ISFM at farm and landscape scales

Too often soil fertility management research is conducted only at the plot or field scale, where interactions among various agricultural enterprises and other land uses are seldom considered. Although most of the current research strength in SSA remains at the plot level, the diversity of forces impinging upon it naturally draws attention towards a hierarchical or nested systems-based approach that is extended to higher scales, particularly the whole farm and landscape. The rationale for working at the farm scale is the need to improve nutrient use efficiency through better allocation of limited organic and inorganic resources among different enterprises, taking into consideration inherent soil variability within the farming system (Okalebo et al. 2003, Vanlauwe et al. 2006). Inadequacies in supplies of both organic and inorganic nutrients have created strong fertility gradients even within the smallest farms. Smallhold farmers typically remove harvest products and crop residues from their food producing outfields and devote their scarce soil inputs to their smaller market infields, resulting in large differences in soil productivity over time between these two field types. Understanding how to manage the limited nutrient supplies across such fertility gradients is a key component in raising productivity in fields of staple crops. In most regions, fertilizer recommendations remain focused on the maximum yields attainable for broad agroecological regions (see Chapter 1), whereas localities, farms and farmers’ production objectives are highly heterogeneous. Fertilizer response by crops also varies with soil type (see Chapter 2). For example, P is a limiting nutrient in a Nitisol while N is the most limiting nutrient in Vertisol (see Table 2.4). These results point to the need to effectively target fertilizer to ensure use efficiency on the different soil types occurring within an agricultural landscape. Different fertilizer responses have been observed in various parts of the same field due to soil fertility gradients. Prudencio (1993) observed such fertility gradients between the fields closest to the homestead and those furthest. Fofana et al. (2006), in a study in West Africa, observed that grain yields averaged 0.8 t ha -1 on outfields and 1.36 t ha -1 on infields. Recovery of fertilizer N varied considerably and ranged from 17 to 23% on outfields and 34 to 37% on infields. Similarly, average recovery of applied fertilizer P was 31% in the infields compared to 18% in the outfields. These results indicate higher inherent soil fertility and nutrient use efficiency in the infields compared to the outfields and underlines the importance of soil organic carbon and secondary and micronutrients in improving fertilizer use efficiency. Once soils are degraded and depressed in organic matter, the response to fertilizer is lower and the recovery of applied fertilizers is reduced

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