| Integrated soil fertility management (ISFM) utilizes locally available organic resources (ORs) in combination with mineral fertilizers for optimal crop productivity while maintaining the natural resource base. However, the optimal benefits of ISFM are governed by availability and quality of ORs, soil and climatic conditions. The goal of this dissertation was to evaluate how combining ORs with N fertilizers influences crop yields, aggregate C and N dynamics; and how these are regulated by OR quality, soil texture and climate.;Greater yields were observed where ORs were combined with N fertilizer, both in the meta-analysis and field experiment. While greater yields were observed with high quality ORs, there were no extra yield benefits of combining them with N fertilizer, resulting in negative interactive effects. Positive interactive effects were seldom, except where low quality ORs were incorporated in sandy soils under dry climate.;Organic resources increased soil organic C (SOC) and N in the clayey soil but there were no long-term differences among OR quality. In the short-term, low quality ORs resulted in greater aggregation and accumulation of macroaggregate C and N, but this was negated by the addition of N fertilizer. In the sandy soil, stabilization of C and N was greater with high polyphenol OR. In conclusion, preservation of OR derived N is affected by the chemical recalcitrance of the OR in sandy soils whereas macroaggregate protection, and not OR quality, is the major factor in clayey soils. The combined application of OR and N fertilizer offers most potential for long-term soil quality and crop productivity improvement in sandy soils under dry climate.;A meta-analysis was conducted to provide a quantitative review of previously published studies on combined OR and N fertilizer application in sub-Saharan Africa. Subsequently, ORs of varying quality were applied with or without N fertilizer in two field experiments on a clayey and a sandy soil in Kenya. The field experiment was conducted over ten seasons with maize as test crop. After three and four years of the field experiment, 15N labeled OR and/or N fertilizer were applied to soil mesocosms to evaluate aggregate C and N dynamics. |
