Availability of Soil Moisture for Active Absorption in Drying Soil.

There is considerable disagreement as to whether or not water is equally available to plants in drying soil over the entire range from field capacity to wilting percentage. Some workers have repeatedly stated that water is equally available over this range (5, 18, 23, 24) while considerable data have accumulated which indicate that physiological processes in plants are altered as a result of decreasing soil moisture content even before the onset of wilting (1, 2, 4, 6, 7, 12, 16, 21, 25). As indicated by KRAMER (15) some of this disagreement exists because of differences in soil moisture tensionsoil moisture content relations for different textural grades of soil. In certain coarse-textured soils most of the water in the range from wilting percentage to field capacity is held with a tension of less than one atmosphere and probably most of it is readily available. In some fine-textured soils, however, less than half of the available water may be held with a force of less than one atmosphere, and water is withheld from plants with appreciably greater energy over the lower part of the available range than over the upper part. Near the wilting percentage a small decrease in moisture content effects an enormous increase in tension. Data also are available which indicate that active absorption and root pressure specifically are depressed by low soil moisture contents above the wilting percentage. LITVINOV and GEBHARDT (17) have reported a qualitative correlation between drying of soil and cessation of exudation from stumps of detopped plants. KRAMER (14) noted that removal of tops of unwilted herbaceous plants was not always followed by expected exudation. However, exudation usually began soon after the soil was watered. Working with coleus, sunflower and tomato, Kramer found that about 45%o of the soil moisture between the moisture equivalent and the wilting percentage was unavailable to detopped plants. He concluded that the active absorption mechanism does not absorb against a diffusion pressure deficit greater than one or two atmospheres. The proportion of moisture in the "available range" over which exudation does not take place was essentially similar for clay, sand, and sandy loam. The uniformity of the results obtained indicated that the soil moisture content which limits exudation is determined predominantly by soil characteristics and not by plant characteristics. Using sunflower plants and a uniform, fertile sandy loam, McDERMOTT (19) found that within a range of moisture content just above the wilting per1Contribution No. 781, Massachusetts Agricultural Experiment Station, Amherst, Massachusetts.