Evaporation is, however, the cause of soil"s losing the greatest amount of water. Soil water is constantly moving toward the surface on account of capillary action, and is being evaporated. This loss by evaporation must be counteracted, if in arid countries or during dry spells agricultural plants are to be provided with sufficient moisture.
Experiment 56. -Fill full of soil four tin cans having small holes punched in the sides and bottom. Water each with the same amount of water. Cover the first with about an inch of grass and the second with about an inch of sawdust, and weigh carefully. Weigh the third and fourth. Record the weight of each. Thoroughly stir the surface of the third, as soon as it is dry enough, about an inch deep. Keep this stirred. Let the fourth stand undisturbed. Weigh all four every school day for two weeks. Keep a record of the loss of weight of each. Why have they lost weight? How do the grass, the sawdust, and stirring of the earth affect the loss? Suggest ways to keep soils from losing their moisture.
In Experiment 56, it was seen that if a layer of grass or sawdust was put on the top of the soil, the moisture did not evaporate as rapidly as it"DRY FARMING" IN EGYPT.
did when the soil was not covered. The grass could have been replaced by shavings, manure, or any substance which would protect the ground from the sun and wind. Protections of this kind are called mulches. They are most frequently used around trees, vines and shrubs. It is impracticable to use them extensively on growing crops.
It was also found that soil water was not readily evaporated where the top of the soil was kept stirred, so that the little capillary tubes by which the soil water reaches the surface were broken and the sunshine and air were kept from the under part of the soil by a layer of finely divided soil mulch. When the surface of the soil is thoroughly stirred or cultivated the particles are separated so far apart that the water cannot pass from one grain to another, and so is retained in the under layer ready for the plant roots. Thorough tillage of agricultural crops is perhaps the best way to assure the plants sufficient moisture in regions subject to droughts.
In some parts of the arid region of the United States dry farming is practiced. The soil is deeply plowed and the plow often followed by a bevel wheel roller called a soil packer, in order to pack the under soil or subsoil so that the air cannot circulate through it and dry out the upper soil. The surface soil is then most thoroughly cultivated so as to make as perfect a soilmulch as possible. Thus, whatever moisture
falls is kept from seeping below the reach
KAFFIR CORN.
A plant suitable for dry farming.
of the plant roots and from evaporating from the surface. In this kind of farming the aim is to use more than one year"s moisture in growing a crop.
Crops are usually planted only every other year, two years" moisture being retained for one crop. The soil is, however, kept thoroughly cultivated all the time. Of course plants requiring the least amount of moisture are best adapted to dry farming.
Irrigation is the most efficient means of raising crops in regions of insufficient rainfall or of droughts. Water is brought to the land from distant sources, or from flowing artesian wells, or is pumped from wells which have been sunk to an available water table. In this wayIRRIGATION IN SQUARES.
water can be supplied to plants whenever needed. Where the ground is quite level it is often flooded, sometimes in larger or smaller squares, with little ridges separating the squares. A great deal of water is lost in this way by evaporation.
Another way is to plow furrows eight to ten inches deep in the direction of the surface slope and run the water into these from the irrigation ditch. In either case the water is allowed to soak in until the soil is thoroughly wet. The surface is then cultivated soas to check surface evaporation.
IRRIGATION IN FURROWS.
In the last few years the government and many private companies have spent millions of dollars in putting in irrigation plants. By this meansthousands of acres of land which would otherwise have been valueless for agriculture has been made exceedingly productive.
47.Alkali Soils. -In dry regions where the rainfall all sinks intothe ground and after remaining for a time rises to the surface and is evaporated, large areas are foundupon which almost nothing can be made to grow even when sufficient water is provided. Often in the dry season white or brown crusts ap- pear scattered over the surface in large patches. The white crust usu-ally tastes like Epsom salts and the brown like salsoda. The salts form-ALKALI SOIL.
Few plants can grow here because of the excess of alkaline salts.
ing these patches have been dissolved out of the soil by the soil water and left on the surface when it evaporated.
Such substances are not found in wet regions because they are carried away by the water which runs into the streams. About the only way soil of this kind can be treated to make it productive is to irrigate and drain it, thus washing the salts out of the soil. This is just what is done by nature in well-watered regions. Sometimes if there is not much alkaliRECLAIMING ALKALI SOIL IN THE SAHARA.
ROMAN PLOWING.
Showing primitive methods.
deep plowing or the planting and removal of certain plants such as sugar beets, which are capable of growing in such soils, will sweeten it.
48.Soil and Man. -Although nature through countless ages hasbeen preparing the soil, and generation after generation of plants and animals has been contributing to its fertility, yet it will not continue profitably to produce agricultural crops unless carefully handled by man. The materials taken from it must be replaced by manures. It must also be thoroughly tilled in order (1) to keep in the moisture, (2) to prepare a mellow place where the roots of the plants may spread, (3)STEAM PLOW.
Showing modern methods.