Some Moisture Relations of Soils from the Erosion Experiment Stations

In this study of a number of moisture constants of samples from fifty-two layers comprising eleven soil profiles of erosion stations of the United States Department of Agriculture, a search was made for relationships between the moisture constants themselves; also between the constants and other physical and chemical characteristics for which data had been obtained by others (l, 2, 5). The centrifugal moisture was determined by centrifuging four-gram soil samples in a small, air driven centrifuge of the moisture equivalent type at 300,000 gravity. The normal moisture capacity was determined by sampling carefully packed soil columns, eighteen inches long and initially dry, after they had been wetted at one end and then stood on end in the dark for seven days. The quantity of water added was insufficient to wet the entire soil columns. The average moisture content of the nearly uniformly wetted portion of the column was taken as the normal moisture capacity. The minimum water of saturation (3) was found by determining the lowest moisture content at which a glisten of free-water could be obtained upon the surface of a soil sample when manipulated to produce closest packing. The sticky point (4) was taken as the moisture content at which the plastic soil mass barely failed to stick to a steel roller when operated at a standard shearing speed of 5 cm per second. For purposes of comparison, determinations were also made at a shearing speed of 10 cm per second. It was found that sticky points determined by adhesion tests usually do not agree with those obtained by shearing tests. With the procedure used, only plastic soils are sticky. The lower plastic limit was determined by the method of Atterberg, care being taken to drive .the moisture content down to the lowest value at which the soil mass could be made to roll out into a filament. The original data obtained are too voluminous to present but some conclusions derived from them will be given. The centrifugal moisture at 300,000 gravity is lower than the permanent wilting percentage but slightly higher than the water vapor absorption at 99 per cent relative humidity. It closely parallels the latter and appears to furnish as reliable a .measure of the colloid content of the soil. Centrifugal fields of 300,000 gravity probably are too high to furnish a reliable measure of the permanent wilting percentage. The normal moisture capacity is about equal to the moisture equivalent except in the case of sandy soils. For these soils the normal moisture capacity is the higher, and for this reason appears to furnish a more reliable measure of field capacity than does the moisture equivalent. The sticky point and the lower plastic limit of acid soils are lowered by the addition of sodium or ammonium hydroxide. The relationships of the moisture constants to mechanical composition are expressed by the following equations obtained by the method of least squares.