Combining organic and mineral fertilizers for integrated soil fertility management in smallholder farming systems of Kenya : Explorations using the crop-soil model FIELD
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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.
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