INFLUENCE OF TILLAGE PRACTICE AND NITROGEN AND PHOSPHORUS FERTILIZER RATES ON CROP YIELDS IN THE TROPICAL SAVANNA

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Abstract
A 3-year study of the effects of different tillage practices (comprising two no tillage systems, five reduced-tillage systems, and a conventional tillage system) and two N and P fertilizer rates on maize (cv. TZB) and cotton (cv. Sam 71) yields was conducted on a tropical ferruginous soil. The N rates were 99 and 33 kg N/ha for maize and 49.5 and 16.5 kg N/ha for cotton. The rates of P were 27 and 9 kg P/ha (maize) and 13 and 4.5 kg P/ha (cotton). Maize grain and cotton lint yields were significantly (P .ltoreq. 0.05) influenced by tillage practice and fertilizer rate. However, maize yields over the 3 yr under CT and NT did not differ significantly, while cotton yields under the two tillage treatments were significantly different at the high fertilizer rate only. Except for maize yields under MT and DR, and cotton yields under DI, maize and cotton yields under CT and the reduced tillage systems did not differ significantly. This suggests that the additional cost of deep-plowing under conventional tillage could be saved without sacrificing yields, particularly for cotton. Crop yields under NT were higher (P .ltoreq. 0.05) than under ZT (no tillage that was previously under conventional tillage), and increasing the fertilizer application rate failed to improve yields under ZT to yields at low level under NT. Nutrient distribution was significantly influenced by tillage practice in the 0-5 cm depth but not in the lower depths. However, the level of fertilization did not influence nutrient distribution in any soil depth, except for organic C content in the 0-5 cm and available P content in the 5-20 cm depth.