Potassium from the soil

THE UNIVERSITY

OF ILLINOIS

LIBRARY

UNIVERSITY OF lUINOISUWRY

NOV 1 1918
UNIVERSITY OF ILLINOIS

Agricultural Experiment Station

BULLETIN No. 182

POTASSIUM FROM THE SOIL

BY CYEIL G. HOPKINS AND J. P. AUMEB

URBANA, ILLINOIS, MAY, 1915

SUMMARY OF BULLETIN No. 182

1. Potassium can be liberated as needed from the inexhaustible
supply naturally contained in the normal soils of Illinois.

2. The amount of potassium taken up from ordinary Illinois soil
by clover is from two to three times the amount required for plant
growth. The excess probably is merely tolerated, as is sodium and
silicon, both of which are present in the soil solution and are taken up
by plants in considerable amounts, altho neither is essential for plant
growth.

POTASSIUM FROM THE SOIL

BY CYRIL G. HOPKINS, CHIEF IN AGRONOMY AND CHEMISTRY, AND
J. P. AUMEE, ASSOCIATE IN CHEMISTRY

Potash is so widely advertised and commercial potassium has been
given such a prominent place in most of the experiments relating to
soil fertility, both in this country and abroad, that the greatest natural
source of potassium, — the soil, — like the inexhaustible atmospheric
supply of nitrogen, is not generally understood or appreciated.

The fact is that the total amount of commercial potassium applied
annually to all the farms of the United States is no more than is con-
tained in one square mile of common corn-belt land to a depth of six
feet, and the important potash problem is how to liberate it as needed
from the inexhaustible supply already contained in all normal soils.

COMMERCIAL POTASSIUM

Potassium (also called kalium — K) is an important plant-food ele-
ment. It makes up 83 percent of potash (K,0), about 42 percent of
commercial potassium chlorid (KC1), incorrectly called ''muriate of
potash," about 42 percent of commercial potassium sulfate (K2SO4),
and about 10 percent of kainit, a crude salt taken directly from the
German potash mines.

In the Journal of Industrial and Engineering Chemistry, January,
1915, page 59, Mr. W. H. Bowker, President of the Bowker Fertilizer
Company, a branch of the American Agricultural Chemical Company,
makes the following statement :

' ' There are 7,000,000 tons of fertilizer consumed in this country annually, of
which probably 5,000,000 tons are what are called complete fertilizers, that is, con-
taining the three essential elements of plant nutrition — nitrogen, phosphorus, and
potash. ' '

" It may be added that the most common so-called "complete" fer-
tilizer used in this country contains about 1% percent of potassium,
while the most common corn-belt soil contains about 1% percent of
potassium.

In the same article, Mr. Bowker quotes the following statement
concerning the German Kali Works, or Potash Syndicate:

"The Syndicate is spending a million dollars a year in its world- wide cam-
paign, * * * * and any farmer who cares to study the statistics showing the enor-

4 BULLETIN No. 182 [May,

mously increased use of potash in American fertilizers during the past decade must
be convinced that advertising pays."

NORMAL AND ABNORMAL SOILS

"While some abnormal soils, such as the peat, or muck, of certain
Illinois swamp lands and the siliceous sands of the Atlantic and Gulf
Coastal Plains, are positively deficient in potassium, the most com-
mon, or normal, soils are richly supplied. (Normal soils are those
which bear relation in composition to the average of the earth's crust
from which they are formed.) Thus 2 million pounds of the common
corn-belt land, corresponding to the plowed soil of an acre to a depth of
about 6% inches, contains about 35,000 pounds of potassium and 1,200
pounds of phosphorus, while 2 million pounds of the earth's crust con-
tains as an average about 50,000 pounds of potassium and 2,200 of
phosphorus.

While phosphorus is usually deficient, all must admit the abun-
dance of potassium when the total supply of normal soils is considered ;
but the contention is usually made that the potassium of the soil is
unavailable, and, instead of being advised to make it available, the
farmer is urged to purchase soluble potassium salts from commercial
agents.

THE "INSOLUBLE RESIDUE"

The chief purpose of the investigations reported in this bulletin
was to secure information as to the power of decaying organic matter
to liberate potassium from the soil 's abundant supply.

The method of soil analysis commonly employed by many investi-
gators involves the digestion of a certain amount (10 grams) of the
soil with a certain quantity (100 cubic centimeters) of a certain acid
(hydrochloric) of a certain strength (specific gravity 1.115), at a cer-
tain temperature (the boiling point of water), for a certain time (10
hours). The "insoluble residue" is then discarded as valueless, and
analyses are made of only that part of the soil which dissolves under
these specific conditions arbitrarily fixed.

As shown by the many soil analyses reported in Bulletin 123, the
"acid-soluble" potassium found by this method is, as a general aver-
age, only about 15 to 25 percent of the total potassium contained in
the soil. It has seemed highly desirable, therefore, to ascertain
whether growing plants with their roots in constant contact for months
with the soil particles might not secure some potassium even from this
"insoluble residue," especially in connection with the decomposition
products of organic matter. In other words, Can the plant find value
in the "insoluble residue" discarded by the analyst ?

19151 POTASSIUM FROM THE SOIL 5

PREPARATION OF POT CULTURES

This investigation was begun in 1909. The soil was taken from the
Experiment Station Farm of the University of Illinois, which is rep-
resentative of the brown silt loam of the early Wisconsin glaciation,
the most common Illinois prairie land. This soil was found to contain
1.784 percent of total potassium, corresponding to 35,680 pounds in 2
million pounds of soil, but of this, an amount corresponding to 28,560
pounds remained in the " insoluble residue" by the common method of
analysis just described.

By digesting many portions of this soil with the usual proportion
and strength of acid under regular conditions of temperature and
time, a sufficient quantity of the "insoluble residue" was secured for a
series of pot cultures prepared as follows in glass jars of about one-
gallon capacity.

Pots 1 and la were filled with normal soil.

Pots 2, 3, and 4 were filled with the " insoluble residue" from the
acid-digestion of the same kind of soil, and the following chemicals in
powdered form were incorporated with the "insoluble residue":

Pot 2. 120 grams of calcium carbonate, CaCO,

120 grams of calcium phosphate, Ca,(PO4)2
30 grams of calcium sulfate, CaSO4
5^ grams of magnesium sulfate, MgSO4
5 grams of iron chlorid, Fed,
Pot 3. The same as to Pot 2, and in solution .160 gram of ammonium ni-

trate, NH4NO3

Pot 4. The same as to Pot 2, and in solution .143 gram of ammonium ni-
trate, NH4NOS, and .089 gram of sodium ammonium phosphate,

EXPERIMENTS AND RESULTS FOR 1910 AND 1911

Seven seeds of red clover (inoculated) and seven of rape were
planted in each pot. The germination of the clover was poor, and the
plants in the residue pots (2, 3, and 4) soon died. After about two
weeks the rape plants were turned under.

To Pot 2 was then added .200 gram of potassium sulfate, K2S04,
and small amounts of ammonium nitrate and sodium ammonium phos-
phate. Rape and clover seed were again planted in all the pots several
times, but in the extracted soil the plants failed to live, even in Pot 2,
and further small applications of the plant-food solutions did not keep
them alive. The plants in the normal soil were removed and seed
replanted whenever the other pots were replanted.

The extracted soil was so compact and "run together" that the fail-
ure was clearly due to bad physical condition. To remedy this, a quan-
tity of quartz sand was extracted by the usual method of acid-digestion
described above, and a sufficient amount was mixed with the extracted

BULLETIN No. 182

[May,

soil of each pot to fill two pots. These extra pots were numbered 2a,
3a, and 4a. The soil of each of these six pots was then leached, or ex-
tracted, with distilled water, to remove at least part of the small
amounts of soluble potassium, phosphorus, and nitrogen salts that
might have remained from the previous applications.

Five clover seeds, inoculated with clover bacteria, were then planted
in each pot, 2 of red clover and 3 of alsike. With some replanting, an
irregular stand of alsike was finally secured, but the plants which sur-
vived showed very marked differences, or individuality, in their ability
to develop. At the close of the season of 1910 the highest yield of dry
matter from the tops and roots of the plants grown in the extracted
soil where no potassium fertilizer had been added (Pot 4a) amounted
to 61/2 grams. From analyses of subsequent crops this dry matter was
estimated to contain 221/2 milligrams of potassium, while the total po-
tassium added to this pot (in seed, inoculation, and impurities) had
been only 1.4 milligrams. The smallest yield was in Pot 3a, in which
none of the plants developed much growth. The crop of i/2 gram from
this pot was estimated to contain only twice as much potassium as the
total additions.

Both tops and roots of the 1910 crop were incorporated with the
extracted soil in the respective pots, and clover (chiefly alsike) was
grown again in 1911. A perfect stand was not secured, but the plants
grew very much better than in 1910. The clover tops were harvested
on August 9, and again on December 21, when the roots were also re-
moved, air-dried, and weighed.

The more important data for the two years are recorded in Table 1.

TABLE 1. — POTASSIUM APPLIED AND CLOVER PRODUCED IN POT CULTURES WITH
INSOLUBLE RESIDUE FROM SOIL, 1910, 1911

(Expressed in grams)

Pot
No.

Data for 1910

Data for 1911

Potassium
applied

Dry
produce

Potassium
applied

Air- dry
produce

1

la
2
2a

.0024
.0022
.3155
.3155

No record
8.0506
1.7768
2.8305

.00005
.00006
.00004
.00007

32.76
26.73
30.33
18.79

3
3a
4

4a

.0013
.0013
.0015
.0014

1.4146
.5094
1.6135
6.5951

.00006
.00010
.00009
.00007

18.88
13.08
11.33
21.29

Aside from Nos. 2 and 2a, no pots received any potassium except
that contained in the seed planted, in the inoculating material, and as
impurity in the other plant foods used ; and to Pots 2 and 2a potassium
fertilizer was added only for 1910. The different plants grown in 1911

1915}

POTASSIUM FROM THE SOIL

also showed considerable individuality, some making much better
growth than others. The tops and roots were again incorporated as
organic manure with the extracted soil in the respective pots.

EXPERIMENTS AND RESULTS FOR 1912

In 1912 two more pots were added to the series, Nos. 5 and 5a.
These were filled with extracted1 quartz sand. The same plant-food
materials were added to these pots as were originally applied in pow-
dered form to Pots 2, 3, and 4, but quartz contains no potassium, and
thus differs from the "insoluble residue" from soil extraction.

The entire series was planted on February 12, 1912, with five seeds
of alsike to each pot ; and with some replanting a good stand was se-
cured. The growth was fairly normal during the season. The leaves
were gathered from time to time as they seemed to mature, and all were
added to the final harvest on December 12, 1912, the produce from each
pot being kept by itself. Figs. 1 and 2 show photographic views of
these duplicated series of pot cultures with the 1912 clover crop.

The sand pots, 5 and 5a, were planted and replanted several times,
and the solutions of ammonium nitrate and sodium ammonium phos-
phate were also added, but the plants either remained very small or
finally died, neither pot producing sufficient crop to harvest or analyze.

The 1912 clover tops from all but the sand pots were analyzed, and
likewise the roots from Pots la, 2a, 3a, and 4a ; but the roots from Pots
1, 2, 3, and 4 were cut up and returned to those respective pots before
planting the next crop. In Table 2 are recorded the important data
concerning the 1912 crop.

TABLE 2. — POTASSIUM APPLIED, AIR-DRY CLOVER PRODUCED, AND POTASSIUM FOUND
IN TOPS AND EOOTS ANALYZED, 1912

(Expressed in grains)

Pot
No.

Potassium
applied

Air-dry produce

Potassium found

Tops

Roots

In tops

In roots

1
la
2
2a

.00003
.00003
.00003
.00003

25.56
34.20
34.20
41.07

14.29
12.04
16.30
26.80

.2852
.3310
.1487
.1519

.1109

.0257

3
3a

4
4a

.00004
.00003
.00005
.00007

34.40
29.08
25.25
38.51

13.40
10.47
13.70
17.30

.1406
.1206
.0909
.1324

.0215

.0232

With no addition of potassium fertilizer, the "insoluble residue"
in Pots 3, 3a, 4, and 4a produced a larger average yield (31.81 grams)

Tor this extraction dilute sulfuric acid was used.

BULLETIN No. 182

FIG. 1. — CLOVER, 1912. POTS 1 TO 5 FROM LEFT TO RIGHT

in 1912 than was produced from the normal soil (29.88 grams, aver-
age of Pots 1 and la) .

The total potassium added to Pots 3, 3a, 4, and 4a in seed, inocula-
tion, and impurities amounted to 6 milligrams for the three years 1910-
1912, while the amount of potassium found in the 1912 tops and roots
analyzed from these four pots was 529.2 milligrams, of which 523.2
milligrams, or nearly 99 percent, must have been secured from the
"insoluble residue."

The pots used were about 6 inches in diameter, and the total area
of the four pots was not more than one fifty-thousandth part of an
acre, so that potassium was secured from the "insoluble residue" at
the rate of 26,160 grams, or 57 pounds, per acre. The air-dry hay har-
vested in 1912 from these four pots amounted to 127.24 grams, or to
about 7 tons per acre, while the yield from the normal soil was about
6y2 tons per acre.

The results indicate that after two years of green manuring, suf-
ficient potassium was liberated from the "insoluble residue" to enable
the clover to be benefited by the lime and phosphate fertilizers so as to
outyield the crops on the normal soil, to which no such fertilizers had
been applied.

The clover hay produced on the normal soil contained about three
times as much potassium per gram as was contained in crops from
the "insoluble residue," which indicates that the actual requirement
for potassium by clover may be very much less than has been estimated
from the composition of hay grown on ordinary soils. In other words,
much of the potassium commonly found in crops may not be required
but merely tolerated, being taken up by the growing plants because of
the abundance in the soil.

Thus the facts established by these investigations tend to support
the suggestion made in Bulletin 123 (page 216) and in the Appendix

1915}

POTASSIUM FROM THE SOIL

FIG. 2. — CLOVER, 1912. POTS IA TO 5 A FROM LEFT TO RIGHT

of the County Soil Reports (under "The Potassium Problem"), that
the benefit sometimes produced by potash fertilizers, when applied to
soils very deficient in decaying organic matter, may be due in part at
least to the power of the soluble potash salt to increase the availability
of phosphorus or other elements.

EXPERIMENTS AND RESULTS FOR 1913 AND 1914

As already stated, the only organic matter incorporated with the
soil in preparing for the 1913 planting was the clover roots in Pots 1,
2, 3, and 4, and the same plan was followed for 1914 ; that is, the tops
from all pots and the roots from la, 2a, 3a, and 4a of the 1913 and 1914
crops were used for analysis, the same as in 1912. The important data
for 1913 and 1914 are combined in Table 3.

TABLE 3. — POTASSIUM APPLIED, AIR-DRY CLOVER PRODUCED, AND POTASSIUM FOUND
IN TOPS AND ROOTS ANALYZED, 1913 AND 1914

(Expressed in grams)

Pot
No.

Potassium
applied

Air- dry produce

Potassium found

Tops

Roots

In tops

In roots

1
la
2
2a

.00012
.00015
.00016
.00019

67.73
78.29
51.05
33.38

4.81*
9.821
15.23
6.66

.8758
.7849
.1979
.2526

.0404

.0159

3

3a
4
4a

.00020
.00021
.00012
.00018

50.99
16.84
76.33
43.82

24.80
3.90
25.22
12.72

.1960
.1390
.2909
.2083

.0071

.0158

*The roots in the normal soil (Pots 1 and la) were found largely decayed at
harvest time and could be only partially saved for analysis.

10 BULLETIN No. 182 [May,

The drainage outlet from Pot 3a became clogged, which probably
accounts for the very poor growth, but it was apparent in both 1913
and 1914 that the clover grew better where the roots from previous
crops were incorporated with the ' ' insoluble residue. ' ' However, the
authors have no explanation for the larger amount of potassium recov-
ered from Pot 2a than from 2, unless it was the irregularity of subse-
quent leaching of the potassium applied to those pots in 1910. The
total potassium secured from Pots 3, 3a, 4, and 4a for the two years
1913 and 1914 was 857.1 milligrams, or 428.5 milligrams per year, as
compared with 529.2 milligrams for 1912.

The potassium secured in the clover tops from Pots 3 and 4 was as
follows :

231.5 milligrams in 1912

255.3 milligrams in 1913

231.6 milligrams in 1914

718.4 milligrams in three years

The total amount of potassium added to Pots 3 and 4 from 1910 to
1914 was 3.4 milligrams, so that 715 milligrams were secured from the
"insoluble residue" by the clover tops analyzed from those two pots,
where the roots were turned back each year. This corresponds to 158
pounds of potassium per acre in three years' crops. A 100-bushel
crop of corn contains about 73 pounds of potassium in the grain, stalks,
and cobs.

It seems plainly evident that potassium need not be purchased for
use on normal soils for the production of the staple farm crops, but
that it may easily be liberated in abundance by means of decaying or-
ganic matter, such as green manures, crop residues, and farm manures ;
and, of course, these materials, if applied in sufficient quantity, will
supply nitrogen and liberate phosphorus from the phosphates natur-
ally contained in the soil or applied to it where needed.

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UNIVERSITY OF ILLINOIS-URBANA