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CONSERVATION TILLAGE AND SUBSOILING EFFECTS ON COTTON

D.H. RICKERL and J.T. TOUCHTON

Auburn University, Agronomy and Soils Department, Auburn, AL 36849 (U.S.A.)

In the southeastern U.S.A., conservation tillage systems reduce soil erosion and conserve moisture during droughty summer months. However, dense subsoils may prevent adequate root growth for tap-rooted crops such as cotton. In 1984, field studies were begun on a sandy loam soil in the Coastal Plains of Alabama. Cotton planted into wheat cover with conventional (chisel plow-disk) and no-tillage systems was split into subsoiled and non-subsoiled plots. Starter fertilizer was placed 2x2 or in the subsoil track and included applications of 0-0-0, 15-0-0, 15-15-0 and 15-15-15. Seed cotton yieldslwere generally higher in no-till than in tilled plots (4956 and 4864 kg ha , respectively), but the largest increases came from subsoiling. Til~d plots averaged 5032 and 4572 kg ha- , and no-till plots 5075 and 4769 kg ha seed cottton for subsoiled and non-subsoiled plots, respectively. Averaged over tillage and subsoil treatments, starter fertilizer improved yields by approximately 5%.

RESPONSE OF SORGHUM TO GROWING SEASON PRECIPITATION WITH CONSERVATION TILLAGE

P.W. UNGER, J.L. STEINER, and O.R. JONES

USDA-ARS, Conservation and Production Research Laboratory, P.O. Drawer 10, Bushland, TX 79012 (U.S.A.)

In studies with dryland grain sorghum (Sorghum bicolor (L.) Moench)_~n the semi-arid southern Great Plains, grain yields increased by 17.0 kg ha per additional mm of stored soil water above a threshold amount needed to initiate grain production under conditions of clean tillage. Studies where residues from a previous wheat (Triticum aestlvum L.) crop were managed by conservation tillage instead of clean tillage methods, have shown greater soil water storage during fallow and subsequently higher sorghum grain yields. Yield increases were primarily attributable to higher soil water content at sorghum planting, but the results also indicated that greater yield responses to preclpiatation resulted when residues remained on the surface during the sorghum growing season. The objective of this study was to evaluate the contribution of growing season residues to sorghum yields. Relationships among soll water contents at planting, precipltation~ and sorghum yields were determined f~r field plots having low (<i t ha-t), medium (1-2.9 t ha-l), or high (>3 t ha ) levels of wheat residue on the soll surface at planting. Data for the analyses were obtained from studies conducted at Bushland, Texas, from 1972-1984. Linear regressions indicated that grain yields increased by 14.6, 23.3, and 25.5 kg ha per ram of precipitation for the period from planting to floral initiation with the respective residue levels. Based on the period from floral initiation to the end o~ anthesis, indicated grain yield increases were 11.6, 17.2, and 19.3 kg ha per mm of precipitation. These results confirm that crop residues not only enhance soil water storage at planting, but also the effectiveness of growing season precipitation for increasing grain sorghum yields.