Restorative Potential of Residual Soil Amendments and One-Year Fallow on Top Soil Chemical Properties of a Tropical Ultisol

Quantitative information on the contribution of residuals of soil nutrient management on natural nutrient recovery capacity of soil through fallowing is scanty. To evaluate the contributions, soil samples were collected from soils amended with 50 tha-1 boiler ash (BA),10 tha-1 poultry droppings (PM10), 150 and 300 kg ha-1 NPK fertilizer, combinations of BA and PM or NPK and a control. The samples were collected at the end of first and second year of maize cultivation and after one year of fallowing to assess changes in pH, total nitrogen, available phosphorus, % organic carbon, exchangeable bases (Ca, Mg, Na, K), cation exchange capacity and % base saturation. The study showed that type and quantity of soil nutrient recovered through fallowing significantly (p=0.01) differed in respect to the type of amendment previously applied. High pH level (7.0) and increase in Ca obtained in plots treated with BA50 either sole or combined was sustained up to the end of the fallow year. . Recovery of organic carbon and available P was more in plots with residuals of BA50 + PM10 while, nitrogen (0.26%) and exchangeable K (0.26 cmolc Kg-1) were obtained in BA50 +NPK150. The highest CEC and % BS values (23.85 cmolc Kg-1 and 76.7%), respectively were obtained from the control plots. Significant fallow effect on soil pH occurred only in NPK treated plots as % OC depleted in NPK300 and BA50 +PM10. Compared to the second cropping season, one year of fallow on the average resulted in an increase in the top soil pH, % OC, N, Ca, K and %BS; but reduced available P, Mg, Na and CEC. A combination of BA and PM or NPK had synergistic effects that exceeded the effect of applying them alone.