POTENTIAL SOIL-WATER EXTRACTION BY SUNFLOWER ON A RANGE OF SOILS

A conceptual framework of the time course of soil water depletion under water limiting conditions was used to quantify water extraction by sunflower (Helianthus annuus L.) on a wide range of soil types. The framework takes account of the maximum plant-available soil water content (MAWC) in each layer, the rate at which the soil water extraction front descends through the soil profile (EFV) and the rate of water extraction within each soil layer (kl). Total plant-available soil water for the profile (TPAW) is defined as the sum for all layers of the difference in volumetric soil water between the drained upper limit (theta(u), determined in a separate infiltration experiment) and the lower limit (theta(l), determined at the end of the experiment after water extraction by the crop). To quantify parameter values, soil water content was measured frequently in sunflower crops grown entirely on stored soil moisture. Measurements were made in an environment with high temperature and vapour pressure deficit to determine the potential rate of water extraction. The parameters theta(u), theta(l) and kl were determined for each 20-cm depth increment for each of five soil types. The rate of progression of the soil water extraction front through the profile (EFV) and time at which the extraction front commences its descent (t0) were also determined for each soil type. The framework described actual water extraction on each soil type well. Values for TPAW ranged from 77 to 210 mm for the five soils. This was a result of differences in maximum depth of extraction (EF(max)) and MAWC. Values for MAWC in the surface were related to clay content and were greatest (24%) at clay contents near 55%. On heavy textured soils, MAWC decreased linearly with depth whereas on soils low in clay content it did not change with depth. The decline was caused by an increase in theta(l) with depth. It is possible that preferred root pathways resulted in a lower than expected water extraction at depth and deeper soil layers were consequently not fully exploited. Therefore, values for TPAW are a combination of plant and soil factors. Maximum depth of water extraction (EF(max)) ranged from 80 to 180 cm. The potential EFV across soil types was 3.6 cm day-1. Values of EF(max) related to either crop factors (time to flowering) or soil factors (decline in MAWC). The extraction rate kl increased linearly with clay content of the soil profile. Cumulative water extraction varied among soil types depending on MAWC, EF(max) and kl.