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.