11. Capillary conductivity, which is the effective flow velocity when the water-moving force is one g, changes rapidly with suction. This physical property makes possible a quantitative treatment of soil moisture phenomena as observed in the field.

The capillary conductivity function should play an increasingly useful role in understanding and applying available information to the moisture problems that enter into soil management. Clay soils, for example, have low conductivity at low suctions compared with sandy soils, but at high suction values the fine-textured soils have much higher capillary conductivity than the coarse-textured soils.

Consider the problem of the salinization of surface soil from a shallow ground water table. The movement of water from a water table at a depth of I meter in a sandy soil in response to surface evaporation may be completely negligible, whereas, in a soil like the Pachappa, the capillary conduction of water to the soil surface from a 1 -meter water table is more than adequate to maintain an evaporation rate that is limited only by the external evaporative conditions. That is, the evaporation of water from the soil surface would be at the same rate as from a free water surface. The movement of water to the soil surface from a shallow water table can now be predicted from theory, and the theoretical approach will play an increasingly useful role in field operations relating salinity, leaching, and drainage.

SOIL WATER MEASUREMENTS, for management purposes, involve two aspects.

One is the volume or quantity of water present in the soil that can be used by crops. This information has been obtained in the past by oven-drying soil samples to determine the quantity of water present. Information on the wilting point or the minimum to which the soil moisture is reduced by the crop is necessary for each individual sample in order to determine the available water. Field samplings of soil-moisture content are still widely made and are quite useful for guiding management practices, particularly in dry-farm areas. Newer methods involving neutron scattering are now being used in experimental work and may prove to be feasible in practical agricultural operations.

12. Readings of two instruments, plotted against time, indicate film flow in the tensiometer suction range. One tensiometer cup (solid line) was in soil permeated by roots. In adjacent soil, another tensiometer was enclosed in a guard such that no roots came closer than 2 1/2 cm. to the tensiometer cup (dashed line).

The other aspect of measuring soil moisture relates to the physical status of the water in soil. In saline soils, this must involve the concentration of the soil solution as expressed in terms of osmotic pressure, but in nonsaline soils, a measurement of soil suction gives an indication of the tenacity with which the water is held and an indication of the availability of the water to plants. Because it gives a direct measure of soil suction, and also because of the very large significance of suction readings to the hydraulics of soil water, the tensiometer will always be a standard soil-moisture instrument.

Tensiometers are commonly used in irrigation agriculture as an aid in irrigation control. The question sometimes is raised as to the representativeness of a tensiometer reading and the extent to which the reading is disturbed by plant roots at the surface of the cup.

Some information on this point is given by the curves in the accompanying figure that show the reading of a tensiometer in soil directly in contact with roots and the reading of a tensiometer in the same root zone where the tensiometer was enclosed within a larger porous cup so that no plant roots came within 2.5 cm. of the tensiometer cup.

The curves indicate that for this sandy loam the reading of the tensiometer that was protected from direct root action increased almost as rapidly as the nearby tensiometer, which was subjected to direct root action. The capillary conductivity for this soil in the tensiometer range of suction apparently was adequate to move water for some distance during the interval between irrigations.

In sandy soils having low contents of silt and clay, the capillary conductivity decreases rapidly as the suction increases. The suction developed in such soils at the plant roots where the moisture is absorbed is not transmitted rapidly to the cup, and wilting symptoms may develop before the tensiometer attains its maximum reading. Tensiometers will work reliably only up to suctions of about 850 millibars. This limitation must always be kept in mind, but apparently there is still a wide range of usefulness for these instruments in research on the control and measurement of moisture.

Several types of electrical resistance blocks give readings that in general respond to the soil water suction. In other words, in nonsaline soils, an electrical resistance unit comes to about the same reading, independent of the kind of soil, when a certain value of the suction is attained. There is still considerable room for improvement in instruments to measure moisture.

The subject of soil moisture is growing and developing. The flow and distribution of water in soils can be described in terms of hydraulic laws and intrinsic soil properties. Better instruments for measuring soil moisture condition and soil properties are still needed, but progress toward more efficient utilization of water in agriculture will be accelerated as the basic laws and principles relating to soil moisture become more widely used.