Effect of direct nitrogen and potassium and residual phosphorus fertilizers on soil chemical properties, microbial components and maize yield under long-term crop rotation

A study was conducted in a long-term field experiment initiated in 1939 at the University of Pretoria, South Africa. The aim was to assess the long-term effect of direct N and K and residual P fertilizers on chemical and microbial properties of soil and grain yield of maize in rotation with field pea. Long-term fertilization resulted in decreased total organic C (TOC) and basic cation contents, and had an acidifying effect on soil. The decrease in TOC was greater in simple fertilizer treatments (N, P, or K) whereas basic cation contents and pH declined more in balanced fertilizer treatments (NPK). Levels of total N were higher in the balanced than in simple fertilizer treatments. Soil microbial biomass and numbers of bacteria, actinomycetes and fungi were influenced by, and exhibited qualitative changes in response to, long-term fertilization. Crop rotation also exerted effects on chemical and microbial properties of the soil. Maize grain yield showed significant increases due to balanced fertilizer treatments. Responses of maize to simple fertilizer applications were not beneficial in terms of yield returns. The results suggested that judicious use of inorganic fertilizers may, in the long-term, maintain soil quality and productive capacity.