Soil Moisture

L. A. Richards and S. J. Richards.

The efficient use of the available supply of soil moisture is usually a major aspect of soil management. It may dominate crop selection, time and rate of planting, tillage operations, weed control measures, and programs of fertilization.

Soil moisture is seldom ideal for best crop yields. Some soils are too wet, even for a part of the year, and artificial drainage may be profitable. In some places a lack of soil moisture limits crop yields, and irrigation or moisture-conserving practices are used.

Statistical studies for semiarid and subhumid climates usually show a significant correlation between effective precipitation and crop yields. Irrigation often is feasible, even when the investment is considerable. Without irrigation in semiarid and sub-humid regions, soil-management practices strongly emphasize the efficiency of use of the available moisture supply.

In humid climates, if other factors are favorable, deficiency of soil moisture may limit crop growth oftener than is generally realized. The possibility of a significant deficiency depends on the crop and the soil as well as the climate. Studies of evapotranspiration in relation to climate indicate that yield-depressing droughts occur with almost statistical regularity in many climates, even though the average precipitation during the cropping season may exceed the total amount needed by the crop.

Supplemental irrigation in humid climates is often feasible because of proximity to a water source and is becoming more widely practiced.

Technicians have gained much information on the effect of soil-management practices in determining the amount of water that enters the soil and the amount that runs off and is lost by surface drainage. Terracing, contour furrows, contour cultivation, and basin listing are used for reducing runoff. Crop residues are left to protect the soil surface from puddling and evaporation. All cultural practices affecting soil structure, at the surface and below, are involved. These and other subjects related to infiltration, erosion, and evaporation have a direct and controlling effect on soil moisture, and are treated in later chapters. Chapters are devoted also to irrigation and drainage. This discussion deals with the general principles relating to the retention and conduction of water by soil principles that apply to soil management under a variety of conditions of climates, crops, and soils.

1. A system for measuring the water content of soil in relation to the suction supplied by a hanging column of water.

THE RETENTION of water by soil is related to the size and arrangement of the soil pores: In the soil pore system water moves and is retained for plant use. The ratio of the volume that is not occupied by soil particles to the bulk volume of the soil is called the porosity.

Fine-textured soils tend to have higher porosity than coarse soils when all of the pores are filled with water, a fine soil usually contains more water than a coarse soil. Sandy soils tend to have a preponderance of large pores. Clay soils, which contain many fine particles, tend mainly to have small pores. During and following the entry of rain and irrigation water, sandy soils with their large pores usually conduct water more rapidly than fine-textured soils. For that reason and because they contain less water to begin with, sandy soils retain less water for plant use.

If a bucket of soil is connected to a water reservoir as at A, the soil will approach saturation. When the reservoir is lowered to B, some (but not all) of the water drains back into the reservoir. The soil retains a certain amount, depending on interconnection and size distribution of pores in the soil. With such an arrangement, each lowering of the reservoir will produce an additional outflow of water from the soil. A layer of permeable material with fine pores, such as a porous ceramic plate, can be placed in the bottom of the bucket to keep air from getting into the suction line. The suction of the vertical column of water extending down to the free water surface in the reservoir is counterbalanced after each outflow increment by an increased suction developed in the water films remaining in the soil. This suction depends on force action at the soil and water surfaces.

The suction required to empty a soil pore depends on its size. The volume of water retained in a soil at a given suction is therefore related to the volume of pore space smaller than a given size. The water content of a soil at a specified suction and structure is called retentivity. The water retention curve is based on a number of retentivity values. Such curves provide a picture of pore size distribution.

The terms "soil suction" and "soil moisture tension" have identical meaning and may be used interchangeably. In this example, the suction is controlled, and the soil moisture makes a corresponding adjustment. Under field conditions, the soil moisture content changes and the suction changes accordingly. Suction is reduced when water is added to the soil by rainfall or irrigation and, conversely, suction is increased when soil moisture is depleted by drainage, root absorption, or evaporation.

Suction is related to the relative wetness of soil and, over a restricted range, can be measured with a tensiometer. This is a standard soil moisture instrument consisting of a porous cup, which is connected to a vacuum gage and filled with water. Commercial instruments usually employ a Bourdon gage, as shown at the left in the illustration.

2. Tensiometers use dial gages, mercury manometers, or water columns for measuring soil suction. The length H on the water manometer represents suction head in the soil adjacent to the cup.

A mercury manometer, mounted above the ground on the tensiometer tube, often is employed for suction readings in experimental work. The tensiometer unit on the right in the drawing is shown with a water manometer attached. Suction is read directly from the gage or scale in pressure units and sometimes is called soil moisture tension. Pressure in the manometer at A is the same as in the porous cup at the same level, so the suction in the cup, and in the soil adjacent to it, is equal to that produced by the vertical length of water column from A to B. This equivalent length of water column H is called suction head and is a convenient measurement for relating suction differences to the water-moving force in soil. If the porous cup of a tensiometer were installed in the soil bucket of the preceding figure, the instrument would give a reading corresponding to the height H.

Suction values are often expressed in terms of cm. (centimeter) of water column. A larger unit is sometimes more convenient. The barometer is used for measuring atmospheric pressure and the bar is a metric unit of pressure, which is approximately equal to the pressure of the atmosphere at sea level. One bar corresponds to 1,021 cm. of water column at 20 C. One millibar therefore corresponds closely to 1 cm. of suction head.

MOISTURE RETENTION CURVES show the relationship between soil moisture and suction.

Moisture content may be expressed in various units. Moisture percentage on a dry basis is commonly used and is the weight of water per 100 units of weight of dry soil. For field applications, expressing soil moisture content as the water ratio is convenient. This is the volume of water per unit bulk volume of soil and is numerically the same as the surface depth of water per unit depth of soil. (The water ratio is determined by drying a known bulk volume of soil or by multiplying the moisture percentage, dry weight basis, by 1 percent of the bulk density of the soil when the latter is expressed in grams per cubic cm.) The equivalent surface depth of water in a soil depth interval is found by multiplying the soil depth by the average value of the water ratio in the interval. This is convenient because rainfall and irrigation water are also expressed in terms of the equivalent surface depth of water.