Carbon, nitrogen and phosphorus dynamics under continuous cultivation as influenced by farmyard manure and inorganic fertilizers in the savanna of northern Nigeria

Changes in the carbon (C), nitrogen (N) and phosphorus (P) status of a savanna Alfisol under continuous cultivation complemented with farmyard manure and inorganic fertilization for 45 years were determined. The objective of the study was to determine the effect of farmyard manure (D), N + P and N + P + K fertilizers and their combinations on soil quality under continuous cultivation. Six field plots from the long-term DNPK experiment at Samaru were sampled for the study: D, N + P, N + P + K, D + N + P, D + N + P + K fertilized plots, a control plot with no fertilization and a native site soil. Continuous cultivation caused significant losses of C, N and P. The N + P or N + P + K fertilization could not mitigate these losses. Losses of N and P were mainly in organic forms. However, combination of farmyard manure with N + P and N + P + K fertilization enabled C, N and P to be maintained equal to, or greater than, the native site soil. It was concluded that farmyard manure applied alone, or in combination with N + P or N + P + K fertilization, was effective in maintaining soil quality under continuous cultivation in the savanna. In contrast, continuous inorganic fertilization was deleterious to soil quality because of depletion of organic matter, the reservoir of plant available N and P in weathered, tropical soils. (C) 1997 Elsevier Science B.V.