Contributions of soil and crop factors to plant available soil water capacity of annual crops on Black and Grey Vertosols

Improved methods for field measurements of plant available soil water capac ity (PAWC) of Black and Grey Vertosols in Australia's north-eastern grain r egion were employed to characterise 83 soil-crop combinations over 7 depth intervals to 180 cm. Soil sub-order was shown to influence all components o f PAWC (means of 224 and 182 mm in Black and Grey Vertosols, respectively) with drained upper limit (DUL), bulk density (BD), and crop lower limits (C LL) showing clear separation between soil sub-orders and a trend with soil depth. In addition to soil sub-order and soil depth effects, CLL showed cro p effects such that expected PAWC of various crops when adjusted for soil s ub-orders were: cotton 240 mm; wheat 233 mm; sorghum 225 mm; fababean 209 m m; chickpea 197 mm; barley 191 mm; and mungbean 130 mm. A total of 549 meas ured CLL values were used to develop a predictive model for estimating CLL from the soil sub-order, depth, DUL, and crop by predicting a CLL as a func tion of DUL and a depth-dependent variable for each crop-soil sub-order. Th e model CLL = DUL * (a + b * DUL) explained 85% of observed variation in th e measured data with no significant bias between observed and predicted dat a. While properly measured data would be more reliable than estimated data, where specific site accuracy is less critical, this model may be used to e stimate PAWC with an acceptable degree of accuracy.