SOME SOILS ARE DROUGHTY

99

Moisture Supply Better in Silt Loams Than in Sand.-Sandy soils give up their water much more easily and completely than silt loams and clays, nevertheless the latter soils generally furnish to crops a much better moisture supply.

The figures in the last column of the table seem to indicate that crops would suffer less for want of water on a coarse sand than on a light colored silt loam during a drought. On the contrary, crops suffer much more on the sand, for two main reasons:

FIG. 42.-A droughty soil. A fine loam about 14 inches deep underlaid by coarse sand and gravel.

(1) Sand gives up its water more easily than silt loam and hence plants are more lavish with it.

(2) In the coarse sand, roots cannot secure moisture by capillarity from depths below the root zone.

Thus it is that corn on sand grows faster than on a silt loam and shows no injury because of lack of rain during the beginning of a dry period, but suffers much for want of water later on as the dry weather continues.

Some Soils are Droughty. Soils which are unable to furnish crops with sufficient moisture during short dry periods are called droughty"-a deep coarse sand is a good example (Fig. 42). Often the best appearing loam or silt loam proves droughty, because it is underlaid at a shallow depth by a coarse and porous subsoil,

such as coarse sand or gravel. During a dry period the main source of moisture that crops can draw on is the capillary water stored in the shallow surface stratum, since no water is supplied to the seed bed by capillarity from lower depths. Furthermore, roots extending into a gravelly subsoil can secure but little moisture. In contrast to this, a silty clay subsoil, for example, furnishes much water to the seed bed, and to the deep roots extending down into it. Moreover, a seed bed underlaid by a fine textured subsoil can hold more capillary water than if the subsoil were sand or gravel.

In buying a farm it is important to examine into the nature of the subsoil to ascertain whether or not it can furnish the seed bed with any capillary water and supply the deep roots with sufficient moisture; and whether or not it can permit percolation sufficient at least to favor proper drainage.

MOISTURE CONSERVATION AND CONTROL, WITH SPECIAL REFERENCE TO HUMID FARMING

The loss of water from rainfall may occur by running off the surface of the soil, by percolating through it, and by evaporating from it. Roughly speaking, only about twenty-five to thirty per cent of the rainfall in a humid climate is used by crops-the rest is lost (Fig. 43). Moisture conservation and control consists (a) in preventing surface run-off as much as possible; (b) in increasing water-holding capacity of soils; (c) in aiding capillary rise of soil moisture; (d) in lessening the loss by evaporation either from the soil or through weeds; (e) in draining wet lands, and (f) in supplying water through irrigation.

How Run-off May be Lessened.-Loss of rainfall through surface run-off may be much lessened by aiding soils to catch or trap water. This may be accomplished in several ways: (a) by fall plowing; (b) deep hillside plowing; (c) by loosening up any hard and compact soil and subsoil; (d) by plowing at right angles to the slope or hillside, and (e) by terracing. Any one of these methods permits more of the rainfall to soak into the ground, and that which is not stored or retained by the soil passes on down and away. In some sections, and especially on hillsides, the storage of water by the soil during late fall, winter and early spring, or at any other time, is greatly hindered, because no provision is made whereby soils can catch the rainfall. Any other method whereby soils may be aided in "trapping" and storing water from rainfall is worthy of consideration.

How Water-holding Capacity May Be Increased. Since the amount of water retained against gravity by most soils is determined largely by texture, organic matter and structure, it is plainly seen that the water-holding capacity of a sand, for example, cannot be increased by improving its texture. Texture of a sand, or of any soil, remains practically the same, and the structure of a sand cannot be materially changed in a short time. Thus the only course open, in case of sand, is to increase the organic matter.

In case of a "heavier" soil, water-holding capacity may be improved by increasing the organic matter and by loosening it if it is very compact. The addition of organic matter also tends to develop a crummy structure which is favorable to water-holding.

Organic matter may be increased by plowing under clover or green rye, by plowing under a good sod as often as possible, through the application of farm manure and through the application of peat, if convenient.

Aiding Capillary Rise of Water. Good contact between the soil particles favors capillary rise of water. This is an important reason why a firm seed-bed is generally desirable. Not only should the seed bed be firm, but there should also be good contact between the seed bed and the subsoil-in this respect fall plowing is advantageous, also working the land and rolling it.

A seed bed too loose and lacking good contact with the subsoil is not a favorable environment in which to plant seeds, especially if they be small like clover and alfalfa. If frequent rains do not occur when such conditions exist, germination is poor and the crops may be failures.

Special attention should be given to compacting the seed bed when deep plowing is done just before seeding or planting-particularly when the soil is sandy or loamy.

Subsoiling heavy soils to enable them to trap and store more water is to be recommended in some instances, but subsoiling land having sand or sandy subsoil is to be discouraged, since it is desirable to maintain compactness of a sand subsoil to favor capillary rise of soil moisture, to retard loss of water through percolation, and to lessen excessive leaching.

Lessening Evaporation of Soil Moisture.-Under field conditions losses through evaporation may be decreased more or less by developing a top layer of loose, dry soil. Such a protective layer is called a "soil mulch" (Fig. 44). A soil mulch may be developed and maintained throughout the growing period only in

HOW A SOIL MULCH CONSERVES MOISTURE

103

fields planted to crops like corn, potatoes, cotton, or any other crop planted in rows so as to permit of intertillage.

What Constitutes a Good Soil Mulch.-On silt loams, clay loams and clays a good mulch consists of a layer of loosened and dry soil composed either of crumbs or a mixture of crumbs and small lumps. A dust mulch is undesirable because it becomes a hard crust when a hot sun shines upon it after a rain, whereas a crummy layer does not crust and bake so easily.

A good mulch on sandy soils consists of a loosened and dry layer of sand

FIG. 44.-How a soil mulch conserves moisture. The footprints are kept moist because the soil moisture is permitted to move to the surface and evaporate. After awhile the footprints will become hard and dry, while the soil beneath the mulch will be mellow and moist.

In a humid climate it is not generally necessary to make a soil mulch deeper than three inches. Many are very effective when only an inch or two in depth, especially in gardens and on heavy soils.

How a Soil Mulch Conserves Moisture. The protective action of a soil mulch in lessening the loss of moisture is based on three principles: (1) The looseness of the soil hinders the movement of capillary water from soil particle to soil particle. (2) The dry surfaces of the soil particles and crumbs offer resistance to the upward movement of water films. (3) A surface layer of dry soil keeps the soil below it somewhat cooler, thus lessening the tendency of the soil moisture to evaporate. When the surface layer of a soil is loosened, therefore, the capillary rise of moisture is