F. Baijukya

Residual phosphorus effects and nitrogen × phosphorus interactions in soybean–maize rotations on a P-deficient Ferralsol

AbstractnLegume-cereal rotations are an essential component of integrated soil fertility management in low-input cropping systems, but strategies are needed to increase phosphorus (P) fertilizer use efficiency in such systems. These may include preferential targeting of P to one of the crops in the rotation cycle, the use of P-efficient genotypes, and the optimization of the rates of P fertilizer used. A field trial was conducted to evaluate the effects of increasing P fertilizer rates (0, 11, 22 and 44xa0kg P ha−1, added as triple super phosphate) applied to three soybean genotypes grown on a P-deficient Ferralsol, on the nitrogen (N) and P nutrition of a subsequent maize crop. In addition, a greenhouse trial was set up to assess N, P and other rotation effects of three soybean genotypes on a subsequent maize crop relative to a maize–maize rotation at high and low P supply. In the field trial, soybean did not respond to increasing P rates, but residual P effects improved maize grain yields by up to 90xa0%. Ear leaf (field trial) and shoot (pot trial) P concentrations increased by applying N to maize, demonstrating important Nxa0×xa0P interactions. The pot trial did not reveal a positive rotation effect of soybean on maize beyond the mere N-benefit, showing that soybean was not able to improve P availability to maize after correcting for the N-effect. No variation in rotation effects on maize among soybean genotypes was observed. Because of the absence of effects of the soybean crop on P availability to maize, opportunities to increase P fertilizer use efficiency in soybean–maize rotations mainly reside in maximizing P uptake by each crop separately and in matching P fertilizer rates with crop demand.

From best fit technologies to best fit scaling: incorporating and evaluating factors affecting the adoption of grain legumes in Sub-Saharan Africa

The success of scaling out depends on a clear understanding of the factors that affect adoption of grain legumes and account for the dynamism of those factors across heterogeneous contexts of sub-Saharan Africa. We reviewed literature on adoption of grain legumes and other technologies in sub-Saharan Africa and other developing countries. Our review enabled us to define broad factors affecting different components of the scaling out programme of N2Africa and the scales at which those factors were important. We identified three strategies for managing those factors in the N2Africa scaling out programme: (i) testing different technologies and practices; (ii) evaluating the performance of different technologies in different contexts; and (iii) monitoring factors that are difficult to predict. We incorporated the review lessons in a design to appropriately target and evaluate technologies in multiple contexts across scales from that of the farm to whole countries. Our implementation of this design has only been partially successful because of competing reasons for selecting activity sites. Nevertheless, we observe that grain legume species have been successfully targeted for multiple biophysical environments across sub-Saharan Africa, and to social and economic contexts within countries. Rhizobium inoculant and legume specific fertiliser blends have also been targeted to specific contexts, although not in all countries. Relatively fewer input and output marketing models have been tested due to public–private partnerships, which are a key mechanism for dissemination in the N2Africa project.

Dilemma of nitrogen management for future food security in sub-Saharan Africa – a review

Food security entails having sufficient, safe, and nutritious food to meet dietary needs. The need to optimise nitrogen (N) use for nutrition security while minimising environmental risks in sub-Saharan Africa (SSA) is overdue. Challenges related to managing N use in SSA can be associated with both insufficient use and excessive loss, and thus the continent must address the ‘too little’ and ‘too much’ paradox. Too little N is used in food production (80% of countries have N deficiencies), which has led to chronic food insecurity and malnutrition. Conversely, too much N load in water bodies due mainly to soil erosion, leaching, limited N recovery from wastewater, and atmospheric deposition contributes to eutrophication (152 Gg N year–1 in Lake Victoria, East Africa). Limited research has been conducted to improve N use for food production and adoption remains low, mainly because farming is generally practiced by resource-poor smallholder farmers. In addition, little has been done to effectively address the ‘too much’ issues, as a consequence of limited research capacity. This research gap must be addressed, and supportive policies operationalised, to maximise N benefits, while also minimising pollution. Innovation platforms involving key stakeholders are required to address N use efficiency along the food supply chain in SSA, as well as other world regions with similar challenges.