Soil chemical changes following 3-year legume or grass leys in west Africa

The influence of 4 tropical grasses (Panicum maximum, Andropogon gayanus broad- (BL) and narrow-leaf (NL) types and Pennisetum purpureum) and 6 legumes (Aeschynomene histrix, Stylosanthes fruticosa, Centrosema pubescens, Mucuna pruriens var. utilis, Cajanus cajan and Leucaena leucocephala) on fertility of the top soil in the Sudanian region of Benin over 3 years was investigated. The plants were sown without fertiliser, harvested under a cut-and-carry regime and soil changes were compared with those under a natural fallow. Soil samples were collected before the study commenced and at the end of the 3-year study. These were analysed for pH, organic C, N, available P and exchangeable cations (Ca, Mg, K and Na). N, P and K contents of aerial parts were determined to estimate the exports of these elements. For the grasses, root biomass, depth and distribution were also measured. Three years after grasses and legumes were sown, the pH under the grasses (6.6-6.7) was higher than under the legumes (6.2-6.4) and C and N concentrations had declined from the initial levels. Owing to their deep rooting systems, A. gayanus BL and P. maximum, and probably C. cajan and L. leucocephala, appeared able to recycle nutrients from deeper soil layers. While these species could be used for ley pastures in savannah regions of west Africa, maintenance fertiliser applications would be required to prevent nutrient depletion under a cut-and-carry regime. Further studies to test the efficacy of farmyard manure in providing these nutrients seem warranted.