Combined Effects of Legumes with Phosphorus Fertilizer on Nutrient Balances and Gross Margins in Maize (Zea mays L.) systems of Kabete sub-County, Kenya

Abstract

Calculation of soil nutrient balances and gross margins (GM) is imperative in ascertaining effect of innovative technologies on soil fertility and farm profitability. A field experiment to evaluate effect of combined legumes and phosphorus fertilizer on soil N, P and K balances and crop GM in maize (Zea mays L.) systems was set up in Kabete Division, Kenya, in the long and short rainy seasons of 2012. The experimental set up was a randomized complete block design (RCBD) with a split plot arrangement. The main plots comprised cropping systems; (i) monocropping (sole maize), (ii) intercropping [white lupin (Lupinus albus L.)/maize (L/M) and chickpea (Cicer arietinum L.)/maize (CP/M)], and (iii) rotation [white lupin-maize (L-M) and chickpea-maize (CP-M)]. The split plots were phosphorus (P) fertilizers; Minjingu phosphate rock (MPR) and triple superphosphate (TSP), and (iii) no P fertilizer applied (CTRL). Soil N, P and K balances and gross margins were analyzed at plot level using NUTrient MONitoring (NUTMON - now known as MonQi) Tool box. Nutrient balances were negative across cropping systems and P sources except for K in M/CP (CTRL and TSP) intercrop. Significantly less negative N balances were obtained in maize monocrop (MPR), CP/M (CTRL) intercrop, CP-M (TSP) rotation, and L/M (MPR) intercrop. L/M (CTRL and TSP) intercrop and L-M (CTRL and TSP) rotation recorded more negative (highest losses) N balances. Across P sources, the maize monocrop, M/L intercrop and L-M rotation had significantly more negative P balances, than CP-M rotation and M/CP intercrop. P balances, across P fertilizers, were significantly less negative in M/CP compared to M/L intercrop. Less negative P balances were recorded in CTRL treatment compared to TSP and MPR across cropping systems. M/L (CTRL and TSP) intercrop system had pronounced negative K balances. In the rotation systems, significantly less negative balances were observed when maize was rotated with chickpea compared to lupin across all P sources. Pronounced GMs were realized in M/L intercrop (TSP) followed by L-M (TSP) and lowest in M/L (TSP and CTRL). The N, P and K nutrient balances in response to P sources and cropping systems exhibited a negative relationship with crop GM. The positive GMs obtained were thus at the expense of soil nutrient mining as treatments with high nutrient losses, case for N and P, had the highest GMs. Considering nutrient balance studies alongside economic analysis has thus demonstrated the hidden environmental costs in the positive crop GMs and by extension the efficiency of such production systems. As a result, increased GMs under introduced technologies are not sustainable unless the same is matched with adequate nutrient replenishments to balance those lost through harvested products and other nutrient loss pathways. Farmers would, actually, go for those technologies that not only maximize yields but also accrue high profits. In the context of this study, and in order of GM (from highest) analysis, M/L intercrop, maize monocrop and L-M rotation with application of TSP are such technologies. In the long-run however these technologies will prove untenable due to nutrient mining. Nonetheless to guarantee efficient production and sustainable maize systems, following application of P fertilizer and legume integration, it is important that profits accrued from farm sales be used to purchase fertilizers and/or support practices geared towards replenishing mined soil nutrients. This way farm profits realized will not be at the expense of nutrient mining.

Keywords: Cropping systems; gross margins; Kabete sub-County; MonQi; Nutrient Balances; Rock phosphates