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Soil Lastire

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Crop Production

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KHAKI UNIVERSITY OF CANADA
A¢i J
Series 1.—No. 5.

KHAKI UNIVERSITY OF CANADA

1918

SOIL MOISTURE »* CROP
~PRODUCTION

Fig. 52.—Corn growing in a soil covered by an excellent dust
mulch,

Reprinted, with adaptations, from ‘‘SOIL MOISTURE
AND CROP PRODUCTION,” by Elmer O. Fippin, in
“Cornell Reading Courses.”

e

Printers and Stationers,

Road,

42-60, Goswell
London, E.C.1

J. C. KING, L>.,

-

1 eh ee

.

SOIL MOISTURE AND CROP PRODUCTION

ELMER O. FIppiIn

The large use of water by plants and its derivation
from the soil are facts commonly understood. The object
of this lesson is tc point out the exteat to which plants
use water and the manner of its us2, and to explain the
ways in which water is collected and conserved in the
soil for this purpose.

USES OF WATER BY PLANTS

The water used by plants serves many purposes. It
is continually given off by the plant surface, especially
by the leaves. This may easily be shown by placing
a glass jar or other closed vessel over a growing plant.
Drops of moisture will collect on the inn2r surface of
the vessel as a result of transpiration. The familiar
phenomenon of wilting is merely an indication that
water is being lost somewhat more rapidly than it is
absorbed from the soil by the roots. As a result of that
precess the soft, herbaceous parts of plants lose their
crisp form and become soft and limp. If the wilting
becomes excessive the plant is likely to die. When
such a condition is reached, all the various functions of
water in the plant and in the soil have ceased to be
performed effectively.

The functions of water in plant growth: (1) It gives
strength and form to the balloon-like cells of which
plants are composed, by keeping them fully distended.

3

(2) It serves as food. Its elements, hydrogen and
oxygen, are separated and built into the structure of the
plant tissues ; and, in addition, water as such is an essen-
tial constituent of many plant materials. Water and
its elements make up abcut fifty per ceat. of the dry
weight of plants and over ninety per cent. of the green
weight. (3) Water dissolves plant-food in the soil and
carries it into the plant. It also distributes the elabo-
rated materials in the plant to the parts where they are
used. (4) By evaporation the plant is protected some-
what against temperatures high Boda Ss to cause injury
to its delicate tissues.

QUANTITY OF WATER USED BY PLANTS

The quantity of water which is used by plants for
these purposes, and which the soil must supply, is very
large, and varies considerably for different plants.
By determining this total quantity and comparing it
with the annual water supply either from rainfall or from
irrigation, and also by determining the capacity of the
soil to hold water for the use of plants and the possibility
of increasing its conservation by appropriate methods
of soil management, one may see to what extent larger
crops are possible with the normal] rainfall.

The most practicable method of determining the use
of water by plants is to measure the amount evaporated
from the plant surface during the entire period of growth
and divide that by the amount of dry plant material
produced. This shows the quantity of water used to
produce a unit of plant material.

Many measurements of this sort have been made.
The quantity of water used varies not only with the kind
of plant, but also with the climate and in a minor way
with many other conditions. The figures in the following

4

table represent the approximate range in the quantity
of water used by a number of common crops under
a climate ranging from moderately humid to semi-arid.
There is also calculated the number of inches of rainfall
that would be required in order to meet that need for
water where large yields are produced. The yields
stated indicate the general growth, but the weight of
the entire top is used as the basis of the calculations.

TABLE 1.—QvuANTITY OF WATER USED IN PRODUCING SOME
ComMMOoN CROPS’

Water |
requirement — Inches of
per pound of rainfall* used
dry matter Yield per acre. in producing
Crop.- in tops crops.
(pounds).

Mini- Maxi- Bushels Totaldry Mini- | Maxi-
mum. mum. ortons. matter. mum. }|mum.

(Bushels) (Pounds )

i Se "250 «375 = —(«100~—S «210,000 | _:11.0 | 16.5
Potatoes.... .300 450 300 6,000 8.0 | 12.0
i Colaliiatl 300 800 40 | 6,000, 8.0 | 21.0
Wheat..... 250 | 500 40 6,500 7.0. 14.0
ii 400 | 600 | 70 7,000 12.0 18.5
(Tons) é
Red Clover. 300 500 4 7,000 9:0 |. 155
Alfalfa..... 600 1,000 6 10,000 26.0 | 45.0

———— !

* One acre of water one inch deep weighs approximately

113.5 tons.

50 Inches of rainfall required to — crop
See Toble J

Maximum 2!
ee

Peas Wheat Oats Fed clover Alfalfa

Corn Potatoes
100 bu 300 bu. 40 bu 40 bu 70 bu +tons G6tons

Fig. 53.— Minimum and maximum rainfall used in producing
some common crops.

From Table 1 it 1s seen that the total quantity of water
required for the plant to produce these large yields ranges
from seven to forty-five inches. If the yields are divided
by two, the amount produced will still be regarded as a
fair yield ; and if the water used is calculated to the mean
of the figures given, the actual quantity required by
crops under average Eastern Canadian conditions will
be found to range from five to eight inches ot rainfall—
except for alfalfa, which uses somewhat more than twice
this quantity, or abouteighteeninches. This large use of
water by alfalfa is probably an important reason for its
deep root system, designed to bring the plant in touch
with the supply of moisture in the deep subsoil.

The depth of the root system and the thoroughness of

6

its distribution are important factors to be taken into
account in controlling soil moisture for different crops.

ANNUAL PRECIPITATION

The annual precipitation in the form of rain and snow
for the greater part of Eastern Canada is thirty to fifty
inches. Limited areas have a smaller or a larger amount
of precipitation, but these figures may be taken to fairly
represent the soil water supply. Of this amount, eight
to tourteen inches fall during the summer, at the rate of
two or three inches a month. Itis clear, therefore, that in
Eastern Canada the rainfall is adequate for the pro-
duction of large crop yields. The limiting factor, so
far as water supply is concerned, is the capacity of the
soil, either naturally or under the management of the
farmer, to retain sufficient water within reach of the plant
roots to carry them through the periods between rains.

MANNER IN WHICH WATER IS HELD IN THE SOIL.

It has been seen in an earlier lesson* in this series that
the soil is normally a porous mass of rock and organic
particles. The water derived from precipitation falls on
the soil and sinks into the pore spaces. Each particle of
the soil holds a film of water on its surface. This is due to
the fact that the attraction of the soil particles at their
immediate surface is great enough to separate the mass
of water and retain some of it ; in other words, at the sur-
face of the soil particles water is attracted more by the
soil than by its own material. ‘This attractive force is
exerted for only a very short distance from the surface
of the particles, and any film thicker than the range ot
that attraction is due to cohesion of the water. The
extent of this cohesion is indicated by the thickness ot the
film of water at the instant when drops form,

*No. 4

7

Since each patticle of soil is covered by a film of water,
the first factor in the determination of the quantity of
water that a soil will retain is the extent of surface of
the soil particles. The greater surface a soil has, the
larger is the quantity of water it may be expected to hold.

The extent of surface in a mass of soil is determined by
its fineness. The finer the soil, the larger is the extent of
surface of the particles in a unit of mass. The extent of
surface in a cubic foot of clean sea sand is less than one
quarter of an acre. In asandy loam soil it is one to two
acres. Inaloam or silt loam soil it is two to three acres.
In a clay loam it is three to four acres, and in heavy clay
it may be more than five acres: When free to do so, all
particles of whatever size will hold the same thickness of
water film on their surface. Consequently loam and clay
soil holds much more water than does sand.

The surface of the soil particles is not the only factor
that determines the retention of water. Due to internal
physical forces the surface of water acts like a stretched
membrane. It is in tension and always tends to assume
the form that has the least surface. This is the reason
why raindrops are spherical. When two soil particles
each bearing a film of water are brought in contact the
water films unite and form a waist, or connecting neck,
at the point of contact. The film of water thickens in
that part, and if one of the soil particles is in contact
with a supply of water this neck will increase in diameter
until it fills the angle between the particles, and the surface
of the water becomes either plane or hemispherical. This
is the phenomenon of capillarity.. A concave, or dished,
film of water exerts suction in its effort to straighten out.
A convex, or bulging, form of water surface exerts
pressure and will force water through the soil. Capillary,
or film, water in the soil normally presents concave sur-

8

faces, which are increasingly dished as the water in the
soilisreduced. If aseries of wet particles are connected,
the water will shift back and forth until all the curves
of the water surfaces are the same ; which means that all
have the same pull and consequently balance one another,
and the water remains at rest. As the quantity of water
in the soil is reduced, the connecting necks of water
become successively smaller ; and finally they are broken,
beginaing where the soil particles are most widely
separated.

In the soil there are many points of coatact between
the particles, and consequeatly many small necks of
water. The smaller the particles, the more numerous are
these points of contact. This is a further reason why
fine-textured soils hold more water than. do coarse-
textured soils. Water held in this way is called film,
or capillary, water.

Capillary water in soit

The quantity of capillary water held by different soils
in good tilth, or physical condition, is shown in the
following table :—

TABLE 2.—QuantTITy OF CAPILLARY WATER RETAINED BY
___Drrrerent Sors

Weight Inches of
per Percentage water
Texture of Soil. | - cubic of water retained in
| foot retained. three feet
| (pounds). depth of soil.
Beaeh sand.......... 110 + 2.5
Light sandy Joam..... 100 10 5.8
Fine sandy loam..... 90 15 7.8
eo 85 20 9.8
Ue See 80 25 11.5
Se 75 (nae 15.0
Heavy elay.......... 70 45 18.0

Rete es fis fo. 70:

In the last column of the table it is seen that a section
of soil three feet in depth is able to hold as much water
as falls in one to six months, depending on the physical
properties of the soil.

Unavailable water in soil

Not all the water in the soil can be used by plants.
Some is permanently unavailable to plants, due to
the tenacity with which it is held by the soil. This occurs
when the films on the soil particles become very thin.
Therefore the quantity of unavailable water is propor-
tioned to the fineness of the soil. It is greatest in clay
soil, which has the greatest extent of internal surface,
and it is least in clean sand and gravel. |

Using the same soils as in Table 2, the approximate
amount of unavailable and available water in a section
thiee feet deep is shown in the following table :

TABLE 3.—AVAILABLE WATER RETAINED BY DIFFERENT
Sorts

|
| Weight Per- | Inchesof Jnches of
per centage unavailable available
Texture of ney cubic of un- waterin | waterin
| foot available three feet three feet

(pounds). | water. depth of soil. depth of soil.

Beach sand.. | 110 abaee 0.6 1.9
Light sandy |

loam...... | 100 4 2.3 | 3:5
Fine sandy | |

loam...... 90 | 7 3.6 4.2
Loa... | 85 | 12 5.8 | 4.0
Silt loam.... | §0 | 1S} 1S 6g 5.5
Clay loam... | 1% | yt ae 8.6 6.4
Heavy clay.. 70 | 30 12.0 6.0
Muck....... Bas 100 8.7 8.3

/

It is evident from Table 3 that any except the very
lightest sand soils are able to-hold enough available
water to largely meet the requirements of crops for
moisture and to bridge over the periods between rains.
It is to be remembered that to these quantities is added
the rainfall during the growing period, amounting
usually to four to teninches. The soils under considera-
tion are supposed to be in perfect physical condition
to a depth of three feet. It has been clearly shown in
investigations in Western States that plants can utilize
water stored at a much greater depth than three feet,

50 Available and unavailable water capacity of sail
and rainfall during the summer months '
See Tables 2and 3

Rainfall June 70 August inclusive
40 12 inches
Unavailable water in soil
30 | Available water retained by soil

20

Beach Light Fine Loam Silt Clay Heavy Muck
Sand sondy sandy loam loam clay
loom loam

Fig. 54.—Diagram representing the amount of total, unavail-
able, and available water retained by a three-foot section of
soils of different texture. To the available water retained is
added the average rainfall within the crop-growing season.
This gives the total amount of water that can be made available
to growing crops.

1]

but this depth may be taken as the ideal for Eastern
conditions. There may be a water-table or other
available forms of water below three feet in depth that —
may be drawn upon to replenish moisture lost from the
upper part of the soil sections.

Control of water capacity of the soil

Insuring absorption of precipitation

The first problem of the farmer is, how to put his soil
in condition to absorb water as it comes in the form of
rain orsnow. If the surface is very compact and sloping
the water may run off over the surface before it can
absorbed. Especially is the maintenance of a loose,
open surface soil desirable during the summer, when the
rainfall is likely to come in heavy, dashing showers
and when the topsoil is dry. If the soil is fine and
compact, the air is trapped and water can find admission
only as the air slowly escapes—much as a bottle can be
filled only as the air escapes. The flow of the water
over the surface may also cause serious erosion.

Increasing retentive capacity of the soil

If the capacity of the soil to absorb water is reduced
by its physical condition, it 1s the function of the farmer
to practise such methods of tillage and management as
will improve that conditicn. The soils that naturally
remain in the best physical condition are the silt and
the sandy soils ; the loam and the clay soils are more
likely to get into bad condition. There are two
extremes to be guarded against: one is the very open
condition typified by coarse sand and gravel; the other
is the very compact, dense condition represented by
puddled clay.

Management of sand soil.—In coarse sand soil the

12

surface of the particles is small in extent, the points
of contact between the particles are few in number,
and the individual spaces are relatively large. Such
a soil has a low water capacity and is so well ventilated
that organic matter rapidly decays. Since the particles
of soil cannot be pulverized, the desired change must
be accomplished in another way. Several courses
are open,

First, such a soil should be kept as compact as possible.
It is better ploughed in fall than in spring, in order that
excess water of the winter may bring the particles
close together. Jf humus is not being applied, shallow
ploughing is better than deep ploughing, since the
operation of ploughing loosens the soil. The roller should
be used freely. Even sand is made up of particles of
different sizes, and if it is ploughed when a little wet it
may be partially puddled, which is a favourable condition
in soil inclined to be very loose. In that case the small
particles are worked into the larger spaces and reduce
their average size to a more effective diameter. This
increases the quantity of water retained.

Second, material may be added to the soil to partially
fill the large spaces and increase the number of points
of contact. The most desirable material for this purpose
is humus—decayed organic matter. Clay would have
a similar effect, but its use is not so practicable and it
also introduces some disadvantages. Increase in the
stock of humus and its deep incorporation is therefore
effective to increase the water capacity of light sand
soil, and, when coupled with deep ploughing and the use
of compacting implements, the best results are attained.
The introduction of the humus more than offsets the
disadvantage of deep ploughing.

_ Management of clay and puddled soils.—In very

13

fine-textured soils, such as clay, and in loams and sandy
loams that are in a puddled condition, the pores are so
small in size, and may also be so small in total volume,
that the soil has a very low capacity to hold available

increased films of available water due to capillary surfaces
resulting from proximity of particles.
Note the uniform curvature of these

Unavailable. water: film

Available water film

Fic. 55.—Diagram representing the retention of capillary water
by soil particles, and the influence of their size and arrangement
on the amount retained. Note the uniform thickness of films
on particles of all sizes where they are sufficiently separated.
The black areas represent the increased thickness of the films
due to the capillary surfaces where the particles are near
enough for the water films to join. On each particle is a thin
film of water unavailable to plants. Water in the interior of
granules ts largely unavailable.

14

_ water. Clay soil is especially prone to be in such a
compact condition, and great care must be exercised
in its management.

It is especially important that these soils be loose and
granular. The finer the soil, the greater should be the
effort to put it in a granular condition. In coarse sand
soils, the individual particles may rest close together
and yet the spaces be too large to be fully effective.
On the other hand, in fine clay, if it is pulverized so that
the individual particles rest close together, the spaces
are so small that they incline to remain full of water,
thus cutting off veatilation and preventing the penetra-
tion of roots. This condition is further aggravated by
the ease with which such a soil is puddled, thereby -
further reducing the size of the spaces. The water
films do not have opportunity to attain their maximum
thickness, and the water that is held is so close to the
surface of the soil particles that it is largely unavailable
to the roots of plants.

The correction for this consists in creating a granular,
or flocculated, condition of the soil, so that a numbe1 of
small particles act together like a single larger particle.
Granulation is tmportant. The most effective size of
granule ranges from the size of corn kernels to that of
timothy seed. Inevitably there will be considerable
finer material, so that the mass will have a loamy
structure.

By loosening the soil, opportunity is given tor the
maximum thickness of films and the largest proportion
of available water.

In a soil that is too fine and compact, the development
of the granular condition is greatly assisted by good
cultural methods applied at the time when t!.e soil is
most friable by having the right moisture content.

15

Fundamental to all of these is good drainage. The
surface soil can be improved to a large extent by tillage
coupled with moderate drainage; but the subsoil, to
the ideal depth of three feet or more, can be improved
only by natural processes, at the basis of which is such
a degree of drainage as will permit the removal of any
free water so that drying and shrinkage is possible.
This change in the physical condition of the soil is the
basis of the statement frequently made, that drainage
increases the available water capacity of the soil. In
addition it admits air, which permits plant roots and
various forms of animal life, such as earthworms, to
penetrate deep into the soil, and these further aid in the
process of granulation.

Tne use of lime where it is deficient, and the generous
introduction of humus, are also effective aids to granu-
lation ; and a further benefit from the presence of humus
is its own large capacity to hold water, as is illustrated by
muck soil.

When clay 01 other paca soil is ploughed, itis likely
to bieak up into large lumps. This is a form of granu-
lation, but, as in sand and gravel, the lumps are too large
and must be reduced in size by pulverizing.

Between these two extremes of soils—those that are
too loose and those that are too compact to have the
maximum ability to hold available water—is every
gradation of condition. The intermediate soils, such as
sandy loams, loams, and silt loams, are easier to mani-
pulate and are adapted to the widest range of crops and
farm practices. The farmer must always use his judgment
as to the particular treatment his soil needs, having in
mind its present condition, the desired change,
and the attendant seasonal conditions and crop re-
quirements.

16

Free, or hydrostatic water

Film water is the only form of soil moisture used
directly by crops. When the films have reached the
maximum thickness, the soil has all the water that it
can retain and this quantity will be permanently held
in the soil. It cannot be removed by drainage. If,
however, the large spaces are filled with water
beyond the range of the films, this added water is
not held by capillary force and is free to pass down-
ward under the influence of gravity unless the lower
part of the soilis impervious. This free water is termed
hydrostatic water, and is the part of the soil moisture
that is removed by drainage. The height to which the
large spaces are filled with water is called the water-
table ; its presence excludes air, and thereby repels the
growth of roots of all the common crops that are not
adapted to grow in saturated soil. Drainage aims to
lower the water-table so as to give a suitable root zone,
which should be at least three feet in depth.

LOSS OF WATER FROM SOIL

It is well known that crops frequently suffer from the
lack of available water. From the above discussion it
is evident that the difficulty is not with the water supply
relative to the quantity used by crops. Nor is it due to
the complete inability of the soil to retain enough of the
precipitation to meet the needs of plants. It must
therefore be due either to the fact that the soil is not in
condition to absorb the precipitation, or to loss after the
water has been absorbed. We have pointed out the
ways in which the capacity of the soil to absorb and hold
water in available form may be increased. It now
remains to consider the ways in which water is lost from
the soil.

17

It is evident from what has been said that when the soil
has taken on the maximum films of water, any excess is
properly permitted to drain away out of the root zone.

Growth of weeds

Film water can be lost only by evaporation. Its
removal by evaporation from the leaves of the crop is
essential and must be provided for. Its waste by
evaporation from the leaves of weed plants is to be
avoided by their destruction. The destruction of weeds
is one important reason for tillage. The larger the weed
plant and the deeper its root system relative to the
regular crop, the more injurious is its growth likely to be
by the exhaustion of the water supply. Since the water
supply also measures the sclution and transfer of plant-
food from the soil to the plant, it is evident that weeds
interfere with the nutrition of the crop as well as with
its water supply. By expanding the evaporating
surface, weed plants greatly increase the loss over what
it would be directly from the soil.

Evaporation from the sot

The phenomenon of the evaporation of water from a
moist soil is a common one. If a soil were kept well
moist throughout the year, the total amount of water
that would evaporate would be sixty or seventy inches,
which is more than would evaporate trom a water surface
because the wet area is increased by the uneven surface
of the soil. This illustrates the large possible waste of
water if some provision is not made for its conservation.
Under average field conditions, the loss of water by evapo-
ration is not so large as the figures just given ; but it does
account for the difference between the fifteen or twenty
inches of water that plants may use and the three, four,

1S

r five inches that they actually do use in their growth.
ne of the big problems of the farmer is to prevent this
useless and expensive loss of water by evaporation from

,the soil.

Mechanism of evaporation

In the process of evaporation, very small particles of
water become detached from the main mass and float off
in the atmosphere. They are detached by their vigorous
vibration at the surface of the water, which is increased
as the temperature increases. Consequently, if other
conditions are equal, the warmer the water the more
rapid is evaporation.

The detached particles of water move about promis-
cuously in all directions in the atmosphere and are said
to diffuse through the atmosphere. Some of them find
their way back to the surface of the soil. If the soil
were absolutely dry, the contact of these moisture particles
with it would make its surface somewhat moist. All
objects, including the dusty-dry soil, carry a certain
quantity of such moisture of condensation, termed
hygroscopic moisture. The quantity of water taken up by
a soil in this way is never enough to support the growth
of crops, although it probably is helpful to the growth
of bacteria, which are exceedingly small plants. From
the last statement it is evident that the common belief
_ that a compacted soil which becomes moist derives its

‘moisture from the air, is wrong, and that this fact must
be accounted for in another way. The moisture that
appears in a footprint in a dry soil comes from below,
not from the air, in accordance with laws that will be
further explained in this lesson.

If the particles of water that are detached from the
moist soil are free to move away in the atmosphere, the

19

loss will continue as long as moisture is available. The
warmer and drier the air, the more rapid is the loss of
water. Winds that hasten the movement of the particles
of moisture and bring @ new supply of dry air-in contact
with the moist soil greatly increase the loss. This is the
basis of the common statement that winds have a very
drying effect.

On the other hand, if the volume of air in contact
with the soil is limited and relatively small—say a layer
a few inches, or perhaps a few feet, in thickness—there
will come a time when no more particles of water can
be taken up. Physicists explain this stage as due to the
fact that as many particles drop back into the liquid as
pass out into the air. An equilibrium is established and
the atmosphere is saturated ; hence loss of water stops.
That is what occurs when a lid is placed over a pail of
water or when a box is turned over a block of moist soil.

Mulches

There are various ways of covering a soil in order to
stop loss of water by evaporation. One method is to
cover the soil with boards. Stones serve the same
purpose, and where other means were not practicable
stones have sometimes been hauled on the soil to form
such a cover. Soils that. are very stony at the top
have been observed to hold moisture well, and the
removal of the stones has resulted in less growth of crops.
Another method is to cover the ground with a mulch
of straw or leaves. There is a method of growing
potatoes by which the tubers are placed on the surface
of the ground and deeply covered with straw. The
potatoes grow, and may reach a satisfactory maturity.
The straw holds the moisture in the soil to the very
surface, so that the roots may develop and find their way

20

7 = the soil for sustenance. A layer of coarse sand or

vel serves the same purpose, and even a very dry
laye1 of finely pulverised soil may be placed on a moist
soil and will prevent the evaporation of moisture in the
same way because it forms a cover.

However, all these methods of covering the soil in
order to save water are impracticable except on a very
smallseale. For the average farm it is necessary to make
the cover out ot the natural soil. It is well known that
such a cover, consisting of a layer of fine, dry soil, is
effective to prevent evaporation. It is called a dust
mulch, dust blanket, soi] mulch, or dust cover.

Simple rules can be given for the general management
of the soil in order to secure and maintain a mulch.
These rules usually involve the frequent stirring of the
topsoil as soon after a rain as is possible without puddling
the soil. The stirring should be repeated as often as the
topsoil has become moist and caked; this condition
indicates that it has largely lost its efficiency as a mulch.

Management of the dust mulch.—In order to understand
the reason for the effective management of a mulch, as
well as to be best able to decide the particular treatment
to be given, it is essential to understand the principles
involved. Throughout Eastern Canada, where there is
a generous rainfall, the mulch must be made from the
moist soil.

Dry soil acts as a mulch because when dry it tends to
remain so and does not readily taken on a new water
film. There are in every soil organic substances of a
fatty or greasy nature, and when once the soil becomes
dry these form a coating on the particles which offeis
some resistance to the absorption of water. Further,
when a soil is dry, moisture creeps over its surface very
slowly, especially if the adjacent soil is only moderately

2)

moist. Any treatment that causes the topsoil to become

dry forms of it a mulch. It is not even necessary that
the mulch be loose if only it is dry, but in practice loose--
ness is necessary in order to quickly securea dry condition.

oP
Sof

= 2
ae
oe
~~ “.

fat

<2
te 0

Rak
PP Aa

ie

Fic. 56.—Diagram representing the relative distribution of
moisture in two sections of soil receiving different treatment :
A, covered by dust mulch, moisture is abundant up to the

under surface of the mulch ; B, without mulch, soil ts dry to
a considerable depth.

In the field a mulch is naturally formed in the follow-
ing way: As soon as rain ceases and the atmosphere

9)»
on ad

ee) ey aa ae

\
}

becomes somewhat dry, evaporation begins from the
surface of the wet soil. As evaporation proceeds and
the top soil becomes less moist than the soil below,
moisture moves up from the adjacent layer to partially
replenish the loss. If, however, evaporation is more
rapid than the rate of movement of water into the
surface inch or two of soil, that soil will gradually be- —
come dry. The drier it becomes, the slower is the
movement upward from below ; so that the more rapidly
the soil becomes dry, the more rapidly is the mulch
developed by which the water in the underlying soil
is protected. If the atmosphere is very dry and warm,
the mulch may quickly form naturally without the loss
of water from more than a few inches of the top soil.
Tais is what happens after an application of water to
the soil in arid regions. In humid regions such as
New York, however, the atmospheric conditions much
of the time, and especially just after a rain, are favourable
for slow evaporation which may very slightly exceed the
movement from below. Hence the top soil will remain
moist for a long time—several days or weeks—and the
loss of water may extend several feet into the soil.
Eventually a very deep mulch would be formed, but
the loss of water in the process is very great. Another
rain may come in the meantime so that the soil is kept
almost continuously in condition for the loss of water.
Further, the moisture condition of the atmosphere
varies. One day of a very drying atmosphere may be
followed by a damp, muggy day in which the movement
of water from the subsoil almost or quite equals the loss
of the two days. This contributes to the waste of water.

By cultivating the top layer of soil its structure is
loosened so that evaporation from that part is hastened.
The loosening also reduces its contact with the moist

23

soil below, so that less moisture is absorbed. Hence
cultivation greatly promotes the formation of a mulch.
.<, It should be understood that the absorption of moisture
by the mulch never entirely ceases, although it becomes
_ very small. Even after the top soil is approximately
dry, a period of very moist atmosphere, such as accom-
panies foggy or very cloudy weather, may permit the
absorption of moisture from the subsoil to partially
destroy the mulch, so that if a period of drying weather
follows, the total loss of water may be considerable,

From these facts it is evident (1) that tillage usually
aids in the formation of a mulch; (2) that the mulch
should be as shallow as is practicable in order that the
least amount of water may be lost inits formation ; and
(3) that when once the mulch is dry further tillage is of
no advantage as long as it remains essentially dry, but
should the mulch become moist and form a crust further
tillage is beneficial.

Sandy soils are easier to mulch and to maintain in ©
a mulched condition than are clay soils, because the
drying is more rapid and the absorption from below is
slower. Normally, if evaporation continues at the
surface of the soil, moisture will be lost to a depth of
two or three feet as a result of the process of equalization
by capillary movement from below. In those soils that
crack seriously the loss from the subsoil is increased by
evaporation into the air in these cracks, from which the
moisture rapidly diffuses or is sucked out by winds
passing over the surface. Clay and muck soils are most
inclined to form large cracks, and it is especially impor-
tant to keep these cracks sifted full of dry soil in order to
reduce the loss of water from the subsoil.

' Depth of soil mulch.—The use of a soil mulch is practi-
cable for all tilled crops. The depth of mulch that is

24

adequate to prevent a large part of the loss by evaporation
may be one half-inch to two or three inches. A half,
or even a quarter, of an inch of fine, dry soil will reduce
evaporation to seventy-five to ninety per cent. of the loss
without a mulch. As indicated above, the climatic
conditions have much to do with the efficiency of a
mulch, and in general a mulch is not so effective in the
humid climate of the Eastern States as in the arid
climate of the Western States. It has, however, been
demonstrated many times that the saving of water
by thorough mulching may make the difference between
a full crop of corn and a half crop or a complete failure.
In orchard management, tillage, with the consequent
mulching, has much to do with the better growth of
fruit, as compared with grass crops. That mulching
is an effective means to prevent loss of soil moisture, and
that sufficient water can be stored in the soil to meet
the needs of crops, is abundantly shown by experience
in the arid and semi-arid sections of the country. Where
water is available for irrigation only in the winter, as
in parts of the North-West, full crops are produced without
any rainfall or application of water during the crop
season. On the eastern slope of the Rocky Mountains,
where the rainfall in one season is often not adequate
to produce a good crop and irrigation is not practised,
the rainfall of two seasons is stored in the soil and held
by a mulch, to be used in producing a crop in alternate
years.

In sandy loam or other soils where a good tilth is easily
maintained, a mulch of a half-inch to an inch in depth is
entirely practicable. Implements having many small
shovels or an equipment of horizontal blades are essential
in order to do good work of this sort. In clay soil
inclined to be cloddy, a somewhat deeper mulch, of one

25

and one-half to three inches, must usually be maintained
in order to get sufficient fine earth to be effective. About
two inches is the depth found to give best results on such
soil. Clods a half-inch or more in diameter do not form
an effective mulch unless it is two or three inches thick.
Distribution of soil moisture under a mulch.—Where
no mulch is established the soil is likely to be relatively
dry for a considerable depth. Plant roots do not find
favourable conditions to get either moisture or plant-
food in that part of the soil section, and a large part
of it is ineffective. On the other hand, in a well-
mulched soil abundant moisture occurs, up to the very
base of the mulch, and roots are able to utilize almost
the entire soil section. |
Mulching crops that are not tilled.—Many crops are not
tilled so that the dust mulch is not applicable. The
treatment here must be even more deep-seated. It
is especially important that such plants be able to

Fic. 57.—Making a dry mulch by the use of a spring-tooth
harrow.

26

\

establish and maintain their roots deep in the soil.
Thorough drainage is therefore very essential. In arid
sections grain crops are sometimes cultivated. Pro-
vided the cultivation begins when the plants are very
small, it may be continued without serious damage until
the heads begin to form.

Taere is opportunity for benefit by the use of available
manure as a top-dressing and mulch on the grain and the
new grass land. Especially is this desirable when grain
is used as a nurse crop for grass. When the nurse crop
is removed in midsummer, the grass plants are weak and
the soil is exposed to rapid drying, both of which con-
ditions are hard on the grass plant. The manure
forms a mulch as well as affords a generous food supply ;
and the practice, which is becoming common in Canada,
is shown to give good return in the succeeding crops
of hay and grain. After a crop has reached the size
at which it effectively shades the ground and holds the

_air in place near the soil so that it becomes saturated

with moisture, the loss by evaporation from’ the soil is
much reduced, except when heavy winds occur that
suck out this moist air and replace it with dry air—which
process cannot be controlled to any extent. Wind-
breaks afford some protection. .

MOVEMENT OF WATER:-IN THE SOIL

It has been pointed out that the soil water on which
plants depend is held in the form of films on the soil
particles, and that these films exist in a state of tension.
The tension is due to the curvature of the water surfaces
in the necks between the soil particles. When a moist
soil has stood for a long time without any loss, the
moisture should be uniformly distributed over the
particles throughout the mass, except that the films

Pa!

are thicker at the bottom than at the top of the section,
due to the pull of gravity on the water. This modifies,
but does not overcome, the capillary, or curvature, pull.
If a sand and a clay soil are in contact, each will take
the same thickness of film on its surface. The amount .
of moisture re-
quired for’ the
same thickness of
film on two soils
of different fine-
ness is called the
moisture eyuiva-
lent. Two soils of
different texture
have come _ to

their moisture fy, 58.—Diagram illustrating unequal

equivalent when
the same plant
wilts on each soil
under the same
atmospheric con-
ditions. Two
soils having the

distribution of capillary water in a mass
of soil. Where the film 7s thick (y) the
curves are flat and exert a small pull, as
at d, e, and f. Where the films are thin
(ax) the surface curves form acute angles
and therefore exert a large pull, as in-
dicated by the length of the arrows ata, b,
and c. The direction of movement of
water ts indicated by the feathered arrows.

same thickness of
film, whether that thickness be great or small, will
appear equally moist, although in each unit volume
one soil may actually hold twice as much water as the
other. :
If for any reason the films of water are thinner in
one part of the soil mass than in the adjacent soil,
the greater curvature of the surface of the films in that
part will draw in water from the wetter part. This
adjustment will take place for an indifferent distance in
all directions. For example, if water evaporates at the

~

28

surface of the soil, the moisture would be drawn up from
the subsoil, As plant roots use the water in contact
with their surfaces, water moves in—drawn by capillarity
—from the more remote, but wetter, particles. It is
‘a general principle that water in the soil moves from
the place where the films are thick to the place where
they are thin. Any change in the structure of a moist
sou that alters the relative thickness of the films in the
' different parts of the mass will start movement of the
moisture to bring about equilibrium.

_ Some treatments that influence the capillary movement of
moisture

Use is made of this principle of capillary, or film,
movement of water. For example, when the soil is
relatively dry and a rainoccurs, capillarity helps to pull
the water into the soil. As the upper soil dries out,
capillarity returns water to the surface. It draws
moisture to meet the roots advancing through the soil.
When the farmer plants very small seeds on the surface
of a dry soil, it is customary to use a roller, which packs
_ the top soil, thereby increasing its capillary pull so that
moisture is drawn up from below to germinate the seed.
When very dry land is ploughed it is sometimes packed
in order to draw moisture from the subsoil. When
the immediate surface is left loose the operation is called
sub-surface packing, for which purpose special imple-
ments have been devised. In many ways the farmer
_ makes use of the principle of capillary movement
and many of his tillage operations are designed to effect
ceapillarity. It is involved in the production of a mulch,
as explained above.

Sometimes the addition of a small quantity of water
to the soil may prove harmful because of this film

29

movement. As has been pointed out, when the soil
is relatively dry, movement is very slow. If a small
quantity of water is added, either by a shower of rain
or by artificial application, it is sufficient to re-establish
rapid capillary movement without causing deeper
penetration. Evaporation is hastened. All the water
applied is lost, and before the process ceases further
quantities of water will be removed, small rains are-
therefore usually injurious, for they add nothing per- —
manent to the soil and necessitate the re-establishment
of the mulch. The common practice of adding a little
water to lawns is to be condemned for this reason. When
an application is made it should be sufficient to penetrate
considerably below the mulch layer. This usually
requires one or two inches of water.

Rate of capillary movement

While differences in the distribution of water in a soil
will adjust themselves over considerable distances,
the rate of movement may be so slow as to make the
process of little value. The particles of water must
move through the thin films. When the films arereduced
so that they are very thin, the friction against the surface
of the soil particles is so great that it stops movement
for all practical purposes. This is why plants wilt
in soil that appears moist. When the films are thick
the outer particles of water are quite free to move
and adjustment is rapid. In a puddled clay soil that
may be wet to the point of stickiness, the water moves
with extreme difficulty because all the films are very
thin, due to the extreme fineness of the particles and
the very small size of the spaces. In a granulated soil
the films in the large spaces are able to beconie thick
and thereby movement is much freer. Because of these

30

extreme possible differences, the tilth of clay soil is
particularly important and puddling is especially to
be avoided.

SOME PRACTICES THAT AFFECT SOIL MOISTURE

A few common farm practices may be discussed in
their relation to the conservation 6f soil moisture

Ridge versus level cultwre.—For the conservation of
soil moisture, level cultivation is now universally recog-
nised as the best practice. Ridges increase the surface
exposed to evaporation, and they also increase the
difficulty of maintaining a good mulch. Even potatoes
are now generally cultivated level, and with better
results in the long run than follows ridge culture.

Early spring ploughing.—In spring the soil is generally
saturated with capillary water, if not also with free, or
hydrostatic, water. ‘The atmosphere is moist, although

Fic. 59.—A mulch of straw, such as is frequently used on
land devoted to small fruits.

dry enough to permit fairly rapid evaporation ; and after

the soil is dry enough to till, any delay may Occasion
the loss of an inch or more of water each week. This

31

quantity would make the difference between a large and
asmallcrop. Land should be ploughed as early in spring
as possible, and if it cannot be ploughed it is sometimes
practicable to partially mulch the surface by discing
or harrowing.

Early summer and fall ploughing.—In the semi-arid
regions where both the summer and the winter rainfall
must be stored, ploughing as soon as the crop has been
removed in the early summer and fall is especially
important. But in New York, where the soil usually
becomes saturated in the winter irrespective of past
treatment, early fall ploughing is not important ; 1n fact,
late fall ploughing is preferable because of its relation
to soil organisms and the physical condition of the soil.

Where a fall crop is to be planted, ploughing as soon
as the previous crop is removed, if accompanied by culti-
vation, may save sufficient moisture to make much-
difference in the starting of the succeeding crop.

Ploughing under green-manure crops.—The drying effect
of crops has been pointed out. If a green-manure crop
is allowed to stand until late in the spring, it may deplete
the soil moisture to such an extent that the succeeding
crop will have difficulty in starting, especially if the
season be a little dry. After a green-manure crop has

commenced to flower, nothing is usually gained from
further growth.

SELECTION OF TILLAGE IMPLEMENTS

The selection of tillage implements for the control of
soil moisture must be guided by the change in soil
structure that is to be made. A large variety of imple-
ments are available, all of which may be grouped into a
few fundamental types. The two main groups of these
are (a) compacters and (b) implements that loosen the

29

soil. Of the latter group ploughs operate to the greatest
depth. Subsoilingis seldom advisable. Deeper ploughing
is generally advisable, especially if it is accompanied by

Fie. 60.—A natural mulch of stone.

the addition of organic matter. Tillage implements are
usually selected with reference to the crop and the
condition of the soil, in addition to their effect in keeping
the surface level.

CONCLUSIONS

It is clear that for the great majority of Eastern Canada
soils and crops, there is sufficient water supply if it is
effectively managed. This is possible to a much larger
extent than is now realized. Our climate is subject
to rather extended periods of dry weather, as well as of
wet weather. The wet periods can be largely oftset by
drainage. Therefore it is the part of wisdom to manage
the soil as if each season were more dry than the normal.

30

If moisture is supplied, growing conditions are better
in dry seasons than in wet, dark seasons. In that
direction is the largest opportunity for the capable
farmer to get results. Irrigation has some place as a
supplement to the natural rainfall and storage capacity
of some soils, but usually only after better methods of
control of soil moisture have proved their inefficiency,

od

DISCUSSION PAPER

The Discussion Paper is planned to help the student
by drawing his attention to the important points of
the subject he is studying. It is intended to develop
thought and self expression of his own ideas. Each
discussion paper, when answered and returned, is care-
fully read by the staff of the Department of Agriculture
and a personal statement is given in connection with
any question that the reader thinks the student has not
fully understood. The student is invited to ask any
questions that will help to give him a more complete
understanding of the course.

GENERAL INSTRUCTIONS

1. Always express your ideas in your own words.
Finish one paper at a time.
3. See that the subject is placed at the top of the
first page of each paper answered.

4. Do not forget to write your NAME, NUMBER, and
COMPLETE ADDRESS, on each set of answers.

5. Number each question, and also each sheet, and pin
them together in their right order.

6. Send in each paper as soon as complete.

35

10.

QUESTIONS

Describe four important functions of water in plants.

Describe the symptoms of plants deprived of suffi-
cient water for normal growth. What happens
to a plant suddenly deprived of water ?

In what different ways is water retained in the
soil ? Mention the water supply that is available
to the plant.

Can the capacity of a soil to retain moisture be
increased ? How ?

In what W2ys may soil water that should be available
for a crop be lost ?

What are the advantages to be gained by applying
a mulch? Discuss the relative merits of the
different mulches described in the text.

Can soil moisture be conserved as easily in a climate
with an annual rainfall of 15 inches as in one
with a rainfall of 45 inches. Why ?

Describe the various movements of water in a
soil. Can they be controlled ?

Willa ridged surface give off more water by evapora-
tion than a smooth surface ? Why ?

Is it good practice to always prepare for a season
drier than normal? Why ?

36