Water extraction by grain sorghum in a sub-humid environment. I. Analysis of the water extraction pattern

Abstract

Under water-limiting conditions, water extraction by a dryland crop is limited by the depth of the root system, and by the rate and degree of water extraction. The water extraction pattern of 6 crops of grain sorghum under continous soil drying in a sub-humid sub-tropical environment was analysed in terms of two components: the rate of descent of the extraction front down the soil profile (extraction front velocity), and the time required to extract 90% of the extractable water from each depth after the extraction front arrived (1/kl90). Extractable water content (θa), at each depth, was defined as the difference between the stable water content (θ) at the start of extraction and the lower asymptote of the exponential decay curve of θ versus time (lower limit). The crops varied in genotype, level of evaporative demand, degree of tillering, and plant population density, and were grown on the same soil type over two seasons. The aim of the study was to test the stability of the extraction front velocity, θa and of 1/kl90 under different agronomic and environmental conditions, to assess their usefulness for modelling water extraction of sorghum.

Extraction front velocity varied little among the 6 crops with an overall average of 3.43 cm day−1. The extraction front of crops that were grown under long, terminal drying cycles continued to descend until early grain-filling, reaching a maximum depth of 190 cm. The value of 1/kl90 in the upper 100 cm of the profile varied considerably across crops. It is shown that this variation can be explained by variation in root length density and level of evaporative demand. For crops exposed to a long, terminal drying cycle, the actual water extracted below 150 cm depth was less than θa in the soil layers above 150 cm. This was due to both a lower θa below 150 cm, associated with low root length density, and also insufficient time between the arrival of the extraction front and maturity, for the crop to extract all the water above the lower limit.