Enhancing phosphorous release from minjingu rock phosphate through integration of legumes in maize based cropping systems of Kabete, Kenya

Nutrient mining through harvested products, without adequate replenishment, has reduced soil nutrient stocks and maize yields in Kabete, Kenya. The use of inorganic fertilizers to replenish soil fertility is expensive and beyond reach for many farmers. Minjingu rock phosphate (MPR) use, under different legume-maize cropping systems, is a low cost and viable alternative that could potentially improve maize productivity. MPR is cheaper than other fertilizers, has high phosphate content and exhibits positive residual effects on crop yields. However, it has a slow phosphorus release pattern. Legumes white lupin (Lupinus albus L. cv. Amiga) and chickpea (Cicer arietinum L.), apart from fixing nitrogen (N), can enhance MPR solubility through protonation, hence increase maize yields. The broad objective of this study was to contribute towards enhanced soil fertility and maize yields through use of alternative low cost legume and fertilizer based technologies. The study was conducted at Kabete Field Station of the University of Nairobi in the long (LRS) and short rain seasons (SRS) of 2012. The experimental set up was a randomized complete block design with a split plot arrangement. The main plots were three cropping systems; maize monocrop, legume-maize rotation and legume/maize intercrop. The split plots were two phosphorous sources: triple super phosphate (TSP) and MPR. Soil and plant samples were collected at seedling, tasseling and maturity stages of maize growth and analyzed for N and P nutrient levels. Soil pH was monitored across maize growth. Maize grain and dry matter yields were measured at harvest. Soil N, P and K balances were determined at termination of experiment using the NUTMON-Toolbox. Available P (mg kg-1) was highest in the rotation systems involving chickpea (16.3) in the SRS and (10.1) in the LRS, implying an overall increase. The highest soil available N (SAN) was recorded in chickpea-maize rotation systems with TSP applied. The pH of soil was highest in the maize monocrop (5.5) and lupin-maize rotation (6.0) with TSP and MPR applied, for LRS and SRS respectively. Plant P was highest in chickpea/maize and lupin/maize intercrops, and lupin-maize (L-M) rotations with MPR applied. This trend was similarly observed for the plant N. Higher grain yields were recorded in MPR-treated compared to the TSP-treated plots in both seasons. A general increase of grain yield (t ha-1) over the seasons was noted i.e. LRS (3.03, 4.04 and 4.16) and SRS (3.78, 4.32 and 5.06) for the control, TSP and MPR respectively. This trend was repeated in the dry matter yield (t ha-1) i.e. LRS (5.01, 5.9 and 6.17) and SRS (5.84, 7.46 and 8.98).White lupin proved superior to chickpea at P mobilization from MPR and availing N in soil. Soil N and P balances recorded were negative i.e. 16.2 and 21.5 kg P/ha/yr. K balances were positive in the C/M control and with TSP. Higher balances were recorded where chickpea was used as either an intercrop or in rotation with maize. The use of MPR as a source of P holds promise as it results in higher crop yields in legumemaize cropping systems and is recommended for improved soil fertility in small holder farms. Improved yield and soil N and P levels were obtained with inclusion of legumes and application of MPR across seasons. In general, MPR was able to yield similar and/or better results for all parameters under study hence recommended for adoption under legume-maize intercrop systems.