Three tillage methods, conventional till, minimum till and deep till versus no till and their respective interaction with three fertilizer levels (0, lx and 2x the recommended amount) were evaluated in a crop rotation that included yam (.Dioscorea alata L.), eggplant (Solanum melongena L.), bean (Phaseolus vulgaris L.) and corn (Zea mays L.). The experiment was established at three ecologically different locations with three different soil orders: Ultisol, Oxisol and Vertisol. Yam, the first crop in the rotation, was planted after soil preparation according to the treatments. The crops that followed in the rotation were planted no till in the same plots. A fifth treatment, in which all crops in the rotation were planted under conventional tillage, was used as a check. Yield response of the rotation crops varied with location. Yam yields in the no till plots were significantly lower than in the conventional till plots in the Ultisol and Vertisol soils, but not in the Oxisol soil. Response to fertilizer was observed only in the Oxisol. Yield response of eggplant with respect to the tillage treatments was similar to that observed for yams, thus suggesting that there was a residual effect of tillage. For the bean and corn crops, the third and fourth crops of the rotation, the residual effect of the tillage practices performed at the beginning of the rotation cycle was little if any.


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