Converging strategies by farmers and scientists to improve soil fertility and enhance crop production in Benin

Keywords: Farmer perception, indigenous knowledge, extensive cassava, earthworm casts, arbuscular mycorrhiza, crop rotation, nutrient uptake, soil fertility, co-research, land tenure.Farmers in the transitional zone of Benin claim that extensive cassava cropping and prior cotton fertiliser enhance yield of subsequent maize. To cope with labour shortage, farmers have adapted fertiliser practices by mixing NPK-SB and urea. We agreed with farmers through a Stakeholder Learning Group to study these innovations. In this process land tenure arrangement was also of interest because it affects long-term soil fertility management. Joint experiments opened a path between farmers' interpretations and scientific explanations of earthworm activities. The presence of earthworm casts is used by farmers as an indicator for soil health. Farmers understand casts to be a kind of 'vitamin' indicating good conditions for crop growth. Cassava cultivars did not significantly change soil chemical properties. However, cassava cultivar, subsequent fertiliser treatment, and farmers1 management significantly affected maize grain yield. The nutrient uptake ratios indicated that P was the main limiting nutrient. Results from a subsequent pot experiment provided evidence that arbuscular mycorrhizal associations may be part of the explanation for the farmers' claim of beneficial effects of cassava. Farmers were knowledgeable about residual effects of fertiliser. Cotton yield, soil chemical properties, subsequent maize yield and nutrient uptake were significantly affected by land use types. The N : P ratios of nutrients taken up indicated that P was relatively more limiting in the egusi-cotton-cotton system, whereas N was relatively more limiting in the cassava-maize-cotton system. This result is consistent with the hypothesis that cassava improves P nutrition of a subsequent crop through mycorrhizal carry-over effects. Co-research activities apparently improved both human capital (farmers' individual knowledge and capacity building) and social capital (group dynamic, space for innovation, interaction, negotiation skills, improvement of cropping practice, improvement of social relationships, and information sharing). Farmers' knowledge on the role of N, P and K nutrients and arbuscular mycorrhizal fungi had been highly improved. With regards to land tenure, the problem to be overcome is how to change mutual perceptions of tree planting as a covert claim to land ownership, since agroforestry is a potential key to soil fertility maintenance. Formal written-down land use rules, including adoption of agroforestry with Gliricidia sepium for permanent yam production and improved soil management practices were negotiated. This document approved by both land owners and migrant farmers does not alter the existing balance of power