EXTRACTABLE SOIL-WATER AND TRANSPIRATION RATE OF SOYBEAN ON SANDY SOILS

Substantial consistency has been obtained in describing plant response to drying soil by expressing plant performance as a function of the f raction of soil water that can be used by the plants, Generally, there is no decrease in plant activity until the soil water has been decrea sed to the level where only about 0.3 of the water that can be extract ed by the plants remains in the soil, There is, however, ambiguity as to whether this relationship is appropriate for sandy soils. The objec tive of this research was to compare methods for describing extractabl e soil water by soybean [Glycine max (L.) Merr.] plants grown in pots with soils differing in sand content, The two methods tested were base d on thermodynamic and physiological definitions of the upper and lowe r end-points of extractable soil water, While differences existed in t he volumetric soil water content for the two definitions of the lower end-point of extractable soil water, these differences were small, In contrast, very large differences existed in defining the volumetric so il water of the upper end-point either thermodynamically (commonly ass umed to be -10 kPa for sands) or physiologically (pot capacity), As a consequence, there were large differences in the response curves obtai ned based on the two expressions of extractable soil water. The thresh old for transpiration expressed as an absolute volumetric soil water w as of course independent of the determination of the end-points, Inste ad of relying on a relative transpiration response to extractable soil water, for sandy soils it may be necessary to use the volumetric soil water content determined under controlled conditions to interpret cro p response under any particular field situation.