Consistency, variability, and predictability of on-farm nutrient responses in four grain legumes across East and West Africa

Grain legumes are key components of sustainable production systems in sub-Saharan Africa, but wide-spread nutrient deficiencies severely restrict yields. Whereas legumes can meet a large part of their nitrogen (N) requirement through symbiosis with N₂-fixing bacteria, elements such as phosphorus (P), potassium (K) and secondary and micronutrients may still be limiting and require supplementation. Responses to P are generally strong but variable, while evidence for other nutrients tends to show weak or highly localised effects. Here we present the results of a joint statistical analysis of a series of on-farm nutrient addition trials, implemented across four legumes in four countries over two years. Linear mixed models were used to quantify both mean nutrient responses and their variability, followed by a random forest analysis to determine the extent to which such variability can be explained or predicted by geographic, environmental or farm survey data. Legume response to P was indeed variable, but consistently positive and we predicted application to be profitable for 67% of farms in any given year, based on prevailing input costs and grain prices. Other nutrients did not show significant mean effects, but considerable response variation was found. This response heterogeneity was mostly associated with local or temporary factors and could not be explained or predicted by spatial, biophysical or management factors. An exception was K response, which displayed appreciable spatial variation that could be partly accounted for by spatial and environmental covariables. While of apparent relevance for targeted recommendations, the minor amplitude of expected response, the large proportion of unexplained variation and the unreliability of the predicted spatial patterns suggests that such data-driven targeting is unlikely to be effective with current data.

The use of soil microbial potassium solubilizers in potassium nutrient availability in soil and its dynamics

Background

An increase in population has led to a higher demand for food. Meeting up this demand has necessitated the use of chemical fertilizers. However, utilization of these fertilizers has a considerable deleterious effect on the soil, plant, human, environmental sustainability, and only increase the cost and reduced profitability. With these identified problems, there is a need for efficient and sustainable methods regarding managing natural resources to enhance food production. Naturally, potassium (K) is an abundant element present in the soil but in an inaccessible form. There is therefore a need to seek an alternative method to improve the K availability to plants noting that K is an essential plant nutrient that plays a major role in plant physiological and metabolic processes. Subsequently, employing microbial potassium solubilizers is an efficient method to enhance the potassium availability in the soil, which in turn improves productivity. Therefore, this review discusses the various types of potassium solubilizing microorganisms in soil, their mechanism of action, and their importance in sustainable crop production.

Main body

Potassium solubilizing microorganisms (KSM) such as bacteria and fungi can solubilize K from an insoluble form to a soluble form to enhance uptake by plants. These microorganisms solubilize K through the production of organic acids such as tartaric acid, citric acid, and oxalic acid to release K from its minerals. Apart from making potassium available, these microbes can improve soil health and crop yield and act as bio-control agents by producing antibiotics. Potassium solubilizing microbes also produce hormones that help plants withstand both biotic and abiotic stresses. Hence, the application of KSM to agricultural soils will reduce the use of chemical fertilizers and enhance the sustainability of food production.

Conclusion

One of the most efficient ways of improving plant utilization of potassium in the soil is to use potassium solubilizing microbes, which can make potassium ions available from minerals of both igneous and sedimentary origins. The use of potassium solubilizing microbes as biofertilizers may be the awaited solution to increasing crop productivity, concerns linked to chemical fertilizer application, and earth resource diminution.

Biological nitrogen fixation and prospects for ecological intensification in cereal-based cropping systems

The demand for nitrogen (N) for crop production increased rapidly from the middle of the twentieth century and is predicted to at least double by 2050 to satisfy the on-going improvements in productivity of major food crops such as wheat, rice and maize that underpin the staple diet of most of the world's population. The increased demand will need to be fulfilled by the two main sources of N supply - biological nitrogen (gas) (N2) fixation (BNF) and fertilizer N supplied through the Haber-Bosch processes. BNF provides many functional benefits for agroecosystems. It is a vital mechanism for replenishing the reservoirs of soil organic N and improving the availability of soil N to support crop growth while also assisting in efforts to lower negative environmental externalities than fertilizer N. In cereal-based cropping systems, legumes in symbiosis with rhizobia contribute the largest BNF input; however, diazotrophs involved in non-symbiotic associations with plants or present as free-living N2-fixers are ubiquitous and also provide an additional source of fixed N. This review presents the current knowledge of BNF by free-living, non-symbiotic and symbiotic diazotrophs in the global N cycle, examines global and regional estimates of contributions of BNF, and discusses possible strategies to enhance BNF for the prospective benefit of cereal N nutrition. We conclude by considering the challenges of introducing in planta BNF into cereals and reflect on the potential for BNF in both conventional and alternative crop management systems to encourage the ecological intensification of cereal and legume production.