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 fraction of soil water that can be used by the plants. Generally, there is no decrease in plant activity until the soil water has been decreased to the level where only about 0.3 of the water that can be extracted by the plants remains in the soil. There is, however, ambiguity as to whether this relationship is appropriate for sandy soil. The objective of this research was to compare methods for describing extractable soil water by soybean [Glycine max (L.) Merr.] plants grown in pots with soils differing in sand content. The two methods tested were based on thermodynamic and physiological definitions of the upper and lower end-points of extractable soil water. While differences existed in the 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 soil water of the upper end-point either thermodynamically (commonly assumed to be -10 kPa for sands) or physiologically (pot capacity). As a consequence, there were large differences in the response curves obtained based on the two expressions of extractable soil water. The threshold for transpiration expressed as an absolute volumetric soil water was of course independent of the determination of the end-points. Instead of relying on a relative transpiration response to extractable soil water, for sandy soils it my be necessary to use the volumetric soil water content determined under controlled conditions to interpret crop response under any particular field situation