Soil Moisture Regime

the aggregate of phenomena responsible for the entry, movement, consumption, and utilization of soil moisture by plants. It is the most important factor in soil formation and fertility. The main source of soil moisture is atmospheric precipitation. Nearby ground water sometimes also plays an important role, and irrigation is of great significance in some agricultural regions. Some atmospheric precipitation and thaw water forms surface runoff, and some enters the soil, where it is consumed by plants. Deep autumn plowing across slopes impedes surface runoff and helps retain and improve the intake of thaw water. Atmospheric precipitation, thaw, and irrigation water penetrate the soil because of its permeability. The more large (noncapillary) intervals there are in the soil, the higher the permeability. Permeability is particularly important for the intake of thaw water. If soil freezes in the fall when very wet, its permeability is usually very slight. Thaw water readily enters the soil in forests, where the vegetation protects the soil against severe freezing, or on fields where snow retention measures are taken early. The entry of moisture into the soil from groundwater depends on the depth of the groundwater and on the water-raising capacity of the soil. Groundwater in clay soils rises considerably—as much as 4 m—by capillary action, but very slowly. It rises more quickly, but not as high, in coarse soils.

The amount of moisture in soil is usually expressed as a percentage of the dry soil mass (gravimetric moisture) or of the volume of intact soil (volumetric moisture). The water supply in soil is expressed in cubic meters per hectare or in millimeters of the water layer. Soil moisture may be vaporous, liquid, or solid (ice). The amount of water vapor in soil air is usually close to full saturation and moves under the influence of temperature differences—from the warmer to the colder layers. The mobility and availability of moisture to plants depend on the bond it forms with the solid soil particles, the size and structure of the soil pores, and the extent to which and manner in which the pores are filled with water. A distinction is made between water that is bound, held by sorption forces, and water that is free and present in the soil pores but not exposed to sorption forces. Bound (sorbed) water is retained on the surface of the soil particles by very great force; such water is virtually unavailable to plants. Free soil moisture may be gravitational—that is, it moves chiefly under the influence of gravity and capillary forces. Above the groundwater lies the capillary fringe, whose moisture readily moves under the combined influence of capillary forces and gravity; such moisture is readily available to plants. The retention of moisture in the capillary fringe corresponds to the capillary water capacity of the soil. If the groundwater is deep, there is an isolated zone of perched water in the upper part of the soil. The maximum amount of this water corresponds to the minimum water capacity of the soil. Some of the moisture in this zone is also available to plants. The capillary and minimum water capacities of the soil have great agricultural significance because they determine the maximum quantity of reliable soil moisture supply (field moisture capacity).

Plants may dry out the soil to such an extent that they begin to wilt. This degree of moistening is usually called the soil moisture content of permanent wilting, and soil moisture above wilting moisture is called productive moisture. All the moisture above the minimum moisture capacity percolates to the upper boundary of the capillary fringe and from there to the groundwater level, from which the outflow takes place by way of the water-impermeable bed. The difference in moisture content between complete saturation and minimum moisture capacity is called the water discharge of the soil. The amount of water discharged varies from 5 percent (in loam and clay soils) to 20-25 percent (in sands).

The water content of the soil determines the technological processes to be used in the cultivation of the soil, the supply of plants with water, and the physiochemical and micro-biological processes responsible for the transformation of nutrients in the soil and their entry into plants with water. Therefore, one of the main tasks of agriculture is to create a moisture regime favorable for crops. This is done by the accumulation, preservation, and rational consumption of soil moisture and, when necessary, by irrigating or draining the land.

The soil moisture regime depends on the soil properties, climatic and weather conditions, and characteristics of natural plant formations and, in cultivated soils, on the characteristics of the crops grown and methods of cultivation used. Maintenance of a stable crumbly structure is important in creating a favorable soil moisture regime. Timely sowing and fertilization also promote the efficient use of soil moisture by the crops. It was found that when fertilizers are properly used, the crops consume less water per centner of dry plant weight—that is, with fertilizers one can decrease the unproductive consumption of moisture by the crops. Likewise, shelterbelts help to reduce the unproductive consumption of soil moisture by crops in arid regions by moderating the force of the wind and increasing the relative moisture content of the ground layer of air on the fields bordered by them.

Seven types of soil moisture regimes are distinguished: cryogenic, leaching (permacidous), periodic leaching, nonleaching (impermacidous), transpiration-seepage, seepage, and irrigation. The cryogenic regime develops in regions where the rocks are permanently frozen. It is characterized by the presence at some depth of a permanently frozen layer above which perched water forms in the summer. The’leaching regime is one in which the soil returns less moisture to the atmosphere than it receives. (The excess moisture percolates down to the groundwater.) The leaching regime is peculiar to the taiga zone, which has podzolic, sod-podzolic, and podzolic-bog soils. With the periodic leaching-type regime, less moisture is returned to the atmosphere than comes from it only in certain years; such a regime is typical of the forest-steppe zone, which has gray forest soils. With the nonleaching regime, the amount of moisture returned to the atmosphere is about equal to that reaching the soil from precipitation. Precipitation does not soak the soil to its entire depth, for a layer with a constantly low moisture content (close to wilting moisture), called the dead horizon of desiccation, forms between the soaked soil layer and the capillary fringe zone. It is found in the steppe zone (which has chernozems and chestnut soils) and in semideserts. The transpiration-seepage and seepage moisture regimes occur in arid regions where the soils are fed not only by atmospheric precipitation but also by moisture from shallow groundwater. The transpiration-seepage moisture regime occurs when the rising groundwater is almost entirely intercepted by plant roots. With the seepage regime, the groundwater reaches the soil surface and evaporates; this often results in salinization. The irrigation regime is created when land is irrigated. Repeated irrigations soak the soil to the depth penetrated by the roots and sometimes—when the soil has to be flushed to get rid of excess salts—even deeper.

The soil moisture regime is regulated to keep enough productive moisture in the root-inhabited layer throughout the growing season. In regulating soil moisture, it is very important that some of the soil pores remain filled with air, which is needed for the life of the plants and normal activity of the microorganisms. This is accomplished by a variety of cultivation and meliorative methods.