Residual Effects of Crop Rotations on Water Intake, Soil Loss, and Sorghum Yield

The duration of the residual effects of crop rotations on the fertility and physical condition of montmorillonitic clay soil is not well established. The purpose of this study was to evaluate the residual effects of three crop rotations on water intake, soil loss, soil organic matter, and crop yield during five growing seasons of continuous grain sorghum (Sorghum bicolor (L.) Moench) grown on Houston Black clay soil. Water intake and soil loss were evaluated in the laboratory by the application of water drops at the rate of 10.2 cm/hour from a height of 1 m to the surface of undisturbed soil cores. Oats (Avena sativa L.)-grain sorghum and fescuegrass (Festuca arundinacea Schreb.)-grain sorghum rotations significantly (5% level) increased water intake and reduced soil loss for 20 months after the rotations were discontinued. The sweetclover (Melilotus alba Desr.)-grain sorghum rotation had no effect on soil loss but had a significant (5% level) residual effect on water intake for 8 months after rotations were discontinued. Grain sorghum rotations with either fescuegrass or sweetclover caused significantly more (5% level) water stable aggregates. However, one growing season of grain sorghum following sweetclover reduced aggregation to near that of soil in continuous grain sorghum. There was no significant change in aggregation in fescuegrass rotation plots after 20 months. Most of the organic matter accumulated in 10 growing seasons of a sweetclover-grain sorghum rotation was lost after 5 growing seasons of continuous grain sorghum. Soil organic matter in former grain sorghum rotations with fescuegrass or oats showed nonsignificant change after 5 years of continuous forage sorghum. Grain sorghum yield was significantly greater (5% level) for four growing seasons on the former sweetclover-grain sorghum plots than on the continuous grain sorghum plots. Nitrogen fertilizer had a significant (5% level) residual effect on continuous grain sorghum yield for 2 years. The residual effects of previous crop rotations can significantly affect (5% level) water intake and soil loss for 20 months and crop yield for 1 to 4 years after rotations are discontinued. Growing sorghum continuously for 5 years reduced water intake significantly and caused a continuous slow decrease in soil organic matter.