Effect of Soil Water Content and a Growing Season Straw Mulch on Grain Sorghum

The results of previous studies with a straw mulch in place during fallow and the growing season of grain sorghum (Sorghum bicolor L. ‘Moench’) suggested that having the mulch in place during the growing season increased the use efficiency of growing season rainfall. The objective of this study was to evaluate the contribution of a growing season straw mulch to growth, yield, grain quality, water use, and water-use efficiency of grain sorghum. Before sorghum was planted in 1977, 1978, and 1979, areas of Pullman clay loam (fine, mixed, thermic Torrertic Paleustolls) were irrigated twice, irrigated once, or not irrigated to simulate high, medium, and low levels of water storage in soil during fallow. After sorghum emergence, wheat (Triticum aestivum L.) straw was placed on the surface at rates of 0 (check), 2, 4, or 8 metric tons/ha. Differences in sorghum response to the high and medium water levels were slight because the second irrigation resulted in relatively little additional water storage in the slowly permeable soil even though the soil was not filled to capacity. Sorghum with the high and medium water levels grew taller, yielded more, and used water more efficiently than sorghum with the low water level at planting. In general, sorghum responded more to soil water content at planting than to mulch rate during the growing season. When significant responses to mulch rate were obtained, they resulted mainly from mulch on the low water level plots. For the 3 years, the growing season mulch at 8 metric tons/ha increased water-use efficiency 19% over the no-mulch treatment, which was less than expected, based on earlier experiments and observations. Apparently, shading from the plant canopy largely substituted for the beneficial effect of a mulch during the growing season. When a mulch is present during both the fallow and the growing seasons, a major effect with respect to water conservation and crops production, therefore, is to enhance water storage in soil during fallow.