Effects of Cowpea-Amaranth Intercropping and Fertiliser Application on Soil Phosphatase Activities, Available Soil Phosphorus, and Crop Growth Response

Abstract

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.

Nitrogen Fixation and Nutritional Yield of Cowpea-Amaranth Intercrop

“…The experimental treatments, layout and the management of the experiment were as described by Mndzebele et al [23]. These entailed the amount of fertiliser applied, seedling preparation, planting, inter-and intra-row spacing as well as irrigation amounts applied.…”

“…To determine symbiotic N 2 fixation, nodulation (number and mass of nodules formed), relative ureide-N (%RU-N), percentage N derived from N 2 fixation (%Ndfa), N-fixed and soil N uptake of cowpea were determined. The relative ureide-N (%RU-N), percentage N derived from N 2 fixation (%Ndfa), N-fixed and soil N uptake were computed as a factor of above-ground biomass shown in Mndzebele et al [23]. The choice of the method was informed by the simplicity in the analyses and less costly equipment.…”

“…The N content, indicating N-accumulation of cowpea was calculated as a product of N% and sample mass [28]. The sample mass to calculate N-content was obtained from Mndzebele et al [23]. The ureide-N and nitrate-N were analysed using the Rimini-Schryver reaction as described by Young and Conway [29], in Unkovich et al [30].…”

“…Nutritional yield (NY) was calculated for cowpea and amaranth using equations 4 to 6 ( Table 1). The computations included the concentrations of Fe and Zn and the above ground edible biomass (AGEB), obtained from Mndzebele et al [23], for each crop. The total NY factoring both amaranth and cowpea were also determined as shown in equations 4 to 6 in Table 1.…”

“…Poor soil fertility can be addressed through exogenous sources of fertilisation such as the application of organic/inorganic fertilisers and sometimes the use of leguminous crops through their symbiotic N 2 fixation, which provide bio-fertilisers in cropping systems [4]. Low soil fertility affects crop production in rural resource-poor farming communities, reducing nutritional food The description of the soils in which the experimental sites were carried out, as well as the rainfall and temperature conditions during the trials were as detailed by Mndzebele et al [23].…”