Role of fertilisation regimes on the yield and nutritional benefiits of cowpeaamaranth intercropping systems.

Abstract

Several African leafy vegetables (ALVs) contribute to food and nutritional security of rural communities, particularly due to their ability to grow in marginal soils. These ALVs such as amaranth and cowpea among others provide valuable macro- and micronutrients that are key to rural household dietary needs. The aim of this study was to assess the effect of fertiliser application on the symbiotic nitrogen fixation, enzymatic phosphatase activity, agro biological properties, nutrition as well as recommended daily allowance in an intercropped Amaranthus cruentus (amaranth) and Vigna unguiculata (cowpea) farming system. The nitrogen fixation and nutritional yield of cowpea-amaranth intercrop study was motivated by limited information relating symbiotic nitrogen fixation and fertilisation of ALVs, such as cowpea and amaranth grown under intercropping system, in addition to nutritional yield. Field trials were conducted at the Agricultural Research Council (ARC), Vegetables and Ornamental Plants campus situated in Roodeplaat, Pretoria, South Africa, during 2014/15 and 2015/16 summer seasons from November to January. The 2 x 4 factorial experiments were laid out in a completely randomized design (CRD) with four replications. The factors evaluated were intercropping (amaranth and cowpea) and fertiliser (control, 25%, 50%, and 100% of the recommended NPK levels). Soil sampling was done before land preparation and soil nutrient analysis was done at the Agricultural Research Council–Soil, Climate and Water (ARC–SCW). The application of nitrogen, phosphorus and potassium were guided by the soil analyses results and recommendations on both seasons. Vigna unguiculata was sown directly in the soils and amaranth was transplanted approximately four weeks after planting amaranth in the nursery. Irrigation was done based on reference evapotranspiration (ET) and a crop factor for each crop. Collected data included acid and alkaline phosphatase activity, phosphorus in the soils, phosphorus in the cowpea and amaranth plants, as well as biomass of cowpea and amaranth at physiological maturity. In the rhizosphere of cowpea and amaranth grown as sole crops, there was a higher acid and alkaline phosphatase activity as compared to those on intercropping. The highest rhizospheric phosphatase activity occurred when both crops were grown without fertilizer or 25% NPK. Applying NPK activates soil-bound phosphorus (P) using root exudates, which is important for the production of ALVs. The results showed a reduction in symbiotic N2 fixation of cowpea with the increase fertiliser addition.

The above ground and above ground edible biomass of amaranth increased proportionately to the rate of fertiliser application up to 100% NPK, but in cowpea it only increased up to 50% NPK. Nutritional yield of iron and zinc increased with the increase in fertiliser application amounts on cowpea and amaranth. The land utilisation values were greater than one, hence an advantage of intercropping. Cowpea was more aggressive, showed high actual yield losses and high competitive ratio relative to amaranth. More income could be obtained from intercropping cowpea and amaranth compared to the respective sole crops at 100% NPK. In the experiment on the potential of intercropped amaranth and cowpea to meet nutritional requirements, the seasonal above ground and above ground edible biomass of amaranth and cowpea increased with fertiliser application up to 100% NPK. More above ground and above ground edible biomass on amaranth and cowpea were obtained in sole cropping when compared to intercropping. Macro and trace nutritional element contents were highest at 100% NPK fertiliser level. The lowest nutritional contents of macro and trace elements was recorded at the control. Overall, amaranth and cowpea contributed to the recommended daily allowance of calcium, magnesium, iron, and zinc, where there was more at the 100% NPK fertiliser level. The research demonstrates the benefits of grain leguminous crops in soil nutrient fertility enhancement and inorganic fertilization with intercropping in managing micronutrient deficiency to meet the nutritional needs of rural communities. Moreover, the study demonstrated the benefit of applying 25%NPK to 50%NPK fertiliser on the above ground and above ground edible biomass of amaranth and cowpea. In sum, macro and trace elements that are crucial for the nutritional health of rural communities were improved, thus contributing more to the recommended daily allowance, which limits food and nutrition insecurity, and fosters sustainable development.