Tillage and fertilizer influenced soil characteristics through their effects on soil properties which in turn affect crop performance. Field experiments were conducted during 2006 and 2007 cropping seasons on an Alfisol (Oxic Tropuldalf) at Owo in the forest-savanna transition zone of Nigeria to evaluate the effect of tillage and fertilizer types on soil properties and sweet potato yield (Ipomoea batatas L.). Two tillage treatments namely: zero tillage (ZT) and conventional tillage (CT) were combined with each of NPK-15-15-15 fertilizer at 250kgha-1, poultry manure (PM) at 10tha-1, combined application of 125kgha-1NPK 15-15-15 fertilizer+5tha-1poultry manure and a control (no fertilizer/manure). The experiment was laid out on a randomized complete block design in a factorial combination of eight treatments that were replicated three times. The surface soil (0-15cm depth) was chemically analysed before and after crop harvest and some selected soil physical properties, leaf nutrient concentrations, growth and sweet potato yield were determined. Conventional tillage plots had significantly lower soil bulk density (1.30Mgm-3) than zero tillage plots and resulted in higher leaf N, P, K, Ca and Mg and tuber yield of sweet potato compared with zero tillage. Soil water content was higher, whereas temperature was lower, in zero tillage than conventional tillage. Zero tillage had significantly higher concentrations of soil pH, organic C, N, P, K, Ca and Mg for surface soil (0-15cm depth). Conventional tillage increased mean tuber yield of sweet potato by 30% compared with planting on untilled zero soil. Application of poultry manure alone and complementary application of NPK fertilizer and poultry manure reduced bulk density and temperature and increased soil water content and porosity, whereas NPK fertilizer did not improve soil physical properties. Compared with control, poultry manure alone, NPK fertilizer alone and complementary application of NPK fertilizer and poultry manure increased soil total N, available P, and exchangeable K, Ca and Mg concentrations as well as leaf N, P, K, Ca and Mg concentrations. Poultry manure tended to improve soil pH, SOC, N, Ca and Mg compared with NPK fertilizer. Combined application of sub-optimal rates of NPK fertilizer and poultry manure gave higher soil and leaf N, P and K concentrations, vine length, vine girth, leaf area and tuber yield compared with NPK fertilizer or poultry manure alone. Compared with control, NPK fertilizer, poultry manure and NPK fertilizer plus poultry manure increased tuber yield by 39, 45 and 83%, respectively. Conventional tillage in combination with NPK fertilizer plus poultry manure gave the highest tuber yield of sweet potato. © 2010 Elsevier B.V.
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