus vulgatus, B. janthinus, and B. proteus vulgaris. Of these, B. mycoides has been very carefully studied, and the findings of Marchal * may be taken as representative of the process of ammonification. He found that when this bacterium was seeded on a neutral solution of albumin, ammonia and carbon dioxide were produced, to- gether with small amounts of peptone, leucine, tyrosine, and formic, butyric, and propionic acids. He concludes that in the process atmospheric oxygen is used, and that the carbon of the albumin is converted into carbon doxide, the sulfur into sulfates, and the hydrogen partly into water, and partly into ammonia by combining with the nitrogen of the organic 1Waksman, 8. A., Soil Fungi and Their Activities; Soil Scei., Vol. II, No. 2, pp. 103-155, 1916. See also, McLean, H. C., and Wilson, G. W., Ammonification Studies with Soil Fungi; N. J. Agr. Exp. Sta., Bul. 270, 1914. 2 Kopeloff, N., The Effect of Soil Reaction on Ammonification by Certain Soil Fungi; Soil Sci., Vol. I, No. 6, pp. 541-573, 1916. > Marchal, E., Sur la Production de l’Ammoniaque dans le Sol par les Microbes; Bulletins de 1’Acad. Royale de Belg., 3 series, T. 25, pp. 727-776; 1893. 414 NATURE AND PROPERTIES OF SOILS substance. Marchal found that B. mycoides was also capable of ammonifying casein, fibrin, legumin, glutin, myosin, serin, peptones, creatine, leucine, tyrosine, and asparagine, but not urea. The following reactions may be cited as indicating the changes that probably occur when albumin and urea undergo ammonification : C,2H,,2.N,,80.. + 770, = 29H,0 + 72CO, + SO, + 18NH, Albumin CON.H, + 2H,0 = (NH,),CO, Urea While ammonification ' seems to proceed to the best advan- tage in a well-drained and aérated soil with plenty of active basic material present, it will take place to some extent under almost any condition, due to the great number of different or- ganisms capable of accomplishing the change. In certain soils, as shown by Russell and Hutchinson? as well as by other authors (see par. 211), protozoa may retard ammoni- fication by feeding on the chief ammonia-producing organ- isms. Such a condition is seldom serious in arable soils. *The ammonifying efficiency of a soil is usually determined by treat- ing a 200-gram sample of fresh soil with cottonseed meal or dried blood carrying 120 milligrams of nitrogen. The mixture is then incubated, usually for seven days, at optimum temperature and moisture. The in- crease in ammonia is taken as a measure of the ammonifying efficiency. The artificial nature of the test detracts largely from its value. See Temple, J. C., The Value of Ammonification Tests; Ga. Agr. Exp. Sta., Bul. 126, 1919. * Russell, E. J., and Darbishire, F. V., Oxidation in Soils and Its Relation to Productiveness. Part 2. The Influence of Partial Steriliza- tion; Jour. Agr. Sci., Vol. 2, pp. 305-326, 1907. Russell, E. J., and Hutchinson, H. B., The Effect of Partial Steriliza- tion of Soil on the Production of Plant Food; Jour. Agr. Sci., Vol. 3, pp. 111-144, 1909. Russell, E. J., and Hutchinson, H. B., The Limitation of Bacterial Numbers in Normal Soils and Its Consequences; Jour. Agr. Sci., Vol. 5, pp. 152-221, 1903. Buddin, W., Partial Sterilization of Soil by Volatile and Non- volatile Antiseptics; Jour. Agr. Sci., Vol. 6, pp. 417-451, 1914. SOIL ORGANISMS 415 229. Nitrification—Some agricultural plants can utilize ammonium salts as a source of nitrogen.t This has been shown to be true for rice, maize, peas, barley, and potatoes (see par. 248). Most plants, however, except for rice, show a decided preference for nitrogen in the nitrate form. Whether these common crops ean thrive as well on ammonium salts as on nitrates has not been definitely demonstrated. In most arable soils the transformation of nitrogen does not stop with its conversion into ammonia, but goes on by an oxidation proc- ess to the formation of nitrous acid. The nitrous acid, after reaction with a base, is farther oxidized, a salt of nitric acid resulting. This process of oxidation is generally spoken of as nitrification. The reactions involved may be written as follows: 2NH, + 30, = 2HNO, + 2H,0 ? 2HNO, + CaH,(CO,), = Ca(NO,), + 2H,O + 2CO, ° Ca(NO,). + O, = Ca(NO,), Kach of these steps is brought about by a distinct bacteri- um, but the groups are closely related. Collectively they are called nitrobacteria. Nitrosomonas and Nitrosococeus are the bacteria concerned in the conversion of ammonia into nitrous acid or nitrites. The former are supposed to be char- acteristic of European, and the latter of American, soils. The organisms concerned in the oxidation of nitrites to ni- * Kelley, W. P., The Assimilation of Nitrogen by Rice; Haw. Agr. Exp. Sta., Bul. 24, 1911. Hutchinson, H. B., and Miller, N. H. J., The Direct Assimilation of Inorganic and Organic Forms of Nitrogen by Higher Plants; Centrlb. f. Bakt., II, Band 30, Seite 513-547, 1911. * Loew states that the reaction is as follows: 2NH, + 20, = 2HNO, + 4H Loew, O., Die Chemischen Verhiltnisse des Bakterienlebens: II. Centrlb. f. Bakt., II, Bd. 9, Seite 690-697, 1891. *TIt has often been suggested that the acid produced by the nitrifying process is of considerable importance in rendering mineral nutrients available. While this may be true, the extent to which the solution phenomenon takes place and its practical significance have never been satisfactorily established by experimentation. 416 NATURE AND PROPERTIES OF SOILS trates are generally designated as Nitrobacter. In practice these bacteria are generally spoken of as nitrite and nitrate organisms. The conditions favoring the two groups are practically the same. As a consequence, nitrification is gen- erally discussed as though the transformation was only one step and depended on one group of organisms.? Just as ammonification follows closely on putrefaction, so nitrification closely accompanies the production of ammonia. In fact, the processes are so well synchronized in a normal soil that only traces of ammonia and nitrites are usually found. The nitrates, however, may accumulate in large amounts. Marked differences have been noted in the nitrifying * 1While it was known from the middle of the nineteenth century that nitrogenous compounds added to the soil quickly produced nitrates, it was not until 1878 that Schloessing and Miintz demonstrated that the process was biological. In 1890 Winogradsky succeeded in isolating the organisms. As they do not develop on ordinary medium, as do the decay and ammonifying bacteria, a special technique was necessary. Winogradsky used silicic-acid-gel plates containing certain inorganic salts, as he found that the presence of even small amounts of organic matter prevented the development of the organisms. In the soil, how- ever, well-decayed organic matter generally stimulates rather than de- presses nitrification. For a review of literature and methods of isolat- ing nitrifying organisms, see Gibbs, W. M., The Isolation and Study of Nitrifying Bacteria; Soil Sci., Vol. VIII, No. 6, pp. 427-471, 1919. 2 Kaserer has isolated an organism, which he called B. Nitrator, that can oxidize ammonia directly to nitrate. He writes the reaction as follows: NH, + H,CO, + 0, = HNO, + H,O + CH,O. He thinks that the energy necessary for the completion of the reac- tion is obtained from the formaldehyde (CH,O) as follows: CH,O + O, = H,O + CO, + Energy The correlation between carbon dioxide production and nitrate accumu- lation lends probability to this theory. Kaserer, H., On Some New Nitrogen Bacteria with Autotrophic Habits of Life; Noted in Exp. Sta. Record, Vol. 18, p. 534, 1905-1906. >The nitrifying efficiency of a soil is usually determined by treating a 100-gram sample held in a tumbler with a suitable amount of ammonia sulfate or some other readily nitrifiable material. After incubation for a suitable period at optimum temperature and moisture, the increase of nitrate nitrogen is determined. This method is merely comparative and measures only the nitrate accumulation. Its value is limited as it does not simulate field conditions. SOIL ORGANISMS 417 power of different soils. Highly productive soils have gen- erally been found to maintain a greater nitrifying efficiency than less productive soils, but this is not always the case, as factors other than available nitrogen may limit the pro- ductiveness of a soil. With the formation of nitrate nitrogen, the main portion of the nitrogen cycle is completed, since plants absorb most of their nitrogen as the nitrate ion. Of this cycle, from pe ANIMAL NITROGEN OF AIR . TRI STS LOSS ISS Wisp NOAA INTIRIRI Fane CHER See ORGANISMS MANERE DECAY PARTIALLY, DECAYED CO NDS Sen oe (el EX COMPOUNDS i * AMMONIFICATION NITRIFICATION REDUCTION ( FREE N NITRATES ~€NITRITES Fic. 59.—Diagram representing the movements of nitrogen between soil, plants, animals and the atmosphere. These transformations are termed the ‘‘nitrogen cycle.’’ plant to soil, and from soil to plant again, the nitrification re- action is the weakest point, since the other biological changes proceed to a certain extent in spite of unfavorable soil con- ditions. Nitrification is easily retarded and may even be brought to a standstill. As a consequence, the factors affect- ing this particular portion of the nitrogen cycle are of special interest. A soil favorable to nitrification is generally wholly favorable to the other desirable processes involving nitrogen transformations. 418 NATURE AND PROPERTIES OF SOILS 230. Relation of soil conditions to nitrification.— 13.1 Cat Oe ais. cee 4.0 FeO, and AlOg se 1.9 CaS iad conkers i TArS SIO; 2G. eu aoe 1.6 OF 5 Ege Me meted caren ta ail Mie)? rah. a ali CacOlhj fic Uses 26.6 BGO 338 hao eee 3 Its odor and the presence of carbon are characteristic. It is intensively alkaline to litmus. In the soil it undergoes a number of very complex changes, urea ultimately being pro- duced. Toxic compounds are present as the reactions pro- ceed. It should, therefore, be placed in the soil some time be- fore the crop is seeded. The carbon seems to aid in the trans- formation as a catalytic agent. The urea quickly breaks down biologically to ammonia: CON.H, + 2H,0 = (NH,), CO, This ammonia is then oxidized to the nitrate form. 251. Basic calcium nitrate (Ca(NO,),-+-)—This fertil- izer, like calerum cyanimid, is produced by the artificial fixa- tion of nitrogen. Air is passed through an electric are of high temperature. Under such conditions a part of the oxygen and the nitrogen are forced together forming nitric oxide. This gas is then oxidized in suitable chambers to the peroxide, which is passed into water, producing nitric acid. The nitric oxide which also results is led back to the oxidizing chambers. The reactions are as follows: N, -+ 0, = 2NO ONO + O, = 2NO, 3NO, + H,0 = 2HNO, + NO The nitric acid is passed into lime-water, giving calcium nitrate. This fertilizer contains from 13 to 16 per cent. of ammonia and is intensely alkaline to litmus. Due to its high COMMERCIAL FERTILIZER MATERIALS 453 deliquescence, it must either be treated in some way, which raises the cost of manufacture, or must be shipped in sealed casks. It is very soluble in water and is immediately available to plants. It leaves no harmful residue in the soil. 252. Other methods of nitrogen fixation.—Calcium ni- trate, because of its cost, cannot compete either with sodium nitrate or ammonium sulfate and is not manufactured in this country. Calcium cyanamid is produced only in amounts sufficient to satisfy the demands of mixed fertilizer manu- facture. Its dry character makes it valuable in such com- pounding. At the present time a number of more efficient methods of artificially fixing nitrogen are known. The Haber process proved extremely successful in Germany, especially when supplemented by the Oswald method of converting ammonia -into nitric acid. In the Haber method a mixture of nitrogen and hydrogen are placed under pressure and moderately heated in the presence of a catalyst. A good yield of ammonia results. N, + 3H, = 2NH, In the Oswald method this ammonia is passed over a cata- lytic agent in the presence of oxygen. NH, + 20, = HNO, + H,0 The advantage of producing both ammonia and nitric acid is obvious, as ammoniun nitrate (NH,NO,), ammonium phos- phate ((NH,),P0,), and potassium nitrate (KNO,) may be produced at one plant. During the war Professor Bucher of Brown University per- fected a simple and inexpensive method of producing sodium cyanide synthetically. Producers gas, formed by passing air over hot coal, is forced through a heated revolving drum con- taining soda ash, iron, and coke. The reaction is as follows: Na,CO, + 4C + N, = 2NaCN + 300 454 NATURE AND PROPERTIES OF SOILS Ammonia may be produced very easily from the sodium cyanide and used as such or changed to nitric acid by the Oswald method. 253. Relative availability of nitrogen fertilizers..—It is very difficult to rank nitrogenous fertilizers on the basis of their rate of availability, since the conditions within the soil so markedly influence the transformations, especially those of a biological nature. Dried blood and ammonium sulfate, for example, will give almost as quick results in a warm, well aérated soil, as far as higher plants are concerned, as sodium nitrate. In general, however, the nitrate fertilizers should be rated as most readily available, followed in order by ammo- nium salts, dried blood, tankage, and similar materials. Such substances as wool, hair, and untreated leather waste should rank last. FERTILIZERS USED FOR THEIR PHOSPHORUS Phosphorus is generally present in nature in combination with calcium, iron, or aluminum. Some phosphates carry or- ganic matter and when thus associated are generally consid- ered to decompose more readily when added to the soil. 254. Bone phosphate (Ca,(PO,),+).—Bones were for- merly applied to the soil in the raw condition, either ground or unground. Most bone as now sold is merely steamed or boiled to remove the fat and nitrogenous matter, which is used in other ways. Bone-meal comes on the market as a dusty powder of characteristic odor. It contains about 27 per cent. of phosphoric acid as tricalcium phosphate. Tankage, which has already been spoken of as a nitrogenous fertilizer, con- tains from 3 to 8 per cent. of phosphoric acid, largely in the form of tricalcium phosphate. All bone phosphates are slow- acting manures, and should be used in a finely ground form and for the permanent benefit of the soil rather than as an 1Thorne, C. E., Carriers of Nitrogen in Fertilizers; Soil Sci., Vol. IX, No. 6, pp. 487-494, 1920. COMMERCIAL FERTILIZER MATERIALS = 455 immediate source of phosphorus. In the soil, water charged with carbon dioxide slowly converts the insoluble tricalcium phosphate into the soluble mono-calcium form: Ca, (PO,).-+ 4C0, + 4H,0 = CaH,(PO,), + 2CaH,(CO,), 255. Rock phosphate’ (Ca,(Po,),-++-).—There are many natural deposits of mineral phosphates in different parts of the world, some of the most important of which are in North America. The phosphorus in all of these is in the form of tricalcium phosphate, but the materials associated with it vary greatly. Rock phosphate may occur in nature as soft phosphate, pebble phosphate, boulder phosphate, and hard rock phosphate. South Carolina phosphate contains from 26 to 28 per cent. of phosphoric acid and a very small amount of iron and aluminum. As these latter substances interfere with the man- ufacture of acid phosphate from rock, their presence is very undesirable, rock containing more than from 3 to 6 per cent. being unsuitable for that purpose. Florida phosphates exist in the form of soft phosphate, pebble phosphate, and boulder phosphate. Such phosphate contains from 18 to 30 per cent. of phosphoric acid, and be- cause of its being softer than most of these rocks it is often applied to the land without being first converted into a soluble form. The other two forms, pebble phosphate and boulder phosphate, are highly variable in composition, ranging from 20 to 40 per cent. in phosphoric acid content. Tennessee phosphate, which is now very important, contains from 25 to 35 per cent. of phosphoric acid. Rock phosphate, or floats as it is often called, appears on the market as a heavy finely ground powder of light gray eolor. It generally carried about 27 per cent. of phosphoric acid as Ca,(PO,),. A typical analysis is as follows: 1Waggaman, W. H., and Fry, W. H., Phosphate Rock and Methods Proposed for Its Utilization as a Fertilizer; U. S. Dept. Agr., Bul. 312, 1915. 456 NATURE AND PROPERTIES OF SOILS Moisture,’ organic (matter ebony iim eal nel) Seb eee 5.06 Ca,(PO, ‘5 Deseo ae Hu RLS ARUBA Ss usr ey hus ok Sa a Oe 77.76 KePO, and ALIPOSMU veer ag) oh el eee 1.50 CaCO, ditt bachelor Hat MPR ce te it Ny ER a 4.43 IND CO oO tes i eae ROR cl OEE re eae ne Sa a 0 CaP, a CaO Paaeeyceer ic Gtoeres eae at oon Mee Oa ee onal aS Jee aicneh sao Reno eels hee dm Oe US nated eee 7G He. OF and: “Al Ores 2 oa. elancn ateietelte Sea meets a ee 3.87 Rock phosphate undergoes the same change in the soil as bone-meal but generally much more slowly, unless the soil is very high in organic matter. Mixing the rock with manure seems to hasten its availability to plants. 256. Acid phosphate’ (CaH,(PO,).+).—Acid phosphate is a dry material of a browning gray color, partially soluble in water, and has a characteristic acrid odor. It is intensely acid to litmus, as it contains certain acid salts. It carries from 14 to 16 per cent. of available P,O, and small amounts of insoluble P,O,. It is made by treating raw rock with sul- furic acid under the proper conditions.” Ca,(PO,), + 2H,SO, = CaH,(PO,),. + 2Ca SO, (insoluble) (water soluble) The acid is never added in amounts capable of quite com- pleting this reaction. Some di-calecium phosphate [Ca,H, (PO,).|, spoken of as citrate soluble or reverted phosphoric acid, is thus produced. Ca,(PO,), + H,SO, = Ca,H,(PO,), + CaSO, (insoluble) (reverted) Acid phosphate consists mostly of gypsum and mono-eal- cium phosphate with some di-calecium phosphate and impuri- 1Chemically, three forms of phosphoric acid are recognized by the fertilizer industry: (1) insoluble (Ca,(PO;,).), (2) reverted or citrate soluble (Ca,H,(PO,).), and (3) water soluble (CaH,(PO,).). The water soluble and citrate soluble phosphates are rated as available to plants. The insoluble form is considered as unavailable. ?Waggaman, W. H., The Manufacture of Acid Phosphate; U.S. Dept. Agr., Bul. 144, 1914. COMMERCIAL FERTILIZER MATERIALS 457 ties. The water soluble and reverted phosphoric acid are both rated as available. The phosphates of acid phosphate when added to the soil quickly revert to an insoluble form: Call) (PO)), -- 2CaH,(CO,), = Ca,(P0,), 4-4C0, + 4H,0 Ca,H, (PO,), ++ CaH,(CO,), = Ca,(PO,). + 2CO, 4+ 2H,0 Plenty of active calcium should be present when acid phos- phate is used to insure this reaction instead of the formation of the very insoluble ferric phosphate (KePO,) and aluminum phosphate (AIPO,). Acid phosphate does not seem to make the soil acid. In fact, it is considered by some investigators to decrease the acidity by rendering aluminum and iron in- soluble. 257. Basic slag ((CaO),.P,0,.S10,-+-).—Iron or steel con- taining over 2 per cent. of phosphorus is too brittle to be useful and, as a consequence, ores of this character were little used until methods of removing this phosphoric acid were discovered. The use of wood in smelting provided a basic ash, thus removing phosphorus from the pig iron. With coal, how- ever, the slag is acid and the phosphorus remains with the ore. In the open-hearth method of smelting the furnaces are lined with a specially prepared dolomitic limestone. Lime is later added as the smelting proceeds. The calcium of the slag unites with the phosphorus of the iron, thus reducing the percentage of that element in the steel. The most prob- able formula for the phosphorus compound in basic slag is (CaO)..P,0,.SiO,. Basie slag contains a large amount of iron and calcium hydroxide. Below is a typical analysis: BAO Mt enc eres Oe ee 45.0 0 hs 0 ee a Pe OW 2 a beef 111 (Fn @ acai ae AiR 6.2 PSKOS cn ee ian ON Gat. 2 6.9 BeEOi-E HesOs 2.5. 2... 17.6 1 EAS G a cts Me a sect ee Ae 18.1 WEAWE Ieee Seer 6 Ok Ie oD Other constituents... 1.0 *Conner, S. D., Acid Soils and the Effect of Acid Phosphate and Other Fertilizers Upon Them; Jour. Ind. and Eng, Chem., Vol. 8, No. 1, pp. 35-40, 1916. 458 NATURE AND PROPERTIES OF SOILS Basic slag comes on the market as a heavy dark gray pow- der, extremely alkaline to litmus, and contains from 14 to 20 per cent. of P,O,. The phosphorus of basic slag is almost all soluble in citric acid and, therefore, is rated as available phosphoric acid. It does not revert in the soil as does acid phosphate, but is immediately attacked by carbon dioxide and rendered rather quickly available. A possible reaction is as below: (CaO),.P,0,.Si0, + 8CO, + 6H,O = CaH,(PO,). + 4CaH,(CO,). + Si0, 258. Relative availability of phosphate fertilizers— Acid phosphate carries most of its phosphoric acid in a water- soluble form and although the phosphates revert to the tri- calcium form immediately when added to the soil, they are rather readily available to plants. This is due to the charac- ter of the freshly precipitated salt and the surface exposed for solution activities. To insure a good distribution in the soil of the phosphoric acid and a rapid influence on crops, acid phosphate should be well mixed with the soil. Basie slag, since its phosphoric acid is largely citrate sol- uble, is generally considered as next to acid phosphate in availability. Steamed bone-meal usually gives better results than raw rock phosphate and rates third, with rock phos- phate fourth in availability. The degree of fineness makes a great difference in the availability of the less soluble phos- phate fertilizers, especially the ground bone and raw rock phosphate. The latter material should be ground fine enough to pass through a sieve having at least one hundred meshes to the inch. 259. Raw rock phosphate versus acid phosphate.—Con- siderable discussion as well as controversy has of late arisen regarding the relative merits of acid phosphate and raw rock phosphate not only when applied on the basis of equal amounts of phosphoric acid but also when compared on the basis of COMMERCIAL FERTILIZER MATERIALS = 459 equal money values. If rock phosphate could be made to equal or nearly equal the availability of acid phosphate, ob- vious advantages would accrue, since raw rock costs much less than acid phosphate and carries about twice as much phos- phorie acid. The availability of the phosphorus of raw rock phosphate varies considerably with conditions. At least four major in- fluences have been recognized: (1) the character of the crop grown, (2) reaction of the soil, (3) the character of accom- panying salts, and (4) the decomposition of organic matter. It is to be expected that the various kinds of plants should not be equally influenced by the phosphorus of tri-calcium phosphate. Prianischnikov! found that lupines, mustard, peas, buckwheat, and vetch responded to fertilization with raw rock phosphate in the order named, while the cereals did not respond at all. He did not include maize in his ex- periments, but that crop is said to respond well to difficultly soluble phosphates. It is generally considered that those plants which have a long growing season are better able to utilize tri-caleium phosphate than are more rapidly growing plants. A number of investigators have stated, as a result of their experimentation, that the availability of raw rock phosphate is greater in acid soils than in those strongly basic. If acidity of the soil is due to the presence of an actual acid, it is con- ceivable that the availability may be due to the solvent action of the soil acid on the calcium of the tri-calecium phosphate, producing the di-calcium salt which appears to be fairly read- ily available to plants. When, however, soil acidity is due to a lack of certain active bases, the case is different. Gedroiz? 1Prianischnikov, D., Bericht iiber Verschiedene Versuche mit Rohphos- phaten unter Reduction; Moscow, 1910. *Gedroiz, K. K., Soils to which Rock Phosphates May Be Applied with Advantage; Jour. Exp. Agron. (Russian), Vol. 12, pp. 529-539, 811-816, 1911. The authors are indebted to Dr. J. Davidson for the translation. 460 NATURE AND PROPERTIES OF SOILS explains this on the basis of the absorptive properties of the so-called acid soil. He regards rock phosphate, not as a chemi- cal compound, but as a solid solution of di-caleium phosphate with lime. According to Gedroiz it is this excessive basicity of the phosphate which is responsible for its unavailability. Absorption of the excess calcium would leave the phosphate in a more readily available condition by forming the di- calcium salt. The presence of certain salts has been found to influence the availability of difficultly soluble phosphates. The subject has been investigated by a large number of experimenters, and it will be possible to summarize their results only in part and very briefly. It has been found, for example, that calcium carbonate decreases the availability of raw rock phosphate and bone-meal. Sodium nitrate reduces the availability of the tri-caleium phosphates, while the ammonium salts increase their availability. Iron and aluminum salts decrease avail- ability. The influence of other salts has not been so well worked out. Prianischnikov,! as the result of his extended experiments on the subject, holds that salts from which plants absorb acid radicals in larger amounts than they do bases decrease availability, or at least do not affect it, while salts from which plants absorb the bases in the greater quantity have a tendency to render the phosphate more available be- cause of the hydrogen ion concentration. There has been great differences of opinion among investi- gators as to the effect of the decomposition of organic matter on the availability of the phosphorus of tri-calcium phosphate. The contention that the availability is increased probably originated with Stoklasa,t whose experiments with bone-meal *Prianischnikov, D., Uber den Einfluss von Kohlensiuren Kalk auf die Wirkung von Verschiedenen Phosphaten; Landw. Vers. Stat., Band 75, Seite 357-376, 1911. ?Stoklasa, J., Duchacek, F., and Pitra, J., Uber den Einfluss der Bak- terien auf die Knochenzersetzung; Centrlb. f. Bakt., Il, Band 6, Seite 526-535, 554-558, 1900. COMMERCIAL FERTILIZER MATERIALS 461 indicate that the availability is increased by decay. A large number of experiments have been conducted with raw rock phosphate composted with stable manure, among which may be mentioned those by Hartwell and Pember' and also by Tottingham and Hoffman,’ who, in carefully conducted experi- ments, failed to find that the availability of the raw phos- phate, as indicated by chemical methods, was increased by fermentation with stable manure. Opposing results have also been obtained, however, and the evidence is somewhat con- flicting. With so many factors active in varying the results, espe- cially those from raw rock phosphate, it is not surprising that satisfactory field data where acid phosphate and raw rock are compared are difficult to obtain. Thorne,® after a critical review of the field experiments where acid phosphate and raw rock were used, comes to the conclusion that, while raw rock phosphate is an excellent fertilizer, acid phosphate is gener- ally superior. He finds that, while raw rock may be used with profit on land materially deficient in phosphorus, acid phosphate has generally proven to be the more effective and the more economical carrier of phosphoric acid for crops. These conclusions, which are corroborated by other in- vestigators,* do not imply that raw rock phosphate is never equal or superior to acid phosphate, nor that raw rock does not have a place as a fertilizer on the average farm. On a + Hartwell, B. L., and Pember, F. R., The Effect of Cow Dung on the Availability of Rock Phosphate; R. I. Agr. Exp. Sta., Bul. 151, 1912. ?Tottingham, W. E., and Hoffman, C., The Nature of the Changes in Solubility and Availability of Phosphorus in Fermenting Mixtures; Wis. Agr. Exp. Sta., Res. Bul. 29, 1913. ®*Thorne, C. E., Raw Phosphate Rock as a Fertilizer; Ohio Agr. Exp. Sta., Bul. 305, 1916. 4 Wiancko, A. T., and Conner, 8S. D., Acid Phosphate versus Raw Rock Phosphate as Fertilizer; Purdue Univ. Agr. Exp. Sta., Bull. 187, 1916. Brooks, W. P., Phosphates in Massachusetts Agriculture; Mass. Agr. Exp. Sta., Bull. 162, 1915. Waggaman, W. H., and Wagner, C. R., Analysis of Experimental Work with Ground Raw Rock Phosphate as a Fertilizer; U. S. Dept. Agr., Bul. 699, 1918. 462 NATURE AND PROPERTIES OF SOILS soil rich in organic matter it may be added to advantage. It is especially useful in reinforcing farm manure, seemingly be- ing about as effective under such conditions as is acid phos- phate. Its higher phosphorus content and lower cost a ton gives it an added advantage. The figures from Ohio,! cover- ing a period of fourteen years in a rotation of maize, wheat, and hay may be taken as evidence regarding these points. The manure, reinforced to the ton with 40 pounds of acid phosphate and raw rock phosphate, respectively, was applied to the corn at the rate of eight tons to the acre. TABLE XCVI A COMPARISON OF ACID PHOSPHATE AND RAW ROCK IN EQUAL WEIGHTS WHEN ADDED TO THE SOIL WITH MANURE. AVERAGE ANNUAL INCREASE TO THE ACRE MANURE MAIZE WHEAT Hay 14 Crops 14 Crops 11 Crops With raw rock............ 25.0 bu. | 12.9 bu. | 1578 Ibs. With acid phosphate....... 30.6 bu. | 15.1 bu. | 1853 Ibs. FERTILIZERS USED FOR THEIR POTASSIUM The production of potassium fertilizers is largely confined to Germany, where there are extensive beds varying from 50 to 150 feet in thickness, lying under an area extending from the Harz Mountains to the Elbe River and known as the Stassfurt deposits. Large deposits of crude potash salts occur in other sections of Germany, and also in France. While small deposits occur in other parts of the world the French and German mines are at present the only ones of any great commercial importance. The World War stimu- lated considerable investigation regarding possible sources of *Thorne, C. E., et al., Plans and Summary Tables of the Experiments at the Central Farm; Ohio. Agr. Exp. Sta., Cire. 120, p. 112, 1912. COMMERCIAL FERTILIZER MATERIALS = 463 potash, especially in the United States. Kelp, saline brines, deposits in old lake beds, and flue dust yielded considerable potassium. Most of these sources, however, are too expensive to compete with European potash in normal times. 260. Stassfurt salts and their refined equivalents.—The Stassfurt salts contain their potassium either as a chloride or as a Sulfate. The chloride has the advantage of being more diffusible in the soil, but in most respects the sulfate is pref- erable. Potassium chloride in large applications has an in- jurious effect on certain crops, among which are tobacco, sugar-beets, and potatoes. On cereals, legumes, and grasses the muriate appears to have no injurious effect. Kainit is the most common of the crude products of the Stassfurt mines and is imported into this country in large amounts. It is generally a greyish vari-colored salt, soluble in water and alkaline to litmus. It carries from 12 to 14 per cent. of K,O, largely as potassium sulfate. Its potash is immediately available to the crop. Below is a typical analysis : ESO Ta Sacer as ak 21.3 Wallis esac et 34.6 LC OU SS ee ae ear 2.0 CasOs Wertoe ver sen sleg/ VTi Svea epee ciao aleasete 14.5 Inisoluibler sean ceotels ee 8 INIA CS At ee ee 12.4 PASO Arter neuen iciersivas 12.7 Silvinit contains its potassium both as a chloride and as a sulfate. It also contains sodium and magnesium chlorides. Potash constitutes about 16 per cent. of the material. Owing to the presence of chlorides, it has the same effect on plants as has kainit. There are a number of other Stassfurt salts, consisting of mixtures of potassium, sodium, and magnesium in the form of chlorides and sulfates. They are not so widely used for fertilizers as are those mentioned above. A great proportion of the crude salts are refined for ex- port purposes, appearing on the market as either the chloride or the sulfate. They usually contain from 48 to 50 per cent. 464 NATURE AND PROPERTIES OF SOILS of potash. The chief impurity is common salt. Some of the potash salts produced in this country carry boron, which is extremely toxic to plants. Such is not generally true of the German and French products. Potassium chloride and potassium sulfate when added to the soil are immediately soluble, being held in the soil solu- tion or absorbed either physically or chemically by the eol- loidal complexes. Due to the selective absorption of the soil for the potassium ion and the fact that plants absorb more of this ion than of the acid radical, an acid residue tends to re- sult from the use of such fertilizers. Some means, such as the use of lime, should be employed to counteract this tendency. 261. Other sources of potash..—For some time after the use of fertilizers became an important farm practice, wood- ashes were the source of most of the potash. They also con- tain a considerable quantity of lime and a small amount of phosphorus. The product known as unleached wood-ashes contains from 5 to 6 per cent. of potash, 2 per cent. of phos- phorie acid, and 30 per cent. of caleium oxide. Leached wood- ashes contain about 1 per cent. of potash, 114 per cent. of phos- phorie acid, and from 28 to 29 per cent. of lime in the form of the hydroxide and carbonate. Unleached ashes carry the oxide, hydroxide, and carbonate forms of calcium. Ashes contain the potassium in the form of a carbonate, (K,CO,), which is alkaline in its reaction and in large amounts may be injurious to seeds. Otherwise this form of potash is very de- sirable, since no acid residue is left in the soil by its use. *Young, G. J., Potash Salts and Other Salines in the Great Basin; U. 8S. Dept. Agr., Bul. 61, 1914. Waggaman, W. H., and Cullen, J. A., The Recovery of Potash from Alunite; U. 8. Dept. Agr., Bul. 415, 1916. Hirst, C. T., and Carter, E. G., Some Sources of Potassium; Utah Agr, Exp. Sta., Cire. 22, 1916. Waggaman, W. H., The Production and Fertilizer Value of Citric- poe Phosphoric Acid and Potash; U. S. Dept. Agr., Bul. 143, Ross, W. H., et al., The Recovery of Potash as a By-Product in the Cement Industry; U.S. Dept. Agr., Bul. 572, 1917. COMMERCIAL FERTILIZER MATERIALS = 465 Ashes are beneficial to acid soils through the action of both the potassium and calcium salts. Insoluble forms of potassium, existing in many rocks usually in the form of a silicate, are not regarded as having any manurial value. Experiments with finely ground feld- spar have been conducted by a number of investigators, but have, in the main, offered little encouragement for the suc- cessful use of this material. Leucite and alunite have given but little better results. An insoluble form of potassium is not recognized as of value when a fertilizer is rated on the basis of chemical analysis. During the World War, since the German importation of potash salts ceased, potassium was sought commercially from a number of sources in this country. Alunite, a hydrous sul- fate of aluminum and potassium, has been experimented with to some extent as have also the green-sand marls which carry glauconite. In a number of cases the recovery of potash from fiue dust has proven commercially profitable. It is esti- mated that 87,000 tons of potash are lost yearly from cement kilns alone in the United States and Canada. During the war considerable progress was made in harvesting and drying the kelp which grows off the coast of southern California. The kelp was later extracted for its potash. This source of potas- sium is rather expensive, however, when brought into com- petition with European products. Perhaps the most reliable sources of domestic potash are the brines of certain alkali lakes of western United States and from the deposits in old lake beds in the same region.1. The exploitation of such sources will, of course, depend upon the price at which German potash can be laid down in this country. *Such salts unless properly prepared are likely to contain borax which is usually toxic when applied at a greater rate than five pounds to the acre, the influence being more intense at low soil moisture. Neller, J. R., and Morse, W. J., Effects upon the Growth of Potatoes, Corn and Beans, Resulting from the Addition of Borax to the Fertilizer used; Soil Sci., Vol. XII, No. 2, pp. 79-105, 1921. 466 NATURE AND PROPERTIES OF SOILS SULFUR AND SULFATES AS FERTILIZERS * The use of these substances as a means of increasing plant growth when applied to soils has recently received much at- tention. While sulfates have been used for centuries as a soil amendment, it is only within the last few years that sulfur itself has been applied to soil. The question of the effect of the latter has received considerable study, not only in France and Germany but in this country as well. The influence of both sulfur and sulfates may be a direct nutrient relationship or the action may be that of a soil amendment. Only in case the former influence occurs could these materials be rated as fertilizers. 262. The use of free sulfur.—Boullanger ” in 1912 added 1 Another group of fertilizers may be mentioned—the so-called catalytic fertilizers. Such materials are supposed to aid plant growth by accelerat- ing natural soil processes. The catalytic action of any material is very difficult to establish when it is added to the soil, since the soil itself carries many substances of a catalytic nature. Manganese has been most seriously considered as a catalytic fertilizer. Konig, J., Hasenbaumer, J., and Coppenrath, E., Einige Neue Eigen- schaften des Ackerbodens; Landw. Vers. Stat., Band 63, Seite 471-478, 1905-1906. May, D. W., and Gile, P. L., The Catalase of Soils; Porto Rico Agr. Exp. Sta., Cire. 9, 1909. Sullivan, M. X., and Reid, F. R., Studies in Soil Catalysis; U. S. Dept. Agr., Bur. Soils, Bul. 86, 1912. Konig, J., Hasenbaumer, J., and Coppenrath, E., Beziehungen zwischen den HKigenschaften des Bodens und der Nahrstoffaufnahme durch die pflanzen; Landw. Vers. Stat., Band 66, Seite 401-461, 1907. Kelly, M. P., The Influence of Manganese on the Growth of Pine- apples; Jour. Ind. and Eng. Chem., Vol. I, p. 533, 1909. Sullivan, M. X., and Robinson, W. O., Manganese as a Fertilizer; U. S. Dept. Agr., Bur. Soils, Cire. 75, 1912. Skinner, J. J., and Sullivan, M. X., The Action of Manganese in Soils; U.S. Dept. Agr., Bul. 42, 1914. Skinner, J. J., and Reid, F. R., The Action of Manganese Under Acid and Neutral Soil Conditions; U.S. Dept. Agr., Bul. 441, 1916. Bertrand, G., The Action of Chemical Infinitesimals in Agriculture ; Address before 8th Inter. Cong. App. Chem., New York, 1912. Ross, W. H., The Use of Radioactive Substances as Fertilizers; U.S. Dept. Agr., Bul. 149, 1914. Hopkins, C. G., and Sachs, W. H., Radiwm as a Fertilizer; Ill. Agr. Exp. Sta., Bul. 177, 1915. ?Boullanger, E., Action du soufre en fleur sur la végétation; Compt. Rend. Acad. Sci. Paris, T. 154, pp. 369-370, 1912. COMMERCIAL FERTILIZER MATERIALS = 467 flowers of sulfur to a soil at the rate of 23 parts per million of soil. He obtained increased growth in all treated soils on which carrots, beans, celery, lettuce, sorrel, chicory, potatoes, onions, and spinach were grown, the weights of the crops on the treated soil being from 10 to 40 per cent. greater than those on the untreated soil. On soils that had been sterilized before applying sulfur, the effect was less marked, from which he concludes that the beneficial effects were due to the influence of the sulfur on the micro-organisms of the soil. There may be some question, however, whether this conclusion is justi- fiable. Sulfur was found by Boullanger and Dugardin' to favor ammonification in soils. Beneficial effects from the use of free sulfur have also been obtained by Demelon,? and by Bernhard,*? while von Feilitzen * found it to be ineffective as a fertilizer. In this country, Shedd ® of Kentucky obtained increases in tobacco yield with sulfur. Perhaps the most marked results with sulfur are reported by Reimer and Tartar ° from Oregon. Alfalfa and clover yields were increased from 50 to 100 per cent. That free sulfur may, under certain conditions, exert a ben- eficial influence on plant growth must be conceded, but that the action is a direct nutritive one remains to be proven. Free sulfur is insoluble and cannot be absorbed as such by plants. It readily undergoes oxidation, however, producing the sulfate, as already explained under sulfofication. As such *Boullanger, E., and Dugardin, M., Mecanisme de l’action fertilisante du soufre; Compt. Rend. Acad. Sci. Paris, T. 155, pp. 327-329, 1912. ?Demelon, A., Sur l’action fertilisante du soufre; Compt. Rend. Acad. Sci. Paris, T. 154, pp. 524-526, 1912. * Bernhard, A., Versuche iiber dis Wirkung des Schwefels als Dung im Jahre 1911; Deutsche Landw. Presse., Band 39, S. 275, 1912. *von Feilitzen, H., Uber die Verwendung der Schwefelblute zur Be- kampfung des Kartoffelschorfes und als indirktes Dungemittel; Fuhling’s Landw. Zeit., Band 62, Seite 7, 1913. * Shedd, O. M., The Relation of Sulfur to Soil Fertility; Ky. Agr. Exp. Sta., Bul. 188, 1914. *Reimer, F. C., and Tartar, H. V., Sulfur as a Fertilizer for Alfalfa in Southern Oregon; Ore. Agr. Exp. Sta., Bul. 163, 1919. 468 NATURE AND PROPERTIES OF SOILS a reaction tends to encourage soil acidity, injurious influ- ences may easily occur on soils already acid or possessing only small quantities of active calcium and magnesium. If sulfur functions as a fertilizer, it is by a change to the sulfate, in which form it is absorbed by plants. 263. The use of sulfate sulfur——The experimental evi- dence regarding the direct fertilizer influence of sulfate sulfur is much more difficult to interpret than that regarding flowers of sulfur. Gypsum has been applied to soils for centuries and marked influences on crop growth are of common observa- tion. Whether this stimulation is due to the sulfate or to the base which accompanies it cannot be determined. Even if the sulfate influence could definitely be proved, there would still remain the question as to whether the action was direct or indirect. 264. Relation of sulfur to soil fertility—The possible deficiency of sulfur in arable soils was first established by Hart and Peterson.t They point out that crops remove more TABLE XCVII POUNDS SULFUR TRIOXIDE AND PHOSPHORUS PENTOXIDE REMOVED TO THE ACRE BY AVERAGE CROPS. POUNDS TO THE ACRE CROP AND YIELD TO THE ACRE SO, ZO: Wihedts(3 Oebiis) Mu bl inp tey alestttiavastenerara: Bye Pal a | ‘Barley (40vbu))\o secures ae oe ee 14.3 20.7 Carts 94D MOU) ar cet ere eae at rauere cient aes 197 Bat Corns (BO wu) iniesas Be Ae esate 12.0 18.0 Altalta(S000Tbs air diny)t &-eeatws ae 64.8 39.9 Turnips (4657 Vbsvamdiy ). 22) cre oe: 92.2 33.1 Cabbage (4800 lbs. air dry)........... 98.0 61.0 Potatoes (@GabOilbssairdiny )ieacer. en: 11.5 21.5 Meadow hay (2822 lbs. airdry)...... i113 PAS * Hart, E. B., and Peterson, W. H., Sulfur Requirements of Farm Crops in Relation to the Soil and Air Supply; Wis. Agr. Exp. Sta., Res. Bul. ich ale COMMERCIAL FERTILIZER MATERIALS — 469 sulfur from the soil than is indicated by the earlier analyses of plant ash, since considerable sulfur was lost by volatization in the former determination. On the basis of their own methods, they present the data given as to the removal of sulfur trioxide and phosphoric acid from the soil by average crops. (See Table XCVII, page 468.) It is to be noted that the amount of sulfur removed by crops is generally about equal to and in some cases much in excess of the phosphorie acid taken from the soil. The fact that soils are generally as low in sulfur as in phosphoric acid lends weight to the argument, that if the latter is a limiting factor in productivity the former should be also. To ascertain whether the supply of sulfur in the soil is really depleted by cropping, Hart and Peterson made parallel determinations of sulfur in five virgin soils and in five soils of the same respective types that had been cropped for sixty years. In each type the cropped soil contained less sulfur than the virgin soil, the average for the former being .053 per cent. SO, and for the latter .085 per cent. SO,. Considerable sulfur is added to the soil every year in the rain-water, largely in the sulfate form, although near cities appreciable amounts of hydrogen sulfide and sulfur di-oxide are formed. The amount of such sulfur is variable. Miller,! at the Rothamsted Experiment Station, reports 17.4 pounds of SO, to the acre, while Crowther and Ruston? near Leeds, England, found 161 pounds of SO, to the acre. Peck * found the addition of SO, to be at the rate of 1 pound to the acre a month at Mt. Vernon, Iowa, while Trieschmann,* over a * Miller, N. H. J.. The Amount of Nitrogen, as Ammonia and as Nitric Acid, and of Chlorine in the Rain-Water Collected at Rotham- sted; Jour. Agr. Sci., Vol. I, pp. 280-303, 1905. * Crowther, C., and Ruston, A. C., The Nature, Distribution and (Effact Upon Vegetation of Atmospheric Impurities In and Near an Industrial Town; Jour. Agr. Sci., Vol. 4, pp. 25-55, 1911. * Peck, E. L., Nitrogen, Chlorine and Sulfates in Rain and Snow; Chem. News., Vol. 116, p. 283, 1917. *Trieschmann, J. E., Nitrogen and other Compounds in Rain and Snow; Chem, News, Vol. 119, p. 49, 1919. 470 NATURE AND PROPERTIES OF SOILS different period at the same place, determined the addition to be less than .2 pound a month. Stewart,! at the University of Illinois, reports the addition of sulfur as SO, over a period of seven years aS amounting to 9.4 pounds of SO, monthly to the acre or 113 pounds yearly. The loss of sulfur expressed as SO, from the Cornell lysi- meters,” due to cropping and drainage combined, amounted, over a period of ten years, to 149.5 pounds from an acre yearly from the rotation tanks. The addition of sulfur in the rain-water at Ithaca amounts to about 65.4 pounds of SO, each year. It is, therefore, safe to assume that rain-water will not replace the sulfur removed by normal cropping and leaching. It must be remembered, however, that in rational soil management, sulfur is returned to the soil in green- manures, crop residues and farm manures. Commercial fer- tilizers are now very commonly used, especially acid phos- phate, which is about one-half gypsum. At the Ohio Experi- ment Station,® plats treated with sulfate bearing fertilizers were found over a period of years to contain considerably more sulfur than soils not so fertilized but cropped in a similar manner. In the light of such data it seems that the sulfur problem is not comparable with or as serious as the phosphorus prob- lem of soil fertility. By the careful utilization of the normal residues produced on the farm there seems little reason for sulfur being a limiting factor in soil productivity, especially if fertilizers carrying sulfur are used in connection with a rational system of soil management. *Stewart, R., Sulfur in Relation to Soil Fertility; Ill. Agr. Exp. Sta., Bul. 227, 1920. Wee ias aag data on these lysimeters will be found in par. 163 of this > Ames, J. W., and Boltz, G. E., Sulfur in Relation to Soils and Crops; Ohio Agr. Exp. Sta., Bul. 292, 1916. CHAPTER XXIII THE PRINCIPLES OF FERTILIZER PRACTICE ? THE USE of commercial fertilizers has increased so rapidly within the last decade that specific knowledge is needed re- garding the various materials offered for sale in order that the most economical results may be attained. The greater the general knowledge, both practical and theoretical, that a person possesses as to the effects of the different nutrient con- stituents on plant growth, the more rational will be the fer- tilizer use. Fertilizer inspection and control, principles of buying and home-mixing, methods of application, mixtures for special crops, are a few of the many phases of economical fertilizer practice. The final and vital consideration is re- garding the financial return from fertilizer application. A fertilizer should always pay. As all fertilizers exert, either directly or indirectly, a resid- ual effect, the problem necessarily broadens into a study of the systems of applying them to a series of crops or to a rota- tion, rather than a study of the effects of one particular fer- tilizer application on one particular crop. 265. Influence of nitrogen on plant growth.2—Of the three elements carried in an ordinary complete fertilizer, *Hall, A. D., Fertilizers and Manures; New York, 1921. Halligan, J. E., Soil Fertility and Fertilizers; Easton, Pa., 1912. Van Slyke, L. L., Fertilizers and Crops; New York, 1912. Fraps, G. S., Principles of Agricultural Chemistry ; Easton, Pa., 1913. ? Discussions of the effects of the various elements on plants may be found as follows: Russell, E. J., Soil Conditions and Plant Growth, Chapter II, pp. 19-50; London, 1912. Also, Hall, A. D., Fertilizers and Manures, Chapters III, IV and VI; New York, 1921. 471 472 NATURE AND PROPERTIES OF SOILS nitrogen! seems to have the quickest and most pronounced effect, not only when present in excess of other constituents, but also when moderately used. It tends primarily to encour- age above ground vegetative growth and to impart to the leaves a deep green color, a lack of which is usually due to insufficient nitrogen. It tends in cereals to increase the plumpness of the grain, and with all plants it is a regulator in that it governs to a certain extent the utilization of potash and phosphoric acid. Its application tends to produce succu- lence, a quality particularly desirable in certain crops. In its general effects it is very similar to moisture, especially when supplied in excessive quantities. The peculiarity of nitrogen lies not only in its absolute ne- cessity for plant growth, its stimulation of the vegetative parts, and its close relationship to the general tone and vigor of the crop, but also in the fact that it was not one of the original elements of the earth’s crust. During the formation of the soil it slowly and gradually became present, brought down by rains and fixed naturally in the soil through the agency of bacterial action. Now it exists in complex nitrog- enous compounds of the more or less decayed organic matter, and becomes available to plants largely through bacterial activity. It may be stated with certainty that one of the possible limiting factors to crop growth is a lack of water-soluble nitro- gen at critical periods in amounts necessary for normal devel- opment. Since soluble nitrogen may be very readily lost from the soil by leaching, the problem of proper plant nutri- tion becomes a serious one. Not only must the farmer be able so to regulate the addition of nitrogen in fertilizers as to obtain the highest efficiency, but he must understand the control and 1 For a discussion of nitrogen in relation to crop yield, see Hunt, T. F., .The Importance of Nitrogen in the Growth of Plants; Cornell Agr. Exp. Sta., Bul. 247, 1907. THE PRINCIPLES OF FERTILIZER PRACTICE 473 encouragement of the natural fixation as well. Due to the practical possibility of keeping up the nitrogen supply of the soil by the proper use of farm manure, crop residues, green- manures, and the utilization of legumes in the rotation, the quantity of nitrogen purchased in commercial fertilizers should be as small as possible if its use is to be profitable. When so purchased it should function more or less as a crop starter rather than as a source of any large amount of the plants’ supply of nutrient. The emphasis placed on all phases of the nitrogen problem serves to reveal its great importance in fertility practices. Because of the immediately visible effect from the applica- tion of soluble nitrogen, the average farmer is prone to ascribe too much importance to its influence in proper crop develop- ment. This attitude is unfortunate, since nitrogen is the highest priced constituent of ordinary. fertilizers and should usually be purchased to a less extent than potash and espe- cially than phosphoric acid. Moreover, of the three common fertilizer elements, it is the only one which, added in excess, will result in harmful after-effects on the crop. These pos- sible and important detrimental effects of nitrogen may be listed as follows: 1. Tt may delay maturity by encouraging vegetative erowth. This oftentimes endangers the crop to frost, or may cause trees to winter badly. 2. It may weaken the straw and cause lodging in grain. This is due to an extreme lengthening of the internodes, and as the head fills the stem is no longer able to support the in- creased weight. 3. It may lower quality. This is especially noticeable in certain grains and fruits, as barley and peaches. The ship- ping qualities of fruits and vegetables are also impaired. 4. It may decrease resistance to disease. This is probably due to a change in the physiological resistance within the 474 NATURE AND PROPERTIES OF SOILS plant, and also to a thinning of the cell-wall, allowing a more ready infection from without. While certain plants, as the grasses, lettuce, radishes, and the like, depend for their usefulness on plenty of nitrogen, it is generally better to limit the amount of nitrogen for the average crop so that growth may be normal. This results in a better utilization of the nitrogen and in a marked reduction of the fertilizer cost for a unit of crop growth. This is a vital factor in all fertilizer practice, and shows immediately whether nitrogen fertilization is or is not an economic success. 266. Influence of phosphorus on plant growth.—lIt is difficult to determine exactly the functions of phosphoric acid in the economy of even the simplest plants. Neither cell divi- sion nor the formation of fat and albumen go on to a suffi- cient extent without it. Starch may be produced when it is lacking, but will not change to sugar. As grain does not form without its presence, it very probably is concerned in the pro- duction of nucleoproteid materials. Its close relationship to cell division may account for its presence in seeds in compara- tively large amounts. Phosphoric acid hastens the maturity of the crop by its ripening influences. This effect is especially valuable in wet years and in cold climates where the season is short. The use of acid phosphate is being advocated in the Middle West, espe- cially for maize, as an insurance against frost-injury and a means of avoiding soft corn. Phosphorie acid also encourages root development, especially of lateral and fibrous rootlets. This renders it valuable in such soils as do not encourage root extension and to such crops as naturally have a restricted root development. Phosphoric acid is especially valuable for fall- sown crops, such as wheat. A sturdy root growth is developed which tends to prevent winter injury and prepares the plant for a rapid spring development. Phosphoric acid decreases the ratio of straw to grain in cereals. It also strengthens the straw, thus decreasing the THE PRINCIPLES OF FERTILIZER PRACTICE 475 tendency to lodge, which is likely to occur especially with oats if too much available nitrogen is present. In certain eases, phosphoric acid decidedly improves the quality of the erop. This has been recognized in the handling of pastures in England and France. The effect on vegetables is also marked. Phosphorus is also known to increase the resistance of some plants to disease, due possibly to a more normal cell development. In this respect phosphoric acid counteracts the influence of a heavy nitrogen ration. Excessive quantities of phosphoric acid ordinarily have no bad effect, as phosphorus does not stimulate any part unduly, nor does it lead to a development which is detrimental. The lack of phosphoric acid is not apparent in the color of the plants as in the case of nitrogen, and as a consequence phos- phoric acid starvation may occur without any suspicion there- of being entertained by the farmer. One of the most important phases to be noted from this comparison of the effects of nitrogen and phosphorus is the balancing powers of the latter on the unfavorable influences generated by the presence of an undue quantity of the former. The possible detrimental effects of too much nitrogen have already been noted. This relationship between the phosphorus and nitrogen in plant nutrition is very important in fertilizer practice, since normal fertilizer stimulation generally results in the most economical gains. 267. Effects of potassium on plant growth.—The pres- ence of plenty of available potash in the soil has much to do with the general tone and vigor of the plant. By increasing resistance to certain diseases it tends to counteract the ill effects of too much nitrogen, while in delaying maturity it works against the ripening influences of phosphoric acid. In a general way, it exerts a balancing effect on both nitrogen and phosphate fertilizer materials, and consequently is espe- cially important in a mixed fertilizer, if the potash of the soil is lacking or unavailable. 476 NATURE AND PROPERTIES OF SOILS Potash is essential to starch formation, either in photo- synthesis or in translocation, and is necessary in the develop- ment of chlorophyll. It is important to cereals in grain for- mation, giving plump heavy kernels. As with phosphorus, it may be present in large quantities in the soil and yet exert no harmful effect on the crop. While potassium and sodium are similar in a chemical way, sodium cannot take the place of potash in plant nutrition. Where there is an insufficiency of potash, however, sodium seems in some way, either directly or indirectly, to be useful.* 268. The element in the ‘‘minimum.’’—In connection with the obvious importance of utilizing, for any particular soil and crop, a fertilizer well balanced as to the three primary elements, two queries naturally arise. These are: (1) What are the proper proportions of nitrogen, phosphoric acid, and potash to apply under given conditions? (2) What would be the effect if any one of these should not be present in suffi- cient quantity as to make it equal in function to the others? The first query cannot be disposed of until the question of fertilizer mixtures has been considered. The second, how- ever, is not affected by so many factors, and is more clearly a question of the function of the elements concerned and is logically discussed at this point. Any element that exists in relatively small amounts as com- pared with the other important nutrient constituents natur- ally becomes the controlling factor in plant development. Any reduction or increase in this element will cause a corre- sponding reduction or increase in the crop yield. This ele- ment, then, is said to be ‘‘in the minimum.”’ In fertilizer practice, ideal conditions would exist if no constituent func- tioned as a decided minimum and the entire influence of each single element was fully utilized. In other words, the fertil- izer would be balanced as to its relationship to normal plant 1 Hartwell, B. L., and Damon, S. C., The Value of Sodiwm when Potassium is Insufficient; R. I. Agr. Exp. Sta., Bul. 177, 1919. THE PRINCIPLES OF FERTILIZER PRACTICE 477 growth. That such a condition is more or less ideal and is seldom realized is obvious, from the fact that the various fer- tilizer carriers undergo more or less radical changes after being applied to the soil. The composition of the soil itself is also a disturbing factor. Nevertheless, the nearer an ap- proach can be made to such conditions, the greater will be the economy in fertilizer practice. Numerous persons have investigated the question as to what effect an increase of an element in the minimum may have on erop yield, and various ideas have been advanced to explain the effect. The idea of a definite law governing the increase of plant growth according as the element in the minimum is increased, was first suggested by Liebig. Wagner? later stated definitely that up to a certain point the increase yield was proportional to the increase in the application. This, however, evidently cannot apply except over a very limited field, since it is a matter of common observation that increased crop yield becomes lower as the lacking element is continu- ously supplied. Mitscherlich ? has formulated a law which is a logarithmic, rather than a direct, function of the increase in the element occupying the position of the minimum. Mitscherlich’s law may be stated concisely as follows: the increased growth pro- duced by a unit increase of the element in the minimum is proportional to the decrement from the maximum. In other words, the increase is proportional to the difference between the actual yield and the possible yield at which the element ceases to be a limiting factor. Mitscherlich has proposed a definite formula for such a *Wagner, H., Bettrige zur Dungerlehre; Landw. Jahr., Band 12, Seite 691 ff., 1883. * Mitscherlich, E. A., Das Gesetz des Minimums und das Gesetz des Abnehmen den Bodenertrages; Landw. Jahr., Band 38, Seite 537-552, Hr Ein Beitrage zur Erforschung der Ausnutzung des im Minimum Vorhandenen NdGhrstoffes durch die Pflanze; Landw. Jahr., Band 39, Seite 133-156, 1910. 478 NATURE AND PROPERTIES OF SOILS growth curve. This formula has been questioned by several investigators,” who have shown that a number of conditions, such as light, heat, and moisture, tend to disturb the applica- tion of such a law. The fact that crop yield is the summation of so many varying factors seems to argue in favor of no hard and fast rule regarding the increased growth due to the added increments of an element in the minimum. It is enough, in the practical utilization of fertilizers, to remember that in order to obtain the best results from fertilizers a mixture should be used that is approximately balanced so far as the effects of the nutrients are concerned, the crop as well as the chemical constitution of the soil being considered. 269. Fertilizer brands.—In an attempt to meet the ae! mands for well-balanced fertilizers suited to various crops and soils, manufacturers have placed on the market a large num- ber of brands of materials containing usually at least two of the important nutrient elements, and nearly always the three; the former being designated as incomplete fertilizers, while the latter are spoken of as complete. These various brands usually have a significant name,* which frequently implies the usefulness of the material for some special crop growing on a particular soil. Oftener, however, the brand name bears no relation either to crop or soil. The name should always be ignored in fertilizer purchase, the availability and composi- tion being the important considerations. bs = (a—y)k. Integrating, log (a—y) = c—kx. y = total yield from any number of increments. x—amount of any particular fertilizer constituent utilized. a= maximum yield and is a constant. k—a constant depending on y and x, variables. 2Pfeiffer, Th., Blanck, E., and Flugel, M., Wasser und Licht als Vege- tationsfaktoren "und ihre "Beziehungen zum Gesetze vom Minimum ; Landw. Ver. Stat., Band 76. Seite 211-223, 1912. Also, Mazé, P., Recherches sue les Relations de la Plante avec les Elements Nutritifs der Sol; Compt. Rend., Tome 154, pp. 1711-1714, 1912. Potato and Corn Fertilizer, Golden Harvest, Ureka Corn Special, Blood and Bone, Harvest King, Soil Builder and ‘the like. THE PRINCIPLES OF FERTILIZER PRACTICE 479 A brand of fertilizer is usually made up of a number of materials containing the important nutrient ingredients. These materials, already described, are called carriers. The making-up of a commercial fertilizer consists in mixing the various carriers together so that the required percentages of ammonia, potash, and phosphoric acid are obtained, care being taken that no detrimental reaction shall occur and that a physical condition consistent with easy distribution shall be maintained. Brands of fertilizer put out by reputable com- panies carry a large proportion of their nutrients in a readily available form. A fertilizer made up principally of dried blood, tankage, acid phosphate, and kainit or muriate of pot- ash is a good example of the ordinary composition of ready mixed goods. The various brands on the market, besides being complete or incomplete, may be designated as high-grade or low-grade as to availability, or high-grade or low-grade as to amount of plant nutrients carried. In the fertilizer trade the terms generally refer to the latter condition. A low-grade fertilizer in the latter sense is always encumbered with a large amount of inert material, called filler, which adds to the cost of mix- ing, transportation and handling. A low-grade fertilizer is generally more expensive a unit of nutrient obtained than are higher grade goods, and consequently should be avoided. Fertilizer concerns have always found it more profitable to sell ready mixed fertilizers than to deal in the separate car- riers, such as dried blood, muriate of potash, and the like. Of late years, however, it has been possible to buy the separate materials. The conditions during the World War greatly encouraged the application directly to the soil of separate carriers, especially acid phosphate, since potash was almost unobtainable and nitrogen fertilizers were very high in price. The use of phosphoric acid alone is often much more eco- nomical and rational than the use of a complete mixture, since the nitrogen removed from the soil by normal cropping and 480 NATURE AND PROPERTIES OF SOILS drainage may be replaced in other and more practical ways. By maintaining the soil organic matter the natural supply of potash may in a loamy or clayey soil often be so influenced as to render a potash fertilizer unnecessary. At least there may be enough soil potash available so that the use of a com- mercial form will not be profitable. 270. Fertilizer inspection and control—From the fact that so many opportunities are open for fraud either as to availability or as to the actual quantities of ingredients pres- ent, laws have been necessary for controlling the sale of fer- tilizers. These laws apply not only to the ready mixed goods but to the separate carriers as well. Most states have such laws, the western laws generally being superior to those in force in eastern states, where the fertilizer sale is heavier. This is because the western regulations are more recent and the legislators have had the advantage of the experience gained where fertilizers have long been used. Such laws are a pro- tection not only to the public but to the honest fertilizer com- pany as well, since spurious goods are kept off the market. Certain provisions are more or less common to most fer- tilizer laws. In general, all fertilizers selling for a certain price or over must pay a state license fee or a tonnage tax and print the following data on the bag or on an authorized tag: 1. Number of net pounds of fertilizer to a package. 2. Name, brand, or trade-mark. 3. Name and address of manufacturer. 4. Chemical composition or guarantee. For the enforcement of such laws the states usually pro- vide adequate machinery. The inspection and analyses may be in the hands of the state department of agriculture, of the director of the state agricultural experiment station, of a state chemist, or under the control of any two of these. In any case, a corps of inspectors is provided, the members of which take samples of the fertilizers on the market throughout the state. These samples are analyzed in laboratories provided THE PRINCIPLES OF FERTILIZER PRACTICE 481 for the purpose, in order to ascertain whether the mixture is up to guarantee. The expense of the inspection and control of fertilizers is usually defrayed by the license fee or the ton- nage tax. If the fertilizer falls below the guarantee,—allowing, of course, for the variation permitted by law,—the manufacturer is subject to prosecution in the state courts. A more effective check on fraudulent guarantees, however, is found in pub- licity. The state law usually provides for the publication each year of the guaranteed and found analyses of all brands inspected. Not only has this proved effective in preventing fraud, but it is really a great advantage to the honest manu- facturer, as his guarantees receive an official sanction. The found analysis of most fertilizers is generally above the guarantee. 271. The fertilizer guarantee—Every fertilizing mate- rial, whether it is a single carrier or a complete ready-to-apply mixture, must carry a guarantee. The exact form is gener- ally determined by the state in which the fertilizer is offered for sale. The content of nitrogen is almost invariably ex- pressed in terms of ammonia (NH,), although the amount of total nitrogen is sometimes required in addition. The phos- phorus is quoted in terms of phosphoric acid (P,0,). In some cases, a bone-phosphate of lime (B. P. L. or Ca,(PO,)2) equivalent is included. The guarantee of a simple fertilizer material is easy to interpret, since the name of the material is printed on the bag or tag. When the amount of the nutrient element carried is noted, the availability and general value of the goods is immediately known. If the material is sodium nitrate at 18 per cent. ammonia, it is apparent that the fer- tilizer is high-grade and should give immediate and definite results when properly applied to a growing crop. The interpretation of a complete fertilizer analysis is not as easy, however, since the names of the carriers are seldom included in the guarantees. The simplest form of guarantee 482 NATURE AND PROPERTIES OF SOILS is a mere statement of the percentages of NH,, P,O, and K,0, as, for example, a 2—8—2.1 This, however, is too brief for a guaranteed analysis on goods exposed for sale, as it gives no idea whatsoever regarding the solubility of the materials. As might be expected, there is a wide range in the character of the guarantees required by the various states. For example, some states insist on the statement of the percentage of both nitrogen and ammonia, while others insist only on the percent- age of nitrogen. Some require the soluble, the reverted, and the total phosphoric acid, while others require only the soluble and the reverted. As to potash, in some cases the soluble must be stated, while in other cases the total must be given.? In general, a guarantee should show not only the amount of the various constituents but also their form or availability. The following outline analysis is excellent in this respect: Percentage of NH, as nitrate. Percentage of P,O,; soluble Percentage of NH, as ammonia. in water. Percentage of NH, total. Percentage of P,O, reverted. Percentage of K,O water soluble. Percentage of P,O, as Percentage of K,O as chloride. insoluble. 272. The buying of mixed goods.—The successful buying of mixed fertilizers on the retail market depends on two things: (1) the selection of a composition suitable to soil and crop with carriers of known value; and (2) the purchase of high-grade goods. The farmer who observes these points will at least have purchased successfully. Whether he obtains a 1In the South, the order is different. An 8-3-2 means 8 per cent. of P,O,, 3 per cent. of NH, and 2 per cent. of K,O. ? Below is the guarantee of a complete fertilizer: INDtTO GEM hase tte chin renee reece evolewetaekt teterspciere tokens 4.2% Higa! to vammoniaiere ceteris ieee rote clevonetentae 5.0 Soluble: SPsORecke acperee vate areterPevetiens ola ciaeiereerersel 4.0 Reverted: PeOen en cexccat oye ste iv ene ere sie el oreees 2.0 Available PsOy rats. tielaete nets ererer eee hotretecva cist 6.0 Tnsoluble | Ps@ es cca s ses ec cashes Coens on see euete tegen ees 1.0 Total PaO ee ed oe artes tesa orclen yee e eta encton toren te 7.0 THE PRINCIPLES OF FERTILIZER PRACTICE 483 profit from the use of the fertilizer depends on the interrela- tion of a number of factors more or less variable from season to season. The selection of a suitable fertilizer, as to carriers and com- position, entails, after the need of the crop and soil are de- cided, a careful study of the guarantee. Should the guarantee be such as that just cited, a large amount of information is at hand concerning the forms of the carriers and the availa- bility of the important constituents. This knowledge, prop- erly correlated with the probable needs of the crop and the soil, will determine whether a particular brand should be pur- chased or not. The real question here is not so much the actual quantities of the elements in a ton of the fertilizer, as it is their balance among themselves. The actual pounds of nitrogen, phosphoric acid, or potash applied to the acre can be governed by the rate at which the mixture is added. The purchase of high-grade goods is the second important point to be considered. Data collected from practically every State show that the higher the grade of the fertilizer, both as to availability and as to the percentage of the constituents earried, the greater is the amount of nutrients obtained for every dollar expended. Avoiding the abnormal war prices, the following data from Vermont! for 1909 seem representative : TABLE XCVIII Cost (IN CENTS) OF ONE PouUND OF| CENTS’ WoRTH oF NUTRIENTS MIXED FERTILIZER RECEIVED FOR NH, E207 K,O EvEerRY DOLLAR EXPENDED Iie (HEIs bob oooDe 32 7.6 8.5 50 Medium grade....... 26 6.3 7.0 60 15 Gife) 1 209246 Gis leo choi = 23 5.7 6.3 67 1 Hills, J. L., Jones, C. H., and Miner, H. L., Commercial Fertilizers ; Vt. Agr. Exp. Sta., Bul. 143, pp. 147-149, 1909. 484 NATURE AND PROPERTIES OF SOILS It is always true that the lower the grade of a fertilizer the higher is the proportional cost of placing the goods on the market. In other words, it costs just as much a ton to market a low-grade material as a high-grade one. This accounts for the fact that the nutrients are cheaper a pound in a high- gerade mixture, and that the value received for every dollar expended is greater. 273. The purchase of unmixed fertilizers——There has always been a tendency among fertilizer manufacturers to discourage the purchase by the farmer of the separate car- riers of fertilizer nutrients. When this was possible the fer- tilizer manufacturer was able absolutely to control the mar- ket. By selling only mixed goods the manufacturer could not only realize a profit on the ingredients themselves but a profit on the mixing in addition. In order to escape these costs many farmers have begun the practice of buying the separate carriers, thus avoiding the extra charges. In many eases, the mixing on the farm costs nothing, as it can be done in winter when the farm work is not pressing. Home-mixing has been greatly encouraged by post-war conditions. In 1920 _ from ten to twenty dollars a ton was often saved on a high- grade mixture by purchasing the carriers separately. In many instances the fertilizing materials purchased sepa- rately need not be mixed at all, thus effecting a considerable saving in time and labor. Acid phosphate is generally added separately, especially to fall wheat. Bone-meal, basic slag, and raw rock give excellent results when applhed with farm manure. Sodium nitrate and ammonium sulfate give good returns as a top dressing on meadows, pastures, and small cereals, especially if phosphates have been added at some other point in the rotation. When farm manure is available, the use of acid phosphate with lime and manure in a legume rotation is generally desirable. Even where little manure is available, the application of sodium nitrate or ammonium sulfate as a top dressing for meadows, with acid phosphate in THE PRINCIPLES OF FERTILIZER PRACTICE 485 its proper place, is feasible. The purchase of expensive ready- mixed fertilizers may thus be avoided without necessitating home-mixing. For vegetable crops, however, especially potatoes, a com- plete fertilizer is generally advisable. Home-mixing is in such cases necessary. Special soils often demand a complete mix- ture. Muck soils generally require both potash and phos- phorie acid, while sandy soils, especially if the organic matter is low, respond to a mixture carrying all three of the fer- tilizer elements. As might be expected, this practice of home-mixing has met with much opposition from manufacturers. In general, it is claimed that the factory goods are more finely ground than those mixed by the farmer, and consequently the ready-mixed goods are not only more uniform but also in better physical condition. Also, the manufacturer is able to treat certain materials with acids, and thus increase their availability. While these reasons are more or less valid, good results may be expected from a fertilizer even though it may not be quite uniform, as the soil tends to equalize this deficiency. More- over, by screening and by using a proper filler, a farmer can obtain a physical condition which will in no way interfere with the drilling of the material. While, obviously, one farm- er alone cannot afford to buy small lots direct from the whole-. sale dealer because of the high freight charges, this objection is being met by organizations of various kinds whereby the single carriers may be purchased in carload lots and shipped directly to the association. It is evident that by purchasing the separate carriers, a farmer is able to obtain pure high-grade material at a reason- able price. Even if the fertilizers are not home-mixed, an educational value enters. The farmer is forced to study the influence of the materials on his crops more closely and is thus placed in a position to make changes that will tend to a higher efficiency of the constituents. The chances are that he will 486 NATURE AND PROPERTIES OF SOILS advantageously alter his fertilizer practice as the rotation progresses and his soil changes in fertility. Such arguments do not always mean, however, that it pays to buy the separate materials. As a matter of fact, in many cases it does not pay, especially where only a small amount of fertilizer is needed and it is impossible to codperate with other farmers. As a general rule, fertilizers should be bought by the method that will give the greatest value for every dollar expended, providing, of course, that the proper material is purchased. Farmers can often avail themselves of the advan- tage of both systems by asking for bids from various manu- facturers on carload lots of mixed goods having a certain composition. The farmers in this case designate the carriers as well as the formula. All the advantages of machinery mix- ing may thus be gained. 274. How to mix fertilizers..—The first step in the buy- ing of the separate fertilizer carriers is to obtain quotations which should state the price a ton, the composition, and the freight rate. With this information, the most desirable car- riers are selected and the amount of each is calculated.? If *Certain materials should not be mixed, especially in large amounts. Thus lime, especially the oxide and hydroxide forms or fertilizers earry- ing lime in considerable amount, should not be mixed with ammonium sulfate and animal manures, since ammonia is likely to be freed. Such materials should be kept away from acid phosphate or the reversion of the latter will occur. Calcium carbonate in small amounts, however, is often mixed with fertilizers carrying acid phosphate. It is not wise to allow moist acid phosphate to lie in contact with sodium nitrate, as nitric acid may be liberated by free sulfuric acid. * Below are three satisfactory mixtures: 2-12-0 400 pounds of tankage. 100 pounds of sodium nitrate. 1500 pounds of acid phosphate (16%P.0;). 2-12-2 320 pounds of tankage. 100 pounds of ammonium sulfate. 1500 pounds of acid phosphate (16%P,0,). 80 pounds of potassium chloride. 4-10-4 150 pounds of sodium nitrate. 100 pounds of ammonium sulfate. THE PRINCIPLES OF FERTILIZER PRACTICE 487 the materials are to be applied separately, the rate to the acre and the number of acres must be known. If a mixture is to be made, the formula of this mixture must be decided on in addi- tion. The pounds of the various carriers necessary to produce a given amount of a certain mixture can now be calculated. All of this is a matter of good judgement and careful arith- metic.* With the separate carriers at hand, the mixing, if necessary, is quickly accomplished. All that is needed may be lsted as follows: (1) a tight floor, (2) a coarse sand screen, (3) a tamper or grinder, and (4) shovels, a rake, and like tools. Since the pounds of fertilizer are quoted on each bag, weigh- ing is unnecessary in making up a given amount of a mixture having a certain formula. Bags may be divided into half or quartered with sufficient accuracy. The bulkiest material is spread on the floor first and leveled uniformly by raking. The remaining ingredients are then spread in thin layers above the first, in the order of their bulk. Beginning at one side, the material is next shoveled over, care being taken that the shovel reaches the bottom of the pile each time. The pile is then again leveled, and the process is re- peated a sufficient number of times to insure thorough mixing. Sometimes a mixing machine may be used for this operation. For storage and general convenience, the fertilizer may be weighed into sacks of 100 to 150 pounds capacity and put in a 240 pounds of tankage. 100 pounds of dried blood. 1250 pounds of acid phosphate (16%P.0,). 160 pounds of muriate of potash. 1A 2-8-2 fertilizer is to be compounded from dried blood containing 12% NH;, acid phosphate carrying 14% P.O, and kainit containing 12% K,O. In one ton of the mixture there should be 40 pounds of NH,, 160 pounds of P ae and 40 pounds of K,O. == ooo lbs: of dried blood. iy ae 14= 1142 lbs. of acid phosphate. 40 — .12— 333 lbs. of kainit. 192 lbs. of filler. 2000 ibs. total. 488 NATURE AND PROPERTIES OF SOILS dry place until needed. Each sack should be labeled, especi- ally if different mixtures are made. A word of caution should be inserted here regarding the concentration of the mixture. Some farmers, in order to les- sen the work of mixing and application in the field, raise the percentage of the elements exceedingly high—a condition very likely to occur when high-grade materials are used. This sometimes is bad practice, in that it may interfere with ger- mination after the fertilizer is applied and may also injure the young plants. Also, it is likely to result in a poor physi- eal condition, which may clog the drill, and in uneven distribu- tion, which will bring about a lowered efficiency of the fertil- izer. The use of sufficient dry finely divided filler will obviate such dangers.’ 275. The choice of a fertilizer—Two primary considera- tions must be observed in the actual utilization of fertilizers. The first of these has to do with the composition of the fer- tilizer and its suitability to soil and to crop. A careful study should be made not only of the percentages of ammonia, phos- phorie acid, and potash but also the availability of these con- stituents. The second consideration in the rational use of fertilizing materials is in regard to the amounts to be applied. As much eare and good judgment are necessary in handling a single carrier as a complete ready-mixed material, especially if the rotation as a whole is considered. It is evident, due to many factors that cannot be controlled, that fertilizer formule for different crops on particular soils are difficult to determine. In fact, such data can never be more than merely suggestive. Further, the best quantity of a mixture to apply, even though it is perfectly balanced, is a figure that can only be approximated. Probably the largest percentage of the fertilizer waste that occurs annually can 1Sand, dry soil, saw dust, dry muck, and even ground limestone, if in small amounts, may be used as fillers. THE PRINCIPLES OF FERTILIZER PRACTICE 489 be charged to this factor. Many farmers make the mistake of applying too much fertilizer. Any information along such lines, however, can only be suggestive, rather than literal, it being understood that the general formule suitable to vari- ous crops, and the quantities ordinarily applied, are subject to wide variations. 276. Fertilizer formulae.'—In the popular mind, the nu- trition of a plant is considered as similar to and as easy as the proper feeding of an animal. With animals, the food is compounded with the correct balance of nutrients and if other conditions are favorable, normal results should be obtained. The nutrition of a plant is by no means as simple as the proper feeding of an animal. In the first place, the plant receives most of its nutrients from the soil and air and not from the fertilizer, since the latter usually merely supplements the nu- trients already present in the soil. Again, the food for the animal remains balanced as it is utilized. In the case of plants, the fertilizer nutrients undergo great changes on addition to the soil, the soil influencing the availability of the fertilizer as well as the fertilizer influencing the soil in a great number of different ways. Moreover, the question of fertilizer resi- dues, especially those of an acid nature, is always paramount when fertilizers are used over long periods. The proper for- mula for a given crop and a given soil under a probable series of weather conditions is thus more or less of a guess and will always remain so. *The following example of fertilizers similarly named but carrying strikingly different guarantees are taken from Bull. 206 of the Vt. Agr. Exp. Sta. Potatoes and Maize Potatoes and Tobacco 4-7-8 2- 6-7 4-8-4 2- 6-4 4-8-0 2-12-0 Vegetables Top Dressings 3- 7-10 7-6-5 4- 8- 4 7-6-2 5-10- 0 7-6-0 490 NATURE AND PROPERTIES OF SOILS In spite of the intangible nature of the question, certain gen- eral rules seem to govern the compounding and use of fertiliz- ers. In the first place, the ratio of the nutrients removed by the average crop bears no relation to the composition of the fertilizer usually added. This is to be expected because of the complex changes that the fertilizer undergoes in the soil and because the different nutrients influence the plant di- versely. TABLE XCIX RATIO OF THE | RATIO OF THE CONSTITUENTS | CONSTITUENTS CONSTITUENTS AS THEY OccuR|CARRIED BY THE IN THE AVERAGE AVERAGE CROP FERTILIZER VANUNIOMTA. 05.5 Secs soe eae ae 4 0-2 Phosphorie(acid: G27. eee nae be 2 16-8 Potashts 2743. Pe OE Oe 3 O-2 It is immediately noticeable that the ratios of the ammonia and potash in fertilizers are low. The ammonia ratio is low because of the ready response of plants to nitrogen and the ease with which this constituent is lost from the soil. The potash ratio is likewise small because potassium is a rather expensive constituent and it is generally better if possible to render available by suitable means that which is already in the soil than to buy it commercially. The phosphoric acid is high in comparison with the ammonia and potash because of its complex reversion in the soil and the tendency of much of it to remain unavailable for long periods due to the high absorptive power of the soil. The following data may now be presented. These for- mule are tentative and suggestive only, being a modification and curtailment of certain analyses standardized for the use of fertilizer manufacturers in the United States. THE PRINCIPLES OF FERTILIZER PRACTICE 491 TABLE C GROUP I: FODDER AND STAPLE CROPS. Wheat (fall) Maize Millet Oats Barley Beans (field) Rye (fall) Buckwheat Peas (field) Som WITHOUT WITH FarM MANURE FARM MANURE RNPIDEL Wy SOMME vey che 4 evo cia, aie mae 2-10-6 0-12-4 / or Acid pany SOUL eyes caress 0s eet 2-10-4 0-12-2 {| Phos. Clay SOM 0s o's ess tinr ons Sai Acid Phosphate TABLE CI GROUP Il: ‘TOP DRESSINGS. WHEAT, RYE TIMOTHY y ? AND OATS PASTURES* Soin Sees Sop FoR Hay = AND oe SPRING EGUMES BOS eee € Sandy sil, ..-.... 7-8-6 fee ee s|p0710-6), OF 8 WWoamy soil... <5. 7-8-3 7-8-0 0-12-4 Bae Clavey soil... 5.’ 7-8-0 7-8-0 0-12-2 Sine : * Note.—Sodium nitrate or ammonium sulfate may be used alone as a top-dressing on all of these crops except legumes. TaBLE CII GROUP III: VEGETABLES. 1. Extensively — Tomatoes, | 2. Intensively—Cabbage, let- sweet corn, beets, cab- tuce, celery, asparagus, bage, ete. ete. Sandynr SOW. cio. 0% +2 3-10-6 ANGy "SOU. ome: 4-10-6 hoamy sows. .5.6< 3-10-4 Loamy soil......... 4-10-4 Glee coil... a Clayey ‘soils isanes. 4-10-2 All root-crops should re- ceive at least 2 per cent. of K,0. The ammonia should be re- duced if farm manure is used. 492 NATURE AND PROPERTIES OF SOILS 3. Miscellaneous. Nandy ‘sell 223)... a. Early potatoes * a2 2. i Loamvy, soil: patie eee 5- Clayey soil.))..Gn eee 4 b. Late potatoes*........ e. General trucking * on sandy soils of Atlantic Seaboard ee 2) i0.)..0 caus ¢ ote eee eee 5-8-7 * Note.—Reduce ammonia if farm manure is used. In this table of suggested formule, it is noticeable that wherever manure is used, the ammonia is reduced or even eliminated. Ammonia is also unnecessary on leguminous crops. With vegetables, the ammonia is usually high. Top dressings for pastures, meadows, and cereals in the spring should always carry large quantities of readily available nitro- gen. In a mixed fertilizer, the phosphoric acid is generally high, for reasons already explained. Due to the absorptive power of a clay, the mixture applied to such a soil should generally carry more phosphorus than that added to a sandy soil. Pot- ash is usually lower in a fertilizer for clayey soils, due to the possibility of liberating potassium from the soil itself by good soil management. 277. Amounts of fertilizers to apply.—The agricultural value of a fertilizer is necessarily a variable quantity, since, in applying fertilizers, a material subject to change is placed in contact with two wide variables, the soil and the crop. Moreover, soil conditions are constantly changing, thus fore- ing a modification of the fertilizer applied to the same soil bearing the same erop at different times. The factors influ- encing the efficiency of a fertilizer application may be listed as follows: (1) seed, crop, and adaptation of crop, (2) weather conditions, (3) physical condition of the soil, including drain- THE PRINCIPLES OF FERTILIZER PRACTICE 493 age, (4) organic content of the soil, and (5) chemical constitu- tion of the soil and its reaction. Although the conditions affecting fertilizer efficiency have thus been so briefly disposed of, it is evident that they are of vital importance in the economical utilization of fertilizing materials. One point of broader scope stands out particularly in this connection—the necessity of putting a soil in any given climate in the best possible condition for plant growth. This means that drainage, lime, organic matter, and tillage, in the order named, must be raised to their highest perfection in order to realize the best results from fertilizers. Such considerations indicate that the decision as to the amount of a single carrier or of a mixed fertilizer that should be applied will be difficult and probably more indefinite than formula selection. In fact, the amount of a fertilizer applied to the acre is more vital than the actual chemical composition, as far as money returns are concerned. With all the groups considered above, except garden and root-crops, the applications are generally relatively light, rang- ing from 150 to 350 pounds to an acre. Where excessive vege- tative growth is required, as in silage, the rate may be in- creased to 500 pounds. In the top dressings of meadows or grains, the rate varies from 100 to 200 pounds an acre. Very often this dressing is sodium nitrate or ammonium sulfate alone. With garden and root-crops, the amount of fertilizer applied is very large, ranging from 800 to sometimes as high as 2000 pounds. The cropping here is intensive, and the ex- penditure for fertilization may be large and yet yield substan- tial profits. 278. The law of diminishing returns.—It must always be remembered that in fertilizer practice the very high yields obtained under fertilizer stimulation are not always the ones that give the best returns on the money invested. In other words, the law of diminishing returns is a factor in the in- fluence of fertilization on crop yield. After a certain point 494 NATURE AND PROPERTIES OF SOILS is reached, the return for each added increment of fertilizer becomes less and less. It is evident, therefore, that with an excessive application of any mixture, the returns to an in- crement will at last become so small that the increased crop fails entirely to pay for even the fertilizer, not to mention Cy ~ @USHELS OF GRAIL Te) POUNDS oF “FLOATS “APPLIED "PER ACRE 2000 2400 Fic. 60.—In the upper diagram the heavy line indicates the increase in the yield of maize due to graduated applications of floats. The lower diagram shows how the cost of the fertilizer approaches and finally exceeds the value of the crop as the applications increase in size. such charges as cost of application, harvesting of increased crop, storage, and the like. The application of moderate amounts of fertilizer is to be urged for all soils until the maxi- mum paying quantity that may be applied to any given crop is ascertained by careful experimentation. Over-fertilization probably accounts for the fact that such a large proportion of THE PRINCIPLES OF FERTILIZER PRACTICE 495 the fertilizer sold to farmers each year not only is entirely wasted, but probably in some cases even becomes detrimental to crop yield. The law of diminishing returns may be illustrated by data from the Cornell University Agricultural Experiment?! Sta- tion. Floats were applied at different rates to plats receiv- ing a uniform dressing of farm manure at the rate of 15 tons to the acre. Table CIII shows the increased yields of maize due to the treatment with the rock phosphate. Pre- war prices were used in the calculations. (See Fig. 60.) TABLE CIII PouNDS OF FLOATS MAIZE MAIZE FLOATS TO THE ACRE (BUS.) (VALUE) (CcOsT) DIBSERENCE 20 ee 7.0 $4.62 $ .90 aes 72 ee tees ss 8.3 5.48 1.80 + 3.68 0S SAS eee 10.2 6.73 3.60 + 3.13 DANO REY hace OS: 12.7 8.38 10.80 — 2.42 279. Method and time of applying fertilizers — Although considerable emphasis has been placed on the selec- tion of the correct fertilizer formulae and on the adequate and economical amounts to use, the method of application must not be lost sight of. A fertilizer is never effective unless uni- formly distributed. It should also be placed in the soil in such a position that it will stimulate the plant to the best advantage. The distribution of the fertilizer by means of machinery is much more satisfactory than is broadcasting by hand, as the former method gives a more uniform distribution. Cereals and other crops are now usually planted with a drill or a planter provided with an attachment for dropping the fertilizer at the same time that the seed is sown, the fertilizer *Lyon, T. L., Soils and Fertilizers; p. 216; New York, 1917. 496 NATURE AND PROPERTIES OF SOILS being by this method placed under the surface of the soil. Broadcasting machines are also used, which leave the fer- tilizer uniformly distributed on the surface of the ground, permitting it to be harrowed in sufficiently before the seed is planted, thus preventing injury to the seed by the chemical activity of the fertilizing material. Corn-planters with fertilizer attachments deposit the fer- tilizer beneath the seed, thus avoiding a possible detrimental contact. Grain-drills do not do this, and, where the amount of fertilizer used exceeds 300 or 400 pounds an aere, it is better to apply it before seeding. Grass and other small seeds should be planted only after the fertilizer has been mixed with the soil for several days. For crops to which large quan- tities of fertilizers are to be added, especially potatoes and garden crops, it is desirable to drop only a portion of the fertilizer with the seed, the remainder having been broad- casted by machinery and harrowed in earlier. 280. Systems of fertilization—During the evolution of fertilizer practice since the middle of the nineteenth century, a number of systems of applying fertilizers have been advo- cated and in many eases actually followed. Perhaps the first plan to be suggested was the single element system. At that time, each crop was supposed to respond largely to one par- ticular element. Thus, nitrogen was supposed to dominate wheat, rye, and oats; phosphoric acid, to dominate maize, turnips, and sorghum; and potash to dominate potatoes, clover, and beans. Present knowledge of plant nutrition and the balancing effects of fertilizer nutrients show this idea to be fallacious. The supplying of abundant minerals as a fertilizer system had its origin from the fact that potash and phosphorie acid are relatively cheap and are rather slowly leached from the soil, while nitrogen is expensive and easily lost in this way. Such a plan, therefore, always provides plenty of potash and phosphorie acid, which are to be balanced each season with THE PRINCIPLES OF FERTILIZER PRACTICE 497 sufficient nitrogen to give paying yields. While this system is not feasible in its entirety at the present time, the prin- ciple involved is worthy of incorporation with more economi- eal plans. A system based on the amount of nutrients removed by crops has received from time to time considerable support. According to this plan, as much plant-food material is added each year as will probably be taken out by the plant, this being determined by chemical analyses of the crop. The system not only overlooks the fact that diverse plants feed differently on the same soil, but that the same crop exhibits marked variability with change of season and change of soil. Moreover, no allowance is made for losses by leaching, which are known to equal at times the losses due to plant absorption. In trucking or in general farming operations, one crop is often the money crop. Naturally its stimulation by heavy fertilization will pay better than applications to crops that bring less on the market. The general plan in this system is to allow the crops following the money erop to utilize the residuum. When this residual influence works out fa- vorably, the system is likely to be a profitable one; but when the following crops fail to respond, the method becomes wasteful in the extreme. 281. Rational fertilizer practice——In the selection of a system that will result in an effective utilization of fertilizers, only two of the plans described above need be considered. In any fertilizer, phosphoric acid and usually potash should always be present in amounts sufficient more than to balance the nitrogen, since the activity of nitrogen is so pronounced. Therefore, a scheme that calls for an abundance of minerals is a sound one. This, coupled with the heavy fertilization of the money crop, does not, however, constitute what might be considered a rational system, since the crops that follow may or may not be adequately supplied with nutrients. Not only must the soil, the crop and the fertilizer formula 498 NATURE AND PROPERTIES OF SOILS and amount receive careful study, but the rotation should be considered in addition. This is a fundamental principle not only with the application of commercial fertilizers but with liming and the use of farm manure as well. The care- ful fertilization of the rotation, with special reference to the money crop, is the only rational system that should ordi- narily be employed, since it not only cares for the crop on the land but also looks to those that are to follow. The atten- tion that must necessarily be paid to the fertility of the soil in such a system insures the establishment of a soil manage- ment which will result in an economical use of the plant nutrients, while at the same time the yields will be raised and a continuous productivity will be provided for. CHAPTER XXIV FARM MANURE * Or all the by-products of the farm, barnyard manure is probably the most important, since it affords a means where- by the unused portion of the crop may become a part of the soil. Its use not only makes possible a return to the land of a part of the nutrients previously removed by the crop but also permits an actual gain of carbohydrate ma- terials, the elements of which the plant obtains not from the soil but from air and water. This country has already entered an era in which the pre- vention of agricultural waste is becoming necessary and a nearer approach to a self-sustaining system of soil manage- ment more and more essential. For the maintenance of fertil- ity, a careful handling and a wise utilization of all the manure *The following publications will be valuable: Ames, J. W., and Gaither, E. W., Barnyard Manure; Ohio Agr. Exp. Sta., Bul. 246, June 1912. Hart, E. B., Getting the Most Profit from Farm Manure; Wis. Agr. Exp. Sta., Bul. 221, June 1912. Thorne, C. E., Farm Manures; New York, 1914. Beavers, J. C., Farm Manures; Purdue Uniy. Agr. Exp. Sta., Cire. 49, Mar. 1915. Burdick, R. T., Concerning Farm Manures; Vt. Agr. Exp. Sta., Bul. 206, June 1917. Fippin, E. O., Farm Manure; Cornell Reading Course for the Farm, Lesson 127, Aug. 1917. Weaver, F. P., Farm Manure; Pa. State Coll., Ext. Cire. No. 67, Oct. 1917. Brodie, D. A., Handling Barnyard Manure in Eastern Pennsylvania; U.S. Dept. Agr., Farmers’ Bul. 978, July, 1918. Wiancko, A. T., and Jones, 8. C., The Value of Manure on Indiana Soils; Purdue Univ. Agr. Exp. Sta., Bul. 222, Sept. 1918. Duley, F. L., Handling of Farm Manure; Mo. Agr. Exp, Sta., Bul. 166, Sept. 1919. 499 500 NATURE AND PROPERTIES OF SOILS produced on the farm are vital. Obviously an understanding is necessary regarding the character and composition of farm manure, its fermentative and putrefactive changes, its losses in handling and storage, and above all its rational use as an amendment and a fertilizer. This need appeals not only to the wide-awake farmer but to the technical man as well, since in the use of farm manures theory and practice widely over- lap. 282. Composition and general characteristics of farm manures.—The term farm manure may be employed in ref- erence to the refuse from all animals of the farm, although, as a general rule, the bulk of the ordinary manure which ultimately finds its way back to the land is produced by cattle and horses. This arises because these animals consume the greater part of the grain and roughage on the average farm, and because the methods of handling such live-stock make it easier and more practicable to conserve their excreta. Yard manure generally refers to mixed manures. The mixing usually occurs during storage, either for convenience in han- dling or for the purpose of checking losses and facilitating fermentation. Thus, horse and cow manures are commonly mixed, since the too rapid putrefaction and consequent loss of ammonia in the former is checked, while at the same time a more rapid and much more complete decomposition is en- couraged in the latter. Ordinary manure consists of two original components, the solid, or dung, and the urine in about the rate of three to one. As these constituents differ greatly, not only in com- position but also in physical properties, their proportions must appreciably affect the quality of the excreta and its agri- cultural value. Litter added for bedding or for absorptive purposes is almost always an important factor, for while it prevents losses of the soluble constituents, it may at the same time lower the value of the product for a unit amount. While compiiations of available data on the composition of FARM MANURE 501 farm manures demand liberal interpretations, they afford considerable light regarding the differences to be expected be- tween excrement from various animals. TABLE CIV THE COMPOSITION OF FRESH MANURE.? PERCENTAGE OF EXCREMENT ~ H,0 NH, EOF K,O Ae eee BUG ak eS. 75 .66 30 40 Horse Urine, DA ote ees 6 aaa 90 1.63 |Trace 1.25 Whole manure.... 78 84 BAS, 5 olide 10%). f oak ors 85 48 .20 10 Cow + Urine, 30%./..... 92 1.21 |Trace 1.35 Whole manure.... 86 12 15) 45 Sold eGo... fas 60 90 50 5 Sheep, Urine, 38%....... 85 1.63 05 2.10 Whole manure..... 68 1.14 BE 1.00 OlidemON Vo Hse ea: 80 .66 .50 40 Swine, Urine, 40%....... oF A8 10 45 Whole manure..... 87 .60 a0. 40 Since the horse does not ruminate its food, the manure is likely to be of an open character. It is also fairly dry, as is that from sheep, the urine in these two manures making up 20 and 33 per cent., respectively, of the whole product. The complete manure from these two animals contains 78 and 68 per cent., respectively, of water—a considerable contrast to the cattle and swine increments. Cattle and swine ma- nures, being very wet, are rather solid and compact. The air, therefore, is likely to be excluded to a large degree and de- composition is relatively slow. They are usually spoken of as cold inert manures as compared with the dry, open, rapidly heating excrements obtained from the horse and the sheep. *Van Slyke, L. L., Fertilizers and Crops, p. 291; New York, 1912. 502 NATURE AND PROPERTIES OF SOILS In every case except that of swine, the urine is much the richer than the dung in ammonia, containing on an average more than twice as much when compared on the percentage basis. The urine is also richer in potash than the solid, aver- aging for the four classes of animals 1.29 per cent. as com- pared to 0.34 per cent. contained in the solid manure. Most of the phosphoric acid, however, is contained in the solid ex- TOTAL TOTAL. TOTAL AULUIONA FHIOSPHOR!/C POTAS/T 0.6% AIC/D OF %. O25 55% 65% OUNG, VKINE. DUNG. URINE. DUNG, URINE. Fig. 61.—Diagram showing the distribution of ammonia, phosphoric acid and potash between the dung and urine of average farm manure. erement, only traces being found in the urine except in the ease of swine. It is, therefore, evident that the urine, pound for pound, is more valuable insofar as the nutrient elements are concerned. The advantage leans heavily toward the urine also in that the constituents therein contained are im- mediately available; this cannot be said of the solid manure. 283. Liquid versus solid manure.—While the urine car- ries more nutrients to an equal weight than the dung, it yet remains to be seen whether in the total excreta voided by an animal there are more nutrients in the urine than in the dung. FARM MANURE 503 In general, more solid manure is excreted than liquid, tend- ing to throw the advantage toward the former as a carrier of plant nutrients. The following table, adopted from Van Slyke,’ bears on this point: TABLE CV DISTRIBUTION OF NUTRIENT CONSTITUENTS BETWEEN THE LIQUID AND THE SOLID OF WHOLE MANURE. PERCENTAGE | PERCENTAGE | PERCENTAGE or TOTAL oF TOTAL oF TOTAL ANIMAL NH; PO; K,0 SOLID ‘LIQUID SOLID | LIQUID moun LIQUID EUGESE ist eng chads Gis uetet 62 | 38 | 100 0 56 | 44 OLONAR eee ie en 49 | 51 | 100 Oma tas 85 STIS) Dis enone, CRN ar ae 52 | 48 95 Dela 10 SN VALI ON Ben Re Gian ierc So. ee owen! 4e PNUERAUE a. 20 Wattal | ieee he 3 a7 | 43 95 a 1 40a" 60 Average for horse and cow | 55 | 45 ; 100 Ov} 35% 4 65 | It is seen here that a little more than one-half the am- monia, almost all the phosphoric acid, and about two-fifths of the potash, are found in the solid manure. Nevertheless, this apparent advantage of the solid manure is balanced by the ready availability of the constituents carried by the urine, giving it in total about an equal commercial and agricultural value with the solid excrement. Such figures are suggestive of the care that should be taken of the liquid manure. Its ready loss of ammonia by fermentation and putrefaction, and the ease with which all its valuable constituents may escape by leaching, should make it an object of especial regard in handling. (See Fig. 61.) 284. Poultry manure.—While poultry manure is often produced on the farm in large quantities, it is not included under the term farm manure, which, as generally used, refers *Van Slyke, L. L., Fertilizers and Crops, p. 295; New York, 1912. 504 NATURE AND PROPERTIES OF SOILS to the excrement of the larger animals. Its general composi- tion is as below, the data being averages from Thorne.* TABLE CVI COMPOSITION OF POULTRY MANURE. PERCENTAGE OF ‘ CONDITION H,O INE |e Or KO Whole manure, fresh.......... 57 1.31 40 00 Whole manure, air dry........ a 2.84 | .86 1.08 It is to be seen that poultry manure in the air-dry state, the condition in which it is applied, has over twice the amounts of nutrients carried by the other classes. It should be applied to the soil at at least one-half the rate commonly recommended for ordinary farm manure. Notwithstanding its ease in handling and its great value, poultry manure re- ceives less care and attention than any other produced on the farm. 285. Farm manure—a direct and indirect fertilizer.— Farm manure, when applied to the land, ordinarily fulfills two functions which are usually not so distinctly developed in one material—that of a direct and indirect fertilizer. Mixed farm manure ready to apply to the land contains on the aver- age .6 per cent. of ammonia, .25 per cent. of phosphoric acid and .5 per cent. potash.? It is obviously a low-grade fertilizer 1Thorne, C. E., Farm Manures, p. 90; New York, 1914. Also, Storer, F. H., Agriculture, Vol. I, p. 613; New York, 1910. Vorhees, E. B., Ground Bone and Miscellaneous Samples; N. J. Agr. Exp. Sta., Bul. 84, 1891. Goessman, C. A., Mass. Agr. Exp. Sta., Bul. 37, 1890, and Bul. 63, 1896. ?See Analyses, Storer, F. H., Agriculture, pp. 237-248; New York, 1910. Thorne, C. E., Farm Manures, pp. 89-93; New York, 1914. Aikman, C. M., Manure and Manuring, pp. 279-292; Edinburgh and London, 1910. ary I. P., The Fertility of the Land, pp. 159-182; New York, 1904, FARM MANURE 505 both as to the amounts of nutrients carried and as to their availability. Because of the large acre applications of ma- nure commonly made, the fertilizer constituents added in ma- nure are considerable. Ten tons of farm manure, even if only one-half its ammonia, one-sixth of its phosphoric acid and one- half of its potash were readily available, are equal in fertil- izing value to 833 pounds of sodium nitrate, 52 pounds of acid phosphate, and 416 pounds of kainit. This equiva- lent to the addition of 801 pounds of a readily available mix- ture of fertilizer salts. This calculation, however, ignores an equal quantity of nutrients which remain in the soil as a residuum and may be used by succeeding crops. This resi- dual effect of manure is generally a paying one during the period of an ordinary rotation. Farm manure acts as an indirect fertilizer in that it adds to the soil organic matter and thus improves the physical condition of the land. While it may not increase the organic matter of the soil, because of the loss of carbon by exhalation and leaching during the period of crop growth, its use materi- ally influences the rate of reduction. Better aération, drain- age and bacterial activity ' of necessity result from such an addition. The influence of manure on the availability of the mineral constituents of the soil is not the least of its indirect actions. The fact that rock phosphate when mixed with manure seems to have a higher availability bespeaks a considerable solvent activity. The tendency of farm manure to alleviate toxic conditions, such as alkali and acid- ity, deserves attention. 286. Outstanding characteristics of farm manure.—As farm manure is essentially a fertilizer, whether it is pro- duced on the farm or purchased outright, it is logical to con- trast it with the ready-mixed materials on the market. In *Conn, H. J., and Bright, J. W., Ammonification of Manure in Soil; Jour. Agr. Res., Vol. XVI, No. 12, pp. 313-350, March, 1919. Fulmer, H. L., and Fred, E. B., Nitrogen Assimulating Organisms in Manure; Jour Bact., Vol. II, No. 4, pp. 423-434, 1917. 506 NATURE AND PROPERTIES OF SOILS such a comparison, five characteristics are outstanding: (1) the moist condition of manure, (2) its low grade, (3) its unbalanced nutrient condition, (4) its variability, and (5) its rapid fermentative and putrefactive processes. These characteristics, neither present nor desirable in ordinary fer- tilizers, place farm manure in a class by itself as to its hand- ling, storage, and field utilization. Of the above points, the first three may be disposed of quickly. Average farm manure, whether fresh or well-rotted, contains from 70 to 85 per cent. water. Sweetser, W. S., The Manurial Value of the Excreta of Milch Cows; Pa. State Coll., Ann. Rep., 1899-1900, pp. 321-351. ® Hall, A. D., Fertilizers and Manures, p. 180; New York, 1921. ™Wood, T. B., Losses in Making and Storing Farm Yard Manure ; Jour. Agr. Sci., Vol. II, pp. 207-215, 1907-08. ®’Thorne, C. E., Maintenance of Fertility; Ohio Agr. Exp. Sta., Bul. 183, p. 202, 1907. ®See Hopkins, C. G., Soil Fertility and Permanent Agriculture, p. 206; Boston, 1910. Also, Fippin, E. O., Live Stock and the Maintenance of Organic FARM MANURE 517 10 and 50 per cent., respectively, for these constituents. While such losses are necessary and are usually compensated by the animal products, their magnitude must be considered in esti- mating the value of manure in the ordinary rotation. 294. Losses due to handling and storage.—| dis ~20% ee -40 » -65» PERCENTAGE OF THE CONS- TITUENTS OF CROP ADDED TO SOIL SOIL Stace aS) oe 2 Bue LEACHING Fig. 63.—Diagram showing the proportion of the harvested crop added to the soil in farm manure under average conditions. It is easy to see that a liberal dressing of manure on the hay and pasture will markedly increase the crop. Neverthe- less, aS manure is available in limited amounts on the average farm and as commercial fertilizers will give almost as good returns on hay, it is generally considered judicious, except in 1Hunt, T. F., General Fertilizer Experiments; Ann. Rep. Penn. hee Exp. Sta,, 1907- 1908, pp. 68-93. Thorne, C. E., and others, Plans and Summary Tables of the gee ments at the Central Farm; Ohio Agr. Exp. Sta., Cire. 120, np. 101- 105, 1912. * Hopkins, C. G., Thirty Years of Crop Rotation in Illinois; Ill. Agr. Exp. Sta., Bul. 125, p. 337, 1908. 534 NATURE AND PROPERTIES OF SOILS TABLE CX XII INFLUENCE OF MANURE ON MAIZE, OATS, AND CLOVER. AVERAGE PERCENTAGE RaTIo VALUE OF INCREASE INCREASE TREATMENT MAIzE AND : MAIZE AND CLOVER OATS CEOS OATS Manure alone.. Hak 92 100 134 Manure, lime and phosphate 30 141 162 206 certain cases, to reserve most of the manure for other crops. The top dressing of meadows is, however, always an allowable practice, especially on new seeding or on hay land that is soon to be plowed for maize. As a food producer, maize has no close rival. Where the climate is favorable, a 75-bushel crop of maize is as easily secured as 40 bushels of wheat or 300 bushels of potatoes to the acre. Moreover, the maize stover may be made more valu- able as roughage than the straw of oats, wheat, or rye. The maize plant must have, however, for its successful growth plenty of available nitrogen. In addition, its response to abundant organic matter indicates the utilization of certain organic compounds. These considerations argue for the use of most of the farm manure on the maize when this crop is important, especially if the supply of manure is limited. Again the maize crop is ready for the manure in the spring and is generally grown on land where the excreta may be distributed during the previous winter and fall. Potatoes are a spring crop and where they are prominent in the rotation may receive liberal applications of manure. If potatoes are the money crop, this should by all means be the practice. Oats, because of the tendency to lodge, gener- ally follow maize or potatoes as a residual feeder, receiving, if necessary, a dressing of commercial fertilizer. If manure is FARM MANURE 539 used on fall wheat, a great loss of manurial value is incurred, due to the necessity of storage during the summer months. Moreover, commercial fertilizers high in phosphorus are so convenient and effective on wheat that the use of manure on this crop is becoming rather uncommon, although manure may be used to advantage as a fall and winter dressing, since it not only stimulates the wheat but is of great value to the new seeding as well. Where cotton and tobacco are the staple erops, they should receive at least a part of the manure pro- duced. The value of manure in orchards should not be over- looked, especially on sandy soils. The up-keep of organic matter, the conservation of moisture, and the nutrients sup- plied are as important here as in any phase of soil manage- ment. 308. Resume.—Barnyard manure, from the standpoint of soil fertility, is the most valuable by-product of the farm. A careful farmer will, therefore, attempt to utilize it in the most economical way. The handling of manure in such a manner that only a minimum waste occurs from the time the manure is voided until it has reached the land is not an easy problem. Manure is so susceptible to the loss of valuable ingredients, both by leaching and by decay, that careful methods must be employed. Tight floors in the stable and covered sheds or manure pits are always advisable. Hauling immediately to the field is the wisest procedure, yet even with the best of care more than 50 per cent. of the fertilizing value is usually lost. The problem of rational manurial utilization is not solved, however, by careful handling and storage alone. Manure must be applied in such a condition, in such amounts and at such a point in the rotation as to realize a reasonable return for every increment applied. The reinforcement of farm manure with phosphoric acid is by no means an unim- portant feature. In fact, all of the principles which are ob- served in the profitable utilization of commercial fertilizers should be adhered to in the use of farm manures. 536 NATURE AND PROPERTIES OF SOILS A permanent system of agriculture evidently cannot be established by merely returning all the manure possible to the land, as approximately only 25 per cent. of the organic matter, 380 per cent. of the ammonia, 50 per cent. of the phos- phorie acid, and 30 per cent. of the potash of the food con- sumed on the farm ever reach the land in the manure. Never- theless, it is certainly worth the while of a farmer to use some eare in handling this product and some thought as to its rational utilization in the field. Even if the manure should aid only in the up-keep of organic matter, the effort would be worth while. Reasonable care in the handling of farm manure will save this country thousands of pounds of manurial fertility which are now utterly lost and at the same time increase by thousands of dollars the food production. CHAPTER XXV GREEN-MANURES From time immemorial the turning-under of a green-crop to supply organic matter to the soil has been a common agri- eultural practice. Records show that the use of beans, vetches, and lupines for such a purpose was well understood by the Romans, who probably borrowed the practice from nations of greater originality. The art was lost to a great extent dur- ing the Middle Ages, but was revived again as the modern era was approached. At the present time, green-manuring is considered a part of a well-established system of soil man- agement, and is given a place, when possible, in every ra- tional plan for permanent soil improvement. 309. Importance of green-manures.—The plowing under of some succulent rapid-growing crop, such as oats, rye, or clover, tends to bring about three desirable soil conditions; additional organic matter, a betterment of the physical con- dition of the soil, and a rise in the nitrogen content of the land, if the crop is an inoculated legume. If conditions are *Penny, C. L., Clover Crops as Green Manures; Del. Agr. Exp. Sta., Bul. 60, 1903. Storer, F. H., Agriculture, pp. 137-175; New York, 1910. Lipman, J. G., Bacteria in Relation to Country Life, Chapter XXIV, pp. 237-263; New York, 1911. Piper, C. V., Leguminous Crops for Green Manuring; U.S. Dept. Agr., Farmers’ Bul. 278, 1907. Spillman, W. J., Renovation of Worn-out Soils; U. S. Dept. Agr., Farmers’ Bul. 245, 1906. Pieters, A. J., Green Manuring: A Review of the American Experi- ment Station Literature; Jour. Amer. Soc. Agron., Vol. 9, No. 2, pp. 62-82, Feb. 1917; Vol. 9, No. 3, pp. 109-126, Mar. 1917; Vol. 9, No. 4 pp. 162-190, Apr. 1917. I 537 938 NATURE AND PROPERTIES OF SOILS favorable, an increase in crop production should result. Where there is a shortage of farm manure, the practice be- comes of special importance since roots and crop residues are usually insufficient to maintain the organic content of the soll. Even where manure is available, a green-manuring erop now and then in the rotation does much towards sus- taining normal production. The effects of turning under green plants are both direct and indirect—direct as to the influence on the succeeding crop, and indirect as to the soil so treated. In the first place, cer- tain ingredients are actually added to the soil by such a procedure. The carbon, oxygen, and hydrogen of plants come largely from the air and water, and the plowing-under of a crop, therefore, increases the store of such constituents in the soil. The compounds that result from crop decay increase the absorptive power of the soil, and promote aération, drain- age, and granulation—conditions that are extremely impor- tant in successful plant growth. If the crop turned under is a legume and the nodule organisms are active, the store of soil nitrogen is markedly augmented, a point of extreme impor- tance in fertilizer practice. Green-manures may function also as cover-crops, insofar as they take up the extremely soluble plant nutrients and pre- vent them from being lost in the drainage water. The nitrates of the soil are of particular importance in this regard as they are very soluble and are absorbed only shghtly by the soil complexes. Besides this, green-manures, especially those with long roots, tend to carry nutrients upward from the subsoil and when the crop is turned under this material is deposited within the root zone. Again, the added organic material acts as a food for soil organisms, and tends to stimulate biological changes to a marked degree. This biological action is espe- cially important in the production of carbon dioxide, am- monia, nitrates, and organic compounds of various kinds, which are necessary in plant nutrition. GREEN-MANURES 539 310. Gain of constituents by green-manuring.—In an average crop of green-manure, from five to ten tons of mate- rial are turned under. Of this, from one to two tons are dry matter, and from four to eight tons water. Of this dry matter, a great proportion is carbon, hydrogen, and oxygen. It might seem at first thought that such an addition is pure gain as far as carbon and carbonaceous matter are concerned. Such is not the case. Large amounts of carbon are lost continu- ously in drainage, to say nothing of that removed by crops or that which is respired by the soil as carbon dioxide. It has already been shown, from results obtained with the Cornell lysimeters, that a heavy soil will yearly lose over 250 pounds of carbon, in drainage alone (see par. 220). This is approxi- mately equivalent to a 2-ton application of green-manure. Although the loss of carbonaceous material is considerable, even during the period that the green-manuring crop is being grown, nevertheless the practice offers a rapid as well as a natural means of increasing the soil organic matter. The mineral parts of the turned-under crop came from the soil originally and they are merely turned back to it again and represent no gain. As they return, however, they are in intimate union with organic materials, and are thus readily available as the decay processes go on. Indeed they are prob- ably more readily available than they previously were, when the green-manuring crop acquired them. The amount of nitrogen added to a soil if the green-manure is a legume‘ is an uncertain quantity. Much depends on the virulence of the organisms occupying the nodules. These bac- *Smith, C. D., and Robinson, F. W., Influence of Nodules on the Roots upon the Composition of Soybean and Cowpea; Mich. Agr. Exp. Sta., Bul. 224, 1905. Hopkins, C. G., Alfalfa on Illinois Soil; Il. Agr. Exp. Sta., Bul. 76, 1902. Hopkins, C. G., Nitrogen Bacteria and Legumes; Tl. Agr. Exp. Sta., Bul. 94, 1904. Shutt, F. T., The Nitrogen Enrichment of Soils through the Growth of Legumes; Canadian Dept. Agr., Rept. Centr. Exp. Farms, 1905, pp. 127-132. 540 NATURE AND PROPERTIES OF SOILS teria are in turn much influenced by plant and soil conditions, such as amount of organic matter, presence of nitrates, acidity and the like. Hopkins' estimates that about one-third of the nitrogen in a normal innoculated legume comes from the soil and two-thirds from the air. He also considers that one-third of the nitrogen exists in the roots. Both of these assumptions are questionable and at best tentative. The amount of nitrogen fixed by legume organisms is extremely variable, probably more so than that assimilated by the azotobacter and allied groups. Again the percentage of the nitrogen held in the roots of legumes is by no means the same for all species. The amount varies within the species with age, degree of maturity and, season. The Delaware in- vestigations ° show that the proportion of the total nitrogen of the plant occurring in the roots may be as low as 6 per cent. in case of cowpeas and as high in the roots of alfalfa as 42 per cent. A range from 6 to 28 per cent. of the total nitrogen of crimson clover was noted in the roots under different condi- tions. According to Hopkins, the nitrogen found in the tops of legumes will be a rough measure of the nitrogen fixed by the nodule organisms. When the crop is turned under, this will represent the gain to the soil. If the preceding assumption is correct, red clover turned under would actually add about 50 pounds of nitrogen for every ton of air-dry substance util- ized, alfalfa about 50, cowpeas 48, and soybeans 53 pounds. These figures, even though they may be far from correct, at least give some idea of the possible addition of nitrogen by green-manuring practices, and show how the soil may be en- riched by such management. As in the case of farm manures, the indirect effects of such a procedure on the physical and bacteriological properties of the soil may over-ride the direct 1 Hopkins, C. G., Soil Fertility and Permanent Agriculture, p. 223; Boston, 1910. ?Penny, C. L., The Growth of Crimson Clover; Del. Agr. Exp. Sta., Bul. 67, 1905. GREEN-MANURES 541 influences, lessening the advantage that lezumes as green- manures are supposed to have over non-legumes, due to their ability to use atmospheric nitrogen. 311. Green-manures as cover-crops.—When green-ma- nures are seeded in the late summer or early fall, they func- tion as cover-crops and may have rather important influences aside from their effects when turned under. Their greatest influence seems to be on the nitrate content of the soil. Nitri- fication is usually checked, a disappearance of nitrates gen- erally following. This reduction in the amount of nitrates probably occurs because of a retardation of nitrification ac- companied by a stimulation of biological utilization of the nitrates. Such an effect is important in conserving the soil nitrogen and is of particular value in orchards,” as it hastens the maturity of the new growth. At Cornell University, green-manures were seeded in July and plowed under in the following spring. Nitrate determinations were made on the soil in July and in October. The figures are five-year aver- ages. (See Table CXXITI, page 542.) 312. The decay of green-manure.—When a green-crop is turned under, the process of its decay is the same as that of any plant tissue that becomes a part of the soil body. The organisms that are active are those common to the soil, to- evether with such bacteria as are carried into the soil on the turned-under crop. The decomposition that results is prob- ably both aérobiec and anaérobic in nature, carbon dioxide be- ing given off continuously. When proper decay has occurred, end products should result which can be utilized as nutrients. *Wright, R. C., The Influence of Certain Organic Materials upon the Transformation of Soil Nitrogen; Amer. Soe. Agron., Vol. 7, pp. 193- 208, 1915. Martin, T. L., The Decomposition of Green Manures at Different Stages of Growth; Thesis for degree of Doctor of Philosophy, Cornell University, 1919. *Lyon, T. L., The Formation of Nitrates in Soil Under Grass; Proce. West. N. Y. Hort. Soc., pp. 82-87, Jan., 1915. Lyon, T. L., Relation of Certain Cover Crops to the Formation of Nitrates in Soil; Proce West. N. Y. Hort. Soc., pp. 32-34, Jan., 1917. 4 i) NATURE AND PROPERTIES OF SOILS On TABLE CX XIII EFFECT OF VARIOUS CROPS ON THE NITRATE NITROGEN OF THE SOIL DURING OCTOBER, 1916-1920." PERCENTAGE RE- DUCTION OF NITRATES IN OcTOBER COMPARED NITRATES IN THE Sor. IN OCTOBER. 1 GREEN-MANURING CROP fecay IDM AS 100 WITH JULY Uys h e S ss rece ERE Te 100 37 Oatsins coo aces oe 73 44 Wretelia tcc. hariee: teen es 73 57 PCa ated acter aes ies 83 10 Raye ang. Vetch are eeger. 74 58 Rivevatid ease. « ooh teers ee: 75 58 Oye tein ence tet RRO yg hae 6 0 The intermediate compounds that are formed should yield an organic matter carrying a black pigment, should readily split up into simple compounds, and should be in general beneficial, both directly and indirectly, to plant growth. Plenty of moisture is essential when green-manures are de- caying, not only to hasten the transformation itself but that the normal soil processes may not be interrupted by a lack of water. The caution with which green-manures must be utilized in semi-arid regions arises because of the drying influ- ences of rapid decay and the danger of filling the soil with undecomposed plant residues. Even in humid regions, green- manures may be detrimental if dry weather sets in before a major portion of the decay processes is completed. As plant tissue decays in the soil, there seem to be two general groups of forces at work which produce three distinct stages of organic destruction.? In the first stage, humus pro- *Unpublished data. Dept. Soils. Cornell University. * Martin, T. L., The Decomposition of Green-Manures at Different Stages of Growth; Thesis for degree of Doctor of Philosophy, Cornell University, 1919. GREEN-MANURES 543 duction is dominant and the amount of the humous materials increases. In the second stage, humus production and humus destruction are more or less balanced, while in the third stage humus destruction is in the ascendant. The amount of humus is on the decrease in the latter stage. The length of these stages will vary with the season, with soil conditions,! and with the character of the crop turned under. Obviously, the influence of decomposing green-manure on the chemical and biological activities of the soil will vary as the decay cycle progresses. In general, over one-half of the organic matter of the average green-manure disappears during the first nine months after application. 313. Influence of decaying green-manure.—In the first stage of decay, which should be a rapid one, many complex compounds are generated along with carbon dioxide and other simple products. The complex materials, which result partly from protein decomposition and partly from the breaking down of easily attacked carbohydrates, may be harmful to ordinary crops. Germinating seeds and young plants are especially susceptible, and detrimental influences are some- times noticed immediately after the turning under of a green- manure. Fred? found that the germination of oily seeds, such as cotton and soybean, was much reduced. Starchy seeds, such as maize, oats, and wheat, were little affected. The germination of flax, hemp, mustard, and clover was some- what reduced. An actual contact of the seed with the de- caying material was usually necessary for serious damage. The detrimental influence always occurred during the first two or three weeks after the green-crop was turned under. Obviously the more succulent the crop, the shorter will this period be. + Russell, E. J.. and Appleyard, A., The Influence of Soil Conditions on the Decomposition of Organic Matter in the Soil; Jour. Agr. Sci., Vol. VIII, Part 3, pp. 385-417, 1917. ? Fred, E. B., Relation of Green Manure to the Failure of Certain Seedlings; Jour. Agr. Res., Vol. V, No. 25, pp. 1161-1176, Mar., 1916. 544 NATURE AND PROPERTIES OF SOILS Not only do the products of the first stage of decay influ- ence the crop growing on the soil, but they affect the biological activities as well.t Nitrification in particular seems to be in- fluenced, as nitrates do not begin to appear until the process of humification is well advanced. Nitrification, however, is probably not entirely suppressed as it is possible for soil or- ganisms to use up the nitrates as rapidly as they are formed. Zz w ro) 5% wtf vr Fel a [man n= Ee > Zz =n VU wy < Le J TIME AFTER APPLICATION Fic. 64.—Diagram illustrating the three stages in the decay of a green-manure. I, humus production dominant; II, a balance be- tween humus production and destruction; III, humus destruction dominant. A depression in nitrate accumulation generally occurs in stage I followed by an increase. (After Martin.) As the humus destruction gradually dominates over humus production, the end products of the decay become prominent. The complex proteid decomposition is practically completed and cellulose destruction is slowly progressing. Of the sim- ple nutritive products, the nitrates are of particular impor- tance. In fact, they have been chosen by a number of in- 1 Briscoe, C. F., and Harned, H. H., Bacterial Effects of Green Manures; Miss. Agr. Exp. Sta., Bul. 168, Jan. 1915. Hutchinson, H. B., The Influence of Plant Residues on Nitrification and on Losses of Nitrates in Soil; Jour. Agr. Sci. Vol. IX, Part 1, pp. 92-111, Aug. 1918. GREEN-MANURES 545 vestigators+ as a measure of humification, since a favorable environment for nitrification probably does not occur until the more rapid decomposition processes are completed. In general, the more rapid the decay of the green-manure, the sooner will nitrification be active again. Besides affecting the bacterial activity of the soil, the de- ecaying green-crop influences the solubility of the soil min- erals. Jensen* found that the addition of 3 per cent. of ereen-manure raised the solubility of lime and phosphoric acid 30 to 100 per cent. This was over and above the mineral constituents which came directly from the decomposing green- crop. Magnesium and iron were also markedly influenced. 314. Crops suitable for green-manures.—An ideal green- manuring crop should possess three characteristics: rapid growth, abundant and succulent tops, and the ability to grow well on poor soils. The more rapid the growth, the greater the chance of economically using such a crop as a means of soil improvement. The higher the moisture content of the crop, the more rapid the decay and the more quickly are bene- fits obtained. As the need of organic matter is especially urgent on poor land, a hardy crop has great advantages. The crops that may be utilized as green-manures are usually * Hutchinson, C. M., and Milligan, S8., Green-Manuring Experiments, 1912 and 1913. India Agr. Res. Inst. Bul. 40, Pusa, India, 1914. Maynard, L. A., The Decomposition of Sweet Clover as a Green- Manure under Greenhouse Conditions; Cornell Agr. Exp. Sta., Bul. No. 394, 1917. Martin, T. L., The Decomposition of Green-Manures at Different Stages of Growth; Thesis for degree of Doctor of Philosophy, Cornell University, 1919. ? Jensen, C. A., Effect of Decomposing Organic Matter on the Solu- bility of Certain Inorganic Constituents of the Soil; Jour. Agr. Res., Vol. IX, No. 8, pp. 253-268, May 1917. See also, Snyder, H., Humus as a Factor in Soil Fertility; Minn. Agr. Exp. Sta., Bul. 41, 1895; and Production of Humus from Manures; Minn. Agr. Exp. Sta., Bul. 53, 1897. Hopkins, C. G., and Aumer, J. P., Potassium from the Soil; Tl. Agr. Exp. Sta., Bul. 182, 1915. Hopkins, C. G., and Whiting, A. L., Soil Bacteriology and Phosphates ; Ill. Agr. Exp. Sta., Bul. 190, 1916. 546 NATURE AND PROPERTIES OF SOILS grouped under two heads, legumes and non-legumes. Some of the common green-manures are as follows: LEGUMES NON-LEGUMES Annual Biennial Cowpea ‘Red clover Rye Soybean White clover Oats Peanut Alsike clover Mustard Vetch Alfalfa Mangels Canada field pea Sweet clover Rape Velvet bean Buckwheat Crimson clover Hairy vetch When other conditions are equal, it is of course always bet- ter to choose a leguminous green-manure in preference to a non-leguminous one, because of the nitrogen that may be added to the soil. However, it is so often difficult to obtain a catch of some of the legumes that it is poor management to turn the stand under until after a number of years. Again, the seed of many legumes is very expensive, almost prohibit- ing their use as green-manures. Among the legumes most commonly grown as green-manures, cowpeas, soybeans, and peanuts may be named. Many of the other legumes do not so fit into the common rotations as to be turned under conven- iently as a green-manure. For the reasons already cited, the non-legumes have, in many eases, proved the more popular and economic as green- manures. Rye and oats are much used because of their rapid, abundant, and succulent growth and because they may be accommodated to almost any rotation. They are hardy and will start in almost any kind of a seed-bed. They are thus extremely valuable on poor soils. Often the value of such a ereen-manure as oats is greatly increased by sowing peas with it. The advantages of a legume and a non-legume are thus combined. It has already been shown that the nitrate production in a GREEN-MANURES 547 soil may be used as a rough measure of the rate of decay of ereen-manures. Admitting such a criterion, certain data from Cornell University become particularly interesting. In a five-year continuous test, green-manuring crops were seeded in July and plowed under in the early part of the succeeding May. The nitrate content of the soil was determined at a number of times during the spring, summer, and fall. A de- crease in nitrates always occurred in the autumn, while an increase began soon after the crops were turned under in the spring. In the following table the rye crop is taken as 100 in both October and July: TABLE CX XIV RELATIVE INFLUENCE OF GREEN-MANURES ON THE ACCUMULATION OF SOIL NITRATES.! NITRATES IN JULY, NITRATES IN OcT., Som FALLOow SINCE) Soin UNDER Crop GREEN-MANURE May 1. SINCE JULY. RYE TAKEN AS 100)/RYE TAKEN As 100 Ryd, ek ee nee 100 100 (CANS 5 se aera at, ee ae 78 73 TEE GL ages ee A 120 13 LEE) a AO bea, ek a ee 99 83 nyerand Vetch fis. ola 136 74 ye sand peas... 4. ooo ak os 102 75 It is immediately apparent that the succulent rye and vetch that survive the winter give better results, as far as nitrate production is concerned, than the dry and dead oats and peas. This shows clearly the value of succulence and the necessity of turning under a crop partially matured.2, The advantage of the legumes over the non-legumes is not hard to explain. *Unpublished data. Dept. Soils, Cornell University. 7Martin, T. L., The Decomposition of Green Manures at Different Stages of Growth; Thesis for the Degree of Doctor of Philosophy, Cornell University, 1919. 548 NATURE AND PROPERTIES OF SOILS The combination of rye and vetch, both of course in a sucev- lent condition, seems especially efficacious. Sod as a green- manure always appears more or less at a disadvantage. 315. The use of green-manures.—The indiscriminate use of green-manures is of course never to be advised, as the soil may be injured thereby and the normal rotation much interfered with. When soils are poor in nitrogen and organic matter, they are very often in poor tilth. This is true whether the texture of the soil be fine or coarse. The turning-under of green-crops must be judicious, however, in order that the soil may not be clogged with undecayed matter. Once or twice in a rotation is usually enough for such treatments. Proper drainage must always be provided. In regions where the rain- fall is seanty, great caution must be observed in the handling of green-manures. The available moisture that should go to the succeeding crop may be used in the process of decay, and the soil left light and open, due to an excess of undecomposed plant tissue. In western United States, it is still a question whether green-manures have any advantage over summer fallowing. It is generally best to turn under green-crops when their sueculence is near the maximum and yet at a time when abundant tops have been produced. This occurs at about the half mature stage. A large quantity of water is carried into the soil when the crop is at this stage, and the draft on the original soil-moisture is less. Again, the succulence encour- ages a rapid and more or less complete decay, with the maxi- mum production of humus and other products. The plowing should be done, if possible, at a season when a plentiful supply of rain occurs. The effectiveness of the manuring is thereby much enhanced. At Cornell University various green-manures were seeded in the summer and plowed under that fall or the next spring. The experiment was continuous for three years, the nitrates being determined in the soil each year in April and in June. The results are as given on the next page. GREEN-MANURES 549 TABLE CX XV INFLUENCE OF THE TIME OF TURNING-UNDER OF GREEN-MANURES ON THE NITRATE ACCUMULATION IN THE SOIL.+ PARTS PER MILLION oF NITRATES Crop In Apris Just | In June, Som BEFORE THE FALLOWED SINCE SPRING PLOWING PLOWING Rye, fall plowed.......... 58 57 Rye, spring plowed........ 53 67 Oats, fall plowed... ..4.2.. 0. 61 42 Oats, spring plowed....... 36 50 Vetch, fall plowed. f. 2.0.0.6. 719 45 Vetch, spring plowed...... 41 67 Average, fall plowed...... 66 48 Average, spring plowed.... 43 61 It is apparent that the decay of the green-manuring crop is hastened by fall plowing, as the nitrates in every case are higher in April on land so handled. In June, however, the nitrate accumulation has passed its highest point in the fall- plowed soil, leaving the spring-plowed plats, where the decay was initiated later, in the ascendancy. The table also shows the advantage that a legume has over a non-legume in causing nitrate accumulation. Oats fall-plowed appear about on an equality with rye. Spring plowing, since the oats are then dry and dead, gives the rye a marked advantage. All of the points above noted have a very practical field application. In turning under green-manures, the furrow slice should not be thrown over flat, since the green-crop is then deposited as a continuous layer between the surface soil and the sub- soil. Capillary movement is thus impeded until a more or * Unpublished data. Dept. Soils, Cornell University. 590 NATURE AND PROPERTIES OF SOILS less complete delay has occurred, and the succeeding crop may suffer from lack of moisture. The furrow ordinarily should be turned only partly over, and thrown against and on its neighbor. The green-manure is then distributed evenly from the surface downward to the bottom of the furrow. When decomposition occurs, the resulting materials are evenly mixed with the whole furrow slice. Moreover, this method of plowing does not interfere with the capillary movements of water, and in actual practice is a great aid in drainage and aération. 316. Green-manure and lime.—The decay of organic matter in the soil is always accompanied by the production of organic acids of various kinds. The greater the succulence of the material, the more rapid is the accumulation of such products. In spite of this, however, the effect of a green- manure is to decrease the acidity rather than increase * it and later greatly to stimulate nitrification even if the soil origi- nally was quite acid. The decrease in lime requirement may be due to the liberation of mineral constituents from the de- caying organic matter and to the effect of the decomposition on the inorganic constituents of the soil. The ultimate influence of green-manure on acidity is some- what in doubt. The bulk of the evidence available seems to indicate that decaying organic matter, if it has any effect, ulti- mately tends to decrease rather than increase the lime re- quirement of the soil.2 Nevertheless, plenty of active calcium should be in the soil, since it promotes the decay of the plant tissue added and seems to control to a certain extent the pres- ence of toxic materials. Lime may be added to the green- manure seeding and be turned under with that crop. The * White, J. W., Soil Acidity as Influenced by Green Manures; Jour. Agr. Res., Vol. XIII, No. 3, pp. 171-197, April, 1918. * Hill, H. H., A Comparison of Methods for Determining Soil Acidity and a Study of the Effects of Green Manures on Soil Acidity; Va. Poly. Inst., Tech. Bul. 19, April 1919. Ames, J. W., and Schollenberger, C. J., Liming and Lime Require- ment of Soils; Ohio Agr, Exp. Sta., Bul. 306, pp. 381-383, Dee. 1916. GREEN-MANURES 551 amendment would thus be in very close contact with the de- caying vegetable tissue. Ordinarily, however, the application of lime at some point in the rotation is sufficient. Lime, besides its capacity to alleviate toxic residues, tends to hasten organic decay.’ This is a very important function as the first stage of decomposition, during which soil and plant activities may under certain conditions be detrimentally af: fected, is markedly shortened. Such a promotion is indicated in a green-manuring experiment at Cornell University. The ereen-manures were seeded in the fall under two treatments, limed and unlimed. The parts per million of nitrates in the soil are given for two dates on the year succeeding, the green- manures having been plowed under either in the fall or early spring. The data are averages of three years. TABLE CX XVI INFLUENCE OF LIME ON THE NITRATE ACCUMULATION IN A SOIL RECEIVING VARIOUS GREEN-MANURES.” Parts PER MILLION oF NITRATES Crop AND TREATMENT APRIL JUNE yen NOM MMe ye sc. bes 6h: 66 53 Fever IMEC ate esc cweuhnatoss cvs, t 45 fal Gatsnno) Mme sce kia ees 53 43 CALS ewIIMOG Sle). sy sara sicatse 45 50 Weteh no! messi. 5 S02: rah 52 Neteh slimmed iect. ok 43 63 Average, no lime........ oe 65 49 wverdge., MEM... ss. 5.5 ss 44 61 *Lemmermann, O., et al., Untersuchung iiber die zerzetzung der Kohlen- stoff Verbindungen Verscheidener Organischen Substanzen im Boden Spezielle unter dem einfluss der Kalk; Landw. Jahrb., Bd. 41, S. 216- 257, 1911. ? Unpublished data. Dept. Soils, Cornell University. 552 NATURE AND PROPERTIES OF SOILS The effect of lime on nitrification is very noticeable in June. In April the no-lime plats are higher in accumulated nitrates, due to the lesser growth of the green-manuring crop. 317. Practical utilization of green-manures.—Green- manures seem to have their greatest value where a permanent instead of a rotation pasture is used, where a long cycle rota- tion of grain is practiced, or where little or no manure is available. The experimental data bearing on the use of green- manures seems to indicate that such a practice is productive of larger crop yields. The following data from Nappan, Nova Scotia, is from one of the more reliable and conclusive experi- ments.