EFFECTS OF MANURE AND FERTILIZER ON GRAIN YIELD, SOIL CARBON AND PHOSPHORUS IN A 13-YEAR FIELD TRIAL IN SEMI-ARID KENYA

Abstract

Long-term indicators of soil fertility were assessed by measuring grain yield, soil organic carbon (SOC) and soil Olsen phosphorous for a P-deficient soil. In one set of treatments, goat manure was applied annually for 13 years at 0, 5 and 10 t ha⁻¹, and intercrops of sorghum/cowpea, millet/green gram and maize/pigeonpea were grown. Yield depended on rainfall and trends with time were not identifiable. Manure caused an upward trend in SOC, but 10 t ha⁻¹ manure did not give significantly more SOC than 5 t ha⁻¹. Only 10 t ha⁻¹ manure increased Olsen P. Measurements of both SOC and Olsen P are recommended. In another set of treatments, manure was applied for four years; the residual effect lasted another seven to eight years when assessed by yield, SOC and Olsen P. Treatment with mineral fertilizers provided the same rates of N and P as 5 t ha⁻¹ manure and yields from manure and fertilizer were similar. Fertilizer increased Olsen P but not SOC. Management systems with occasional manure application and intermediate fertilizer applications should be assessed. Inputs and offtakes of C, N and P were measured for three years. Approximately 16, 25 and 11% of C, N and P respectively were stabilized into soil organic matter from 5 t ha⁻¹ a⁻¹ manure. The majority of organic P was fixed as soil inorganic P.