Irrigation for crops in a sub-humid environment

Summary

Six irrigation treatments were imposed on two soybean cultivars grown on a grey cracking clay soil in two successive seasons. Irrigation treatments were FF — irrigation after 90 mm of calculated evapotranspiration (or 40% depletion of available moisture), MM — after 135 mm (60%), II — after 180 mm (80%) and NIL (rainfed). Two additional treatments, FM and MF combined different irrigation frequencies before and after mid-flowering.

Irrigation treatments 11 and NIL had less leaf area and total dry weight, but the distribution of dry weight between leaves, stems and pods was identical for all irrigation treatments in both seasons within a cultivar. It was found that net assimilation rate on a unit leaf area basis was less affected by irrigation treatment than was leaf expansion. Treatment FM had the greatest apparent redistribution of dry weight from leaves and stems to pods.

Frequent irrigation during late flowering and pod filling (i.e. FF and MF) was necessary to ensure maximum yield (up to 3600 kg ha⁻¹) in all seasons. Yield reduction in the NIL treatment was up to 58%. Number of seeds m⁻² was the single most important yield component affected by irrigation treatment. It was concluded that one or two fewer irrigations could be applied during the vegetative phase than is the present commercial practice and the water saved used to grow a larger area of crop.