Soil Moisture Studies of Some Great Plains Soils I. Field Capacity and “Minimum Point” as Related to the Moisture Equivalent

Field determinations of soil moisture were made on land either continuously cropped to wheat or alternately fallowed and cropped to wheat, to depths up to 6 feet for periods up to 30 years at 31 stations or locations in the Great Plains. These determinations were made at intervals throughout the period of seedbed preparation and the life of the crop. Two of the major points of interest that developed were the quantity of water that the soil could continue to hold against the pull of gravity, and the portion of it that could not be used by the wheat crop. Late in the 1930's data on these two points were abstracted on all plots — 121 — where enough data were available to make the abstractions reliable. The moisture equivalent was used as an expression of texture in examining these data. The results were assembled but not published. Recently these field determined values have been compared with laboratory vlaues obtained through recently-developed techniques. (Part II of this paper). Their relation to the moisture equivalent is presented partly as a background for these other studies. The moisture equivalent had a good but not an exact relationship to the field capacity throughout the range of soils studied. The regression lines established showed that there was a slight reduction in field capacity with depth, probably due to the manner of wetting. There was a fair agreement between the extent to which wheat reduced the moisture content of the soil, and the moisture equivalent to the depth to which wheat rooted freely. Below a depth of 3 feet the extent to which wheat could remove water was affected by increasingly sparse root development that differed on the several soils. The moisture equivalent served as a means of showing the extent to which actual field reduction fell short of the level established by the soil texture. The quantity of water between the field capacity and the minimum point for wheat decreased from the surface downward, due to a slight reduction in the field capacity and an increase in the quantity of water that wheat could not remove. Results are applicable to dry land conditions in the Great Plains and would not apply to soils wetted by irrigation or by a more continuous or adequate water supply.