Soil Moisture Studies of Some Great Plains Soils: II. Field Capacity as Related to 1/3-Atmosphere Percentage, and “Minimum Point” as Related to 15- and 26-Atmosphere Percentages

Values for field capacity and the “minimum point” were abstracted under arbitrary standards from a large number of soil samples taken over a period of 30 years at 11 locations. Field capacity was correlated with the ⅓-atmosphere percentage and the minimum point with 15- and 26-atmosphere percentages. Spring wheat was the principal crop grown although at some locations winter wheat and rye were included. Although there was a highly significant correlation between field capacity and the ⅓-atmosphere percentages, the regression coefficient was approximately 0.5 for all depths except the surface. The relatively low values for field capacity were attributed to depth of wetting which occurred under intermittent and limited rainfall. A detailed study of field capacity was conducted at two sites where abstracted values of field capacity had been reported previously. Hydraulic head curves indicated that downward movement of moisture practically ceased 4 days after irrigation at Site 1. Similar curves at Site 2 indicated that drainage continued for 10 days after the profile had been thoroughly wetted to a depth of 6 feet. A comparison of 1/10- and ⅓-atmosphere retention values, determined on undisturbed soil cores, indicated that the moisture content at field capacity corresponded more closely to the 1/10-than to the ⅓-atmosphere percentage. Soil moisture tension values from tensiometers at 5 locations observed 4 days after irrigation ranged between 44 and 166 cm. of water for a 6-foot profile. Tension values generally decreased with depth at each location and varied within the same soil at a given depth. Some of the variability was attributed to the presence of coarse-textured layers within the profile. There was a highly significant correlation between the minimum point and the 15- and 26-atmosphere percentages especially in the upper 2 to 4 feet. The greater scattering of points at the lower depths was attributed to limited root distribution at these depths. The data indicated that wheat plants were capable of absorbing moisture at tensions which exceeded 26 atmospheres at depths where roots were well disseminated. The practical significance of these data is discussed.