Soil Moisture Tension and Pore Space Relations for Several Soils in the Range of the “Field Capacity”

E term "field capacity" is here used to indicate 1 the moisture content of the soil after from one to several days of natural drainage following a thorough soaking by rain or by irrigation. The usefulness of the moisture equivalent as an index of this "field capacity" has been demonstrated both in arid and in humid areas. With soils of arid regions it has been shown that the two values are essentially equal at moisture equivalents of about 20%, and with humid soils they correspond at slightly higher values. At lower moistures, i. e., with very sandy soils, the field capacity is recognized as being distinctly higher than .the moisture equivalent. At high values, i. e., with clays, the field capacity tends to be slightly lower than the moisture equivalent. Browning (i) has indicated these relationships and has discussed the literature on the subject. The moisture equivalent is .theoretically equal to about 500 cm of water column tension .(8). This has often been considered as the actual tension on soil water at field capacity. Water in the coarser pores has been visualized as filling and draining out each time the soil is soaked by rain or by artificial means. It has been shown that for certain soils the moisture equivalent corresponds to an actual porous plate tension of about y$ atmosphere, 340 cm of water (6). Also, in arid soils, a comparison has been made between field capacity and the Y$ atmospheric tension by the method found to correspond with the moisture equivalent (3). These determinations corresponded at approximately 25% moisture. Field capacity was higher at the low moisture contents, lower at the high contents than the Yz atmosphere value. It has been suggested that the relation of field capacity to the moisture equivalent or to some fixed tension is as much a matter of the extent of the filling of soil pores as it is of drainage (11). Further, it has been pointed out that irrigation often fails to wet soil up to its field capacity (4). Soil tensiometers have been developed which measure the pull on soil water, and these have been used to follow the changes that occur under natural conditions (6, 7, 9). In the present study, field tensiometers, essentially as described by Richards (6), have been used to determine the soil moisture tensions on the same or similar soils to those used (i) in establishing a relation between field capacity and the moisture equivalent. The study has been designed to test the theory that field capacity under our conditions is as much a matter of pore size distribution, wetting, and "unsaturation" as it is of drainage.