Marc Corbeels

Agro-ecological functions of crop residues under conservation agriculture. A review

Conservation agriculture, which is based on minimum tillage, permanent soil cover and crop rotations, has widely been promoted as a practice to maintain or improve soil quality and enhance crop productivity. To a large extent, the beneficial effects of conservation agriculture are expected to be provided by permanent soil cover with crop residues. Surface crop residues play an important role for crop growth through their benefits on soil-related structural components and processes in the agro-ecosystem, referred to in this study as agro-ecological functions. Through a meta-analysis of the literature, we have studied the relative effects of surface crop residue levels on the performance of a set of agro-ecological functions compared with a no-till bare soil, i.e., without surface residues. The selected agro-ecological functions were soil water evaporation control, soil water infiltration, soil water runoff control, soil loss control, soil nutrient availability, soil organic carbon (SOC) stocks and gains, weed control and soil meso- and macrofauna abundance. The potential effects of crop residue cover were quantified using boundary line models. Our main findings were (1) 8xa0txa0ha−1 of residues were needed to decrease soil water evaporation by about 30% compared to no-till bare soil. (2) To achieve the maximum effect on soil water infiltration, water runoff and soil loss control, residue amounts of at least 2xa0txa0ha−1 were required. (3) The effect of increasing the amounts of surface crop residues on soil nutrient supply (N, P and K) was relatively low; the boundary line models were not significant. (4) The average annual SOC gain increased with increasing amounts of residues, with a mean of 0.38xa0txa0Cxa0ha−1xa0year−1 with 4 to 5xa0txa0ha−1 of residues. (5) Weed emergence and biomass can be reduced by 50% compared to a no-till bare soil with residue amounts of 1xa0txa0ha−1 or more. (6) There was a weak response in soil meso- and macrofauna abundance to increasing amounts of surface crop residues. The maximum effect corresponded to an increase of 45% compared to a no-till bare soil and was reached from 10xa0txa0ha−1 of residues. Our findings suggest that optimal amounts of surface residues in the practice of conservation agriculture will largely depend on the type of constraints to crop production which can be addressed with mulching.

Conservation Agriculture in Sub-Saharan Africa

Specific practices of conservation agriculture (CA) in sub-Saharan Africa are diverse and vary according to local farming conditions. However, despite more than two decades of investment in its development and dissemination, adoption of CA is low. Crop responses to CA are highly variable, and not always positive, which is an important hindrance for adoption, especially for resource-poor farmers who need immediate returns with their investments in CA in order to be able to feed their families. In contrast with commercial farms such as in Brazil, reduced costs with CA on smallholder farms in sub-Saharan Africa are not always observed. Another major challenge with the practice of CA is the use of crop residues for mulching since crop residues are a major source of feed for livestock, especially in semiarid regions, where biomass production is limited and livestock plays a crucial role in farming systems. Studies indicate that the three principles of CA, including mulching, are needed to increase crop yields compared with conventional tillage (CT)-based practices. Among the three principles of CA, mulching is certainly the one that is least observed in past and current cropping practices in Africa. CA has a potential to improve the soil water balance and increase soil fertility, and it is undoubtedly a cropping practice that can result in substantial benefits for certain farmers in Africa. The question is when and how it is the best approach for smallholder farmers in sub-Saharan Africa. In general, CA is more likely to be attractive for farmers with a strategy of intensification than for farmers who struggle to produce food for their family. The latter too often face multiple constraints that limit the possibilities to engage in technological innovations. Some farmers may not be interested in new technologies because they earn their income from off-farm activities. Good markets of input supply and sale of extra produce are a prerequisite condition for adoption of CA as they are for any other new agricultural technology that aims at intensification. In sub-Saharan Africa, there is certainly a need to better target CA to potential end users and adapt the CA practices to their local circumstances and specific farming contexts.