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122. Composition of soils. ВЂ” Soils are made up largely

of insoluble material of no food value for plants. The

amount of actual plant-food in the soil is comparatively

small, but since plants do not use large quantities of this

food, the supply is sufficient for crop production. Hil-

gard has compiled in the following table a great number

of analyses of typical soils.

These analyses show that less than 5 per cent of humid

soils is composed of plant-food and that the remainder is

largely made up of material insoluble even in strong

acid. In arid soils, the proportion of plant-food is some-

what higher, but even there, it comprises less than 10

per cent of the total soil.

The organic matter in humid soils is usually much higher

than that in soils of arid regions; but the low organic

matter of the arid soils is relatively high in nitrogen.

120

The Principles of Agronomy

Table 1. — Chemical Composition of Humid and Arid

Soils. Strong Hydrochloric Acid Analysis В»

Humid Regions

Arid Regions

Average of 696

Average of 573

Samples

Samples

1. Insoluble residue ....

84.17

69.16

2. Soluble sUica (Si02) . .

4.04

6.71

3. Alumina (AI2O3) . . .

3.66

7.21

4. Ferric iron (Fe2Q8) . . .

3.88

5.48

5. Sulfuric trioxide (SO3) . ,

0.05

0.06

6. Manganese (Mn02) . .

0.13

0.11

7. Phosphoric acid (P2O6)

0.12

0.16

8. Lime (CaO)

0.13

0.43

9. Magnesia (MgO) ....

0.29

1.27

ID. Soda (NazO) .....

0.14

0.35

11. Potash (K2O)

0.21

0.67

12. Humus

1.22

1.13

123. The analysis of soils. — In order to determine

the plant-food in a soil, the chemist takes a sample to a

laboratory, where he analyzes it. He does the sampling

very carefully, since the accuracy of the analysis depends

on the accuracy of the samples. If, for example, he

should analyze just the surface inch, his results would not

apply to the lower depths of the soil where roots often

feed ; frequently the soil varies much in composition at

these different depths. Again, if the sample should be

taken from a low place containing considerable organic

matter, it would not represent the entire field. In sam-

pling, therefore, the chemist takes soil from a number of

places in the field and at various depths and mixes all

together in order to get an average sample for analysis.

After the sample has been prepared, the method of

analysis depends on the information desired. If the total

PlanUfood of the Soil 121

plant-food is to be detennined, the soil is treated with

certain acids which dissolve the soluble matter, after

which the chemist can detennine the quantity of the

various elements in it.

124. Available and reserve plant-food. — Only a small

part of the total plant-food of the soil is available to

crops during any one year. Roots penetrate every part

of the soil, but they can absorb only material that is in

solution. Through the carbon dioxide which they give

off, the roots assist in dissolving the minerals of the soil.

Their action is slow, consequently only a small portion of

each compound can be used in any one year. This is

very fortunate, since, if all plant-food were readily dis-

solved, it would be leached out by rains or floods. The

potassium found in such minerals as mica becomes avail-

able only after years of weathering, while that in kainit

can be immediately dissolved. It is impossible, there-

fore, from a chemical analysis, to tell how much of a given

element is available to plants for immediate use without

knowing in what minerals it is contained.

125. Making plant-food available. — The making avail-

able of reserve plant-foods as fast as needed by crops is

one of the chief problems of soil management. This is

done (1) by tillage, which aids weathering agencies in

their action on soil particles; (2) by drainage, which

allows air to circulate more freely through the soil; (3)

by plowing under organic matter, which, in decaying,

helps to make the minerals soluble ; and (4) by numerous

other less important means. The nitrogen present in

the soil is made available by nitrification, which is favored

by tillage and by a desirable moisture content.

126. Quantity of plant-food removed by plants. —

Each crop uses plant-food in varying quantities. The

quantity of mineral foods taken from the soil by different

122

The Principles of Agronomy

crops is expressed by Warington in the following table, which

includes the material found in the entire harvested crop.

Table 2. Mineral Foods removed from the

Soil by Crops

Total

Crop

Yield

Ash

Nitrogen

Potash

Lime

Phosphoric

Acid

Wheat . .

Barley . .

Oats . .

Maize . .

Meadow hay

Red clover

Potatoes .

Turnips

30 bu.

40 bu.

45 bu.

30 bu.

UT.

2T.

6T.

17 T.

172 lb.

157 lb.

191 lb.

121 lb.

203 lb.

258 lb.

127 lb.

364 1b.

48 1b.

48 1b.

551b.

43 1b.

49 1b.

102 lb.

471b.

192 lb.

28.8 lb.

36.7 lb.

46.1 lb.

36.3 lb.

50.9 lb.

83.4 lb.

76.5 lb.

148.8 lb.

9.2 lb.

9.2 lb.

11.6 lb.

32.1 lb.

90.1 lb.

3.4 lb.

74.0 lb.

21.1 lb.

20.7 lb.

19.4 lb.

18.0 lb.

12.3 lb.

24.9 lb.

21.5 lb.

33.1 lb.

The table shows the variation in the relative quantities

of nitrogen, potash, lime, and phosphoric acid used by

different crops.

127. Plant-foods that are scarce. — Of the ten ele-

ments required by plants, only three may be considered

as scarce. These are nitrogen, phosphorus, and potas-

sium. In a few soils calcium and sulfur may be deficient,

but they are usually present in sufficient quantities to

supply the needs of crops for centuries.

Nitrogen is, without doubt, the element most likely to

be lacking in soils, and it is the most expensive element

when purchased; but the fact that it can be added to

the soil by the growth of leguminous plants makes its

maintenance possible in every soil. Phosphorus, which

is used in large quantities by the grain crops, is present

in exceedingly small quantities in many soils. On this

account, it becomes necessary to use phosphorus fertilizers

PlanUfood of the Sail 123

in order to maintain the fertility of these soils. Potas-

sium is usually present in fairly large quantities^ but

since it is, in the main, not available to plants, soils usually

respond to potassium fertilizers. It is probable, however,

that proper methods of increasing the availability of

reserve potassium will do much toward making unneces-

sary the heavy use of this fertilizer.

128. Exhaustion of the soil. — The possible exhaus-

tion of the soil has been discussed for many years;

niunerous different opinions have been held. Some have

contended that the plant-food supply is rapidly being

used up and that it will not be long before the soil is so

impoverished that crops will not grow. Others have

maintained that the soil is being constantly renewed and

as a result will never be exhausted. Experience has

demonstrated, however, that, if the productivity of the

soil is to be maintained at a high standard, part of the

plant-food removed by crops must be returned either as

farm manure or as commercial fertilizers. Since plant-

food is rendered available but slowly, it is probable that

crops never can entirely exhaust the soil. A lessened

supply of available food, however, greatly reduces yields

of all crops.

129. Losses in plant-food result primarily from the

removal of crops from the land, but in regions of heavy

rainfall large quantities are also removed by leaching and

by surface washing. In some of the limestone areas of

the eastern part of the United States, the rock and soil

have been leached so much that the greater part of the

original material has been removed, leaving only the more

insoluble minerals. Naturally, during this process the

more available compounds have, been carried away. In

many sections, surface erosion is responsible for the

destruction of much valuable land. The soil is, in some

124 The Principles of Agronomy

cases, washed entirely away, while in others, the main

part is retained, but the soluble material is leached from

the surface.

130. Plant-food in organic matter. — The organic

matter of the soil is composed almost entirely of dead

plants in various stages of decomposition. These dead

tissues contain a quantity of mineral matter that has

been once in solution, and is, therefore, more likely to

be available to growing plants than the minerals. Nitro-

gen is particularly important in this connection, since

practically all of the nitrogen of the soil is found in the

organic matter. Besides furnishing directly a part of the

plant-food, organic matter assists, by its decay, in render-

ing available the mineral matter of the soil.

131. Relation of plant-food to value of a soil. — In

order that a soil may be valuable, it must have an ample

supply of plant-food ; but this is by no means the only

consideration. Farmers sometimes submit a small sample

of soil to a chemist with the request that he analyze it

and tell what the land is worth. Those who are familiar

with soil study understand that it is impossible by merely

knowing the total quantity of plant-food to tell the exact

value of any land. Such questions as drainage, aeration,

mositure supply, texture, and many other things help

to determine what a soil can produce. All these factors

must be taken into consideration in estimating the value

of land and in outlining methods of management.

SUPPLEMENTARY READING

Soils, Lyon, Pippin, and Buckman, pp. 327-374.

Fertilizers and Crops, L. L. Van Slyke, pp. 105-116.

The Soil, P. H. King, pp. 107-134.

Soils, E. W. Hilgard, pp. 313^21.

Physics of Agriculture, P. H. King, pp. 69-106.

Pirst Principles of Soil Pertility, A. Vivian, pp. 3-46.

CHAPTER XII

MANURES AND FERTILIZERS

Plants require for their growth an available supply

of various mineral foods. These should be present in

the soil in a balanced condition in order that the plants

may find the most congenial environment. Crop yields

are decreased if any one of these necessary elements is

present in exceptionally small quantities. Even if all

other conditions are favorable, the producing power of

most soils could be materially increased by simply chang-

ing the available supply of one or two elements. The

addition of a fertilizer may effect this readjustment.

Where crops are raised continuously on land and re-

moved each year, a certain amount of plant-food is carried

away. An unreplenished deposit of money in the bank,

no matter how large, will in time be exhausted if contin-

ually drawn out. The plant-foods in the soil may be

considered in much the same way, and while this analogy

is not entirely true, yet the same principle holds.

Some soils contain a very great store of plant-food, but

even such will not continue to endure abuse without

protesting by giving reduced yields. If a permanent

system of agriculture is to be maintained on any soil,

no matter how rich, at least a part of the mineral matter

that is removed must be returned either in the form of

farm manure or commercial fertilizers.

125

126 The Principles of Agronomy

Materials are often applied to the soil for their indirect

action as well as for the plant-food which they add.

Fann manure improves the physical condition of the soil ;

lime corrects acidity and flocculates the particles of fine

clay ; other fertilizers help to render available the reserve

store of plant^food in the soil.

132. Types of fertilizers. — The materials added to

the soil either as direct or indirect fertilizers are numer-

ous. By far the most important of these is farm manure,

which is composed largely of animal excreta mixed with

litter. Of the fertilizers purchased from the outside, the

most common are those applied for the nitrogen, phos-

phorus, and potassium they contain. These are usually,

but not always, in the form of mineral salts. Other

materials called amendments are used for their indirect

action on the soil rather than for the direct plant-food

they furnish. In addition to these substances, which

must be hauled to the soil, it is a common practice to

grow certain crops which serve a similar purpose. These

are usually the legumes. In order for them to be of use

as a fertilizer, they are plowed under.

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