Combining Organic and Mineral Fertilizers for Integrated Soil Fertility Management in Smallholder Farming Systems of Kenya: Explorations Using the Crop-Soil Model FIELD

Integrated soil fertility management (ISFM) technologies for African smallholders should consider (i) within-farm soil heterogeneity; (ii) long-term dynamics and variability; (iii) manure quality and availability; (iv) access to fertilizers; and (v) competing uses for crop residues. We used the model FIELD (Field-scale resource Interactions, use Efficiencies and Long term soil fertility Development) to explore allocation strategies of manure and fertilizers. Maize response to N fertilizer from 0 to 180 kg N ha¿1 (±30 kg P ha¿1) distinguished poorly responsive fertile (e.g., grain yields of 4.1¿5.3 t ha¿1 without P and of 7.5¿7.5 t ha¿1 with P) from responsive (1.0¿4.3 t ha¿1 and 2.2¿6.6 t ha¿1) and poorly responsive infertile fields (0.2¿1.0 t ha¿1 and 0.5¿3.1 t ha¿1). Soils receiving manure plus fertilizers for 12 yr retained 1.1 to 1.5 t C ha¿1 yr¿1 when 70% of the crop residue was left in the field, and 0.4 to 0.7 t C ha¿1 yr¿1 with 10% left. Degraded fields were not rehabilitated with manures of local quality (e.g., 23¿35% C, 0.5¿1.2% N, 0.1¿0.3% P) applied at realistic rates (3.6 t dm ha¿1 yr¿1) for 12 yr without fertilizers. Mineral fertilizers are necessary to kick-start soil rehabilitation through hysteretic restoration of biomass productivity and C inputs to the soil.