Agronomic and Physiological Responses of Soybean and Sorghum Crops to Water Deficits II. Crop Evaporation, Soil Water Depletion and Root Distribution

Abstract

The effects of soil water depletion on crop evaporation and root absorption of water were studied in soybean and sorghum crops. Sorghum did not deplete the maximum soil water store by more than 100 mm, whereas rainfed crops of soybeans, cvv. Ruse and Bragg, depleted the soil water store by 130 and 170 mm, respectively. This was sufficient to reduce soybean yields by 35% and hasten maturity in both cultivars when compared with irrigated crops. The post-flowering efficiency of water use by rainfed crops of soybeans was about one-third that of sorghum. The root distribution of Ruse and its pattern of soil water extraction indicated that during bean- fill it was unable to exploit water from much below 80 cm depth, but this effect was offset by its reaching maturity before yield was severely affected by water stress. As Ruse approached maturity, its root densities decreased in soil layers below 10 cm depth, whereas Bragg, which matured 2 weeks later than Ruse, maintained a deep root system and continued to deplete water down to 120 cm. The contrast in root distribution between soybean cultivars also influenced the level of soil water depletion at which crop evaporation fell below the potential rate. Soil and root resistances to water absorption were used to interpret the effects of root density and soil water depletion on water uptake. The regional implications of the results were examined using a water balance model to analyse historical rainfall records. It was concluded that similar soil moisture conditions could be expected about 1 year in 5, indicating that these results have a ready application for irrigation scheduling in this area.