Low available soil phosphorus (P) is associated with its immobility, which renders it unavailable for plant uptake. In addition, farmers normally apply inorganic fertilisers to legumes to activate soil-bound phosphorus using root exudates. Sufficient soil mineral nutrition is key to sustainable crop production, and hence food and nutritional security. The aim of this study was to quantify the acid and alkaline phosphatase activity as an indicator of P supply and availability under varying levels of nitrogen, phosphorus and potassium (NPK) fertilization and different cropping systems. An intercropping (cowpea and amaranth) and fertiliser (control, 25%, 50%, and 100% of the recommended NPK levels) field trial was laid out in a 2 × 4 factorial treatment structure in a completely randomized design (CRD) with four replications. There was higher acid and alkaline phosphatase activity in the rhizosphere of cowpea and amaranth grown as sole crops compared to those from intercropping. The cowpea and amaranth plants grown without fertiliser or 25% NPK had the highest rhizospheric phosphatase activity, while 100% NPK application exhibited the least. The markedly higher phosphatase activity from the low fertiliser application treatments indicates the possible stimulation of microbial activity to supplement P demands for the crops. The study revealed that the application of lower rates inorganic fertilisers in a legume intercrop stimulates the activity of the phosphatase enzymes, which can subsequently liberate soil-bound phosphorus. Plant tissue phosphorus concentration of cowpea and amaranth plants increased proportionately to the increase in fertiliser application up to 50% of the recommended NPK level. The land equivalent ratio (LER) was greater than 1, indicating that it is more beneficial to intercrop cowpea and amaranth as opposed to growing them as sole crops. Overall, the application of NPK fertilizer to amounts of up to 50%, based on the results of this study, appear to be better than 100% in terms of biomass accumulation and phosphate activity.

Nutrient-poor soils coupled with micronutrient deficiency among many rural resource-poor communities remain a challenge in sub-Saharan Africa. Nutrient-poor soils can be managed through various soil amendment or fertilisation strategies. Micronutrients can be supplied through plants. The study was aimed at determining the symbiotic nitrogen fixation of cowpea as well as the contribution of inter-cropping under varying levels of nitrogen, phosphorus, and potassium (NPK) fertilisation. In addition, the amount of micronutrients supplied by cowpea and amaranth were determined. The experiment was laid out in a 2 × 4 factorial treatment structure in a completely randomised design, with inter-cropping (cowpea and amaranth) and fertiliser (control, 25%, 50%, and 100% of the recommended NPK levels) as treatment factors with four replications. Symbiotic N₂ fixation of cowpea decreased from 341–448 kgN.ha⁻¹ to 77–91 kgN.ha⁻¹ for the first year and 557–227 kgN.ha⁻¹ to 92−164 kgN.ha⁻¹ for the second year with fertilisation. The iron and zinc nutritional yield increased (61–210 g.ha⁻¹ for first year and 304–867 g.ha⁻¹, for second year), proportional to fertiliser application to both crops. The research shows the benefits of leguminous crops in soil nutrient fertility and inorganic fertilisation with inter-cropping in managing micronutrient deficiency to meet the nutritional needs of rural communities.

Reducing nitrogen (N) losses and associated nitrate (NO ₃ ^- ) leaching and nitrous oxide emissions from agricultural land is a critical target worldwide. This is particularly urgent in areas with low fertility soils and a climate that increases the risk of N loss, such as the arid and temperate regions of South Africa. Here, we assessed the potential of fodder oats ( Avena sativa ) as a winter catch crop to deplete residual N in a field laid fallow for the previous four years, where vetch had proliferated. The soil presented a high clay content (34-44%), with the main exchangeable bases being calcium and magnesium hence, ammonium (NH ₄ ⁺ ) deposited by the vetch was expected to be rapidly adsorbed and slowly released. A significant decrease in the concentrations of NO ₃ ^- (49%) and NH ₄ ⁺ (30%) throughout the soil profile (0-90 cm) was observed following harvest of the oats compared to the concentrations measured before sowing. The effectiveness of the oats to uptake both forms of N from top and deep soil layers enhances their potential to reduce N losses. Our results are useful to fill current knowledge gaps on N dynamics in understudied, vulnerable soils such as agricultural land in South Africa, and to advance crop rotation strategies that reduce risk of N leaching.