Clay soils in the midsouthern US are usually driest immediately after harvest in late August through early October because plant roots have extracted water from deep in the profile and rainfall is normally low during the late July through October period. Dry soil allows effective deep (15 to 18 inches) tillage. The question is whether or not to subsoil clay soils intended for soybean production.
Fall deep tillage apparently increases water infiltration into the soil profile during the high-rainfall winter and spring months. The benefits are more pronounced on clay soils such as Tunica and Earle than on Sharkey clay. The loamy subsoil texture of Tunica and Earle has more available water than does the constant clay texture of a Sharkey profile. Thus, soybeans growing on Tunica and Earle clay soils will benefit from the increased water availability in the subsoil that results from deep tillage.
The following points serve as a guide for making decisions about fall deep tillage of clayey soils.
For an irrigated production system, do not deep till. Irrigated yields from an early-planted crop will give higher net returns than deep tillage with irrigation regardless of soil type.
For a nonirrigated production system, the following soybean crop must be planted early (late March through mid-April) using early-maturing varieties for maximum subsoiling benefits to be realized.
For less responsive soils such as Sharkey, fall subsoiling should be a breakeven proposition in the worst-case scenario. However, the consistent low yield response (< 5 bushels/acre) will make energy price the controlling factor. In the best-case scenario, fall subsoiling may increase net returns in April but not May plantings. Again, energy price will be a controlling factor. With soils such as Sharkey, planting must be early and prices must be relatively high to expect a consistent positive effect from subsoiling.
Fall subsoiling will not overcome the yield-reducing effects of extreme drought regardless of soil type. It will offer benefit where a short drought period is alleviated by following rain.
Fall subsoiling will not enhance control of perennial weeds such as redvine and johnsongrass when glyphosate-resistant varieties are used. Thus, continuous use of shallow tillage followed by planting glyphosate-resistant varieties and using glyphosate should keep perennial weeds in check. Fall subsoiling results in worse redvine and johnsongrass weed infestations if conventional varieties are used continuously.
Fall deep tillage will result in soft and spongey soils that may preclude early planting the following spring. In order to maintain or enhance conditions for early planting of soybeans following fall subsoiling, surface smoothing should be done in the fall to minimize the number of large holes that may contribute to ponding of water before spring planting. Beds can be formed in the fall following surface smoothing.
For soils that are responsive (Tunica, Earle, and other soils with loamy subsoil), deep-tilling every three years is just as effective as deep-tilling every year. This reduces long-term tillage costs.
High energy prices may offset the benefits of deep tillage unless yield responses are high. High yield responses have only happened on soils such as Tunica and Earle with a following early-planted soybean crop. In fact, as stated in my last column, a bedded system with minimum fall tillage used only to refurbish beds may be the most attractive option to cut energy costs, ensure early planting, and ensure the highest consistent yield over the long term. (Reports with supporting data for this article can be found at http://msucares.com/pubs/index.html , keyword tillage).