162. Reasons for rotation of crops. ВЂ” Some sort of

crop rotation has been practiced for many centuries.

The reasons for this practice were probably not at first

understood, even to-day all the effects of alternate crop-

ping are not known ; but so many reasons are now known

that there seems no good excuse for not practicing some

kind of rotation on almost every farm. All crops do not

require the various foods in exactly the same proportions ;

some use more potash or nitrogen, while others need rela-

tively more phosphorus or lime. If one crop is grown

continuously on the same land, the available supply of

certain elements is reduced and the yield will finally

decrease; but if crops with different requirements are

alternated, the food supply of the soil is kept in a more

balanced condition. Each kind of plant has a different

rooting system and manner of growth. If shallow-rooted

crops are grown continuously, only part of the soil is used,

while an alternation of deep- and shallow-rooted crops

overcomes this difficulty.

One of the chief reasons for crop rotations is the im-

provement of the soil. This is made possible by the use

152 The Principles of Agronomy

of legume crops, which fix nitrogen from the air {Fig. 45).

The nitrogen fixed by these crops can he used by others

which follow in the rotation, but it would be practically

lost if the legumes were raised continuously. The control

of plant diseases, insect pests, and weeds is made possible

by the rotation of crops; indeed, such considerations

often cause the farmer to change his crops when he would

not otherwise do so. Economy in the use of man-labor,

horse-labor, machinery, and irrigation water results

FiQ. 45. — Every rotation should include a nitrogen-gathering crop.

from the raising of a number of crops on a farm. These

considerations alone, without any of the other benefits,

would be sufRcient reason for practicing rotations.

163. Methods of crop rotation. — Careful planning is

required in making a good rotation. The first e-ssential

is to decide on what crops can best be grown under the

conditions. When this is done the quantity of each crop

to raise and the placing of it can be determined.

The following principles should be kept in mind in

Tillage and Crop Rotations 153

planning a rotation: (1) raise about the same acreage

of each crop every year ; (2) have at least one cash crop ;

(3) include a legiune crop in the rotation ; (4) alternate

tilled and non-tilled crops ; (5) alternate deep- and shal-

low-rooted crops; (6) alternate exhaustive and restora-

tive crops; (7) follow the best sequence of crops; and

(8) add manure to the right crops in the rotation. It is

not always possible to conform to all of these principles,

but they are useful guides.

SUPPLEMENTARY READING

Soils, S. W. Fletcher, pp. 46-188.

Soils, Lyon, Pippin, and Buckman, pp. 663-681.

Cyclopedia of American Agriculture, Vol. I, pp. 372-398.

Physics of Agriculture, F. H. King, pp. 223-253.

Crops and Methods for Soil Improvement, A. Agee, pp. 149-158.

The Fertility of the Land, I. P. Roberts, pp. 61-107, 356-372.

U. S. D. A. Farmers' Bulletins :

No. 245. Renovation of Worn-out Soils.

326. Building up a Run-down Cotton Plantation.

421. Control of Blowing Soils.

CHAPTER XV

SPECIAL SOIL PROBLEMS

Every region has certain special soil problems not found

in other places. Some of these are merely local ; others

apply to a comparatively large area. On each farm, soil

conditions are found that are not identical with those

found on other farms of the same neighborhood. These

special conditions make it necessary for each fanner to

study his own soil in order to solve the problems which

it presents. It is not possible to discuss, or even to under-

stand, all the special soil problems.

ALKAU

In arid regions, there are millions of acres of land con-

taining excessively high quantities of soluble salts which

are usually spoken of as alkalies. The soil is rendered

valueless by these salts if they are present in quantities

that prohibit crop growth. Many soils, however, con-

taining considerable alkali will raise good crops until

strong concentrations of salt are brought near the surface

by the evaporation of large quantities of water. In

judging arid soils, it is necessary to know the amount of

soluble salts present and their relation to the quantity

causing injury to crops. In the management of such soils,

the farmer should know how to prevent the accumula-

tion of salts in the strata of the soil that is used ; and in

154

Special Soil ProhUms 155

regions where large quantities of alkali are already pres-

ent, he should know how to reclaim the land. Effects of

alkali on vegetation are shown in Figs, 46 and 47.

164. Kinds of alkali. — Any soluble salt present in

the soil in injurious quantities may be considered an

alkali. The salts that most often cause injury are : sodium

chloride, or common salt ; sodium sulfate, or Glauber's salt ;

sodium carbonate, or sal-soda ; and magnesium sulfate, or

FlQ. 46. — Alkali spot with vegetatian idlled.

epsom Salt. In addition to these, sodium nitrate and a

number of other salts do damage in some districts.

Sodium chloride is injurious to vegetation when present

in lower concentrations than any of the other salts men-

tioned; sodium carbonate, or black alkali, injures the

soil when present tn low concentrations by dissolving the

organic matter and causing a hanl crust to form. Plants

will grow in the presence of relatively large quantities of

the sulfates.

156 The Principles of Agronomy

166. Effect of alkali on plant growth. — The injury

done to vegetation by alkali salts results largely from the

shutting off of water from the plant on account of the soil

solution having a greater concentration than the plant

cells. By the law of osmosis water passes from the dilute

to the more concentrated solution. In a normal soil, the

root has a cell-sap with a higher concentration than the

soil solution ; hence, water passes from the soil into the

Fio. 47. — An orchard being killed by the rise of alkali.

plant. When the soil solution is made too concentrated,

on the other hand, water passes out of the roots into the

soil and the plant dies.

166. Reclamation of alkali lands. — The permanent

reclamation of alkali lands rests on a removal of excessive

salts by drainage. Other means may give temporary

relief, but drainage is the only certain cure. In draining,

the principles discussed in Chapter X are to be followed.

Where the accumulation of alkali results from over-

irrigating higher lands, the remedy is obviously the pre-

vention of percolating water which carries soluble salts

Special SoU Problems 157

from above and concentrates them in the lower lands.

Any practice which reduces evaporation, such as cultiva-

tion, cropping, or the use of manure, tends to reduce the

accumulation of these salts.

ACIDITY

Most crops require for their best growth an alkaline,

or basic, reaction, although some grow better if the soil

is slightly acid. Such important crops as the legumes can

hardly be made to grow on an acid soil, since the bacteria

which fix nitrogen in connection with growth on the roots

of these crops require a basic reaction. Acid soils are

most often found in humid regions where the basic ele-

ments of the soil-minerals have been leached out, leaving

the acid part behind, and in swamp lands where the decay

of large quantities of vegetable matter results in the ac-

cumulation of organic acids. The continuous applica-

tion of ammoniun sulfate as a fertilizer to cultivated soils

also finally results in an acid condition.

167. Indicators of soil acidity. — An acid soil is indi-

cated by the growth of a number of plants, among which

are common sorrel, sour dock, horsetail, and corn spurry ;

also by the failure of alfalfa and other legumes to do well.

Blue litmus paper and a number of laboratory tests may

be resorted to in determining acidity and the amount of

lime necessary to correct the condition.

168. Correction of soil acidity. — Acidity is best

corrected by the use of some form of lime ; and acid soils

usually pay handsomely for the expense of applying lime.

Swamp lands high in organic matter often contain so much

acid that it does not pay to correct the sour condition,

especially since these soils usually contain an abundance

of nitrogen. The kind of lime to use depends on condi-

158 The Principles of Agronomy

tions; burned lime and ground limestone both accom-

plish the result. Ground limestone, however, is usually

cheaper and if fine enough, it is very effective. Ground

limestone also has an additional advantage of destroying

less organic matter than the burned, or caustic lime.

EROSION

One of the chief difficulties with which farmers of certain

sections have to contend is the erosion of the soil, during

which fertility is washed out and at times the entire soil

carried away. Some erosion goes on normally in all parts

of the world ; indeed, it is by erosion that canons and

ravines have been formed. It is much more intense,

however, on land that is under cultivation. Many factors