Differences in soil bulk density and porosity caused by tillage influenced changes in nutrient concentrations and yield of sweet potato. Although untilled zero tillage soil had higher soil nutrient concentrations and water status than conventional tillage, but these did not translate into its having higher yield due to higher bulk density. Conventional tillage improved nutrient availability and enhanced induction and development of storage tuber yield of sweet potato because of reduced bulk density.
Combined application of NPK fertilizer and poultry manure at sub-optimal rates with any tillage method ensured more availability of major nutrients in soil and increased sweet potato leaf nutrient (N, P and K) concentrations and tuber yield compared with full rates of NPK fertilizer or poultry manure alone. Poultry manure or NPK fertilizer increased soil fertility and leaf nutrient concentrations and tuber yield compared with control, but soil conditions and leaf nutrient concentrations were better in poultry manure than NPK fertilizer. Conventional tillage in combination with NPK fertilizer plus poultry manure gave the highest tuber yield of sweet potato and therefore recommended for sweet potato in the forest-savanna transition zone of Nigeria for nutrient availability and productivity conservation.