Population dynamics of mixed indigenous legume fallows and influence on subsequent maize following mineral P application in smallholder farming systems of Zimbabwe

Abstract

Developing soil fertility management options for increasing productivity of staple food crops is a challenge in most parts of Sub-Saharan Africa, where soils are constrained by nitrogen (N) and phosphorus (P) deficiencies. A study was conducted to evaluate the response of indigenous legume populations to mineral P application, and subsequently their benefits to maize yield. Mineral P was applied at 26 kg P ha^-1 before legume species were sown in mixtures at 120 seeds m^-2 species^-1 and left to grow over two rainy seasons (2 years). Application of P increased overall biomass productivity by 20–60% within 6 months, significantly influencing the composition of non-leguminous species. Dinitrogen fixation, as determined by the N-difference method, was increased by 43–140% although legume biomass productivity was apparently limited by nutrients other than P and N. Crotalaria pallida and C. ochroleuca accounted for most of the fixed N. Improved N supply increases the abundance of non-leguminous species, particularly Conyza sumatrensis and Ageratum conyzoides. However, abundance of common weed species, Commelina benghalensis, Richardia scabra and Solanum aculeastrum, declined by up to18%. Application of P did not significantly influence productivity of those legume species that reached maturity within 3 months. There was increased N2-fixation and biomass productivity of indifallows as influenced by specific legume species responding to P application. Compared with natural (grass) fallows, indigenous legume fallows (indifallows) increased subsequent maize grain yields by ~40%. Overall, 1- and 2-year indifallows gave maize grain yields of[2 and 3 t ha^-1, respectively, against \1 t ha^-1 under corresponding natural fallows. Twoyear indifallows with P notably increased maize yields, but the second year gave low yields regardless of P treatment. Because of their low P requirement, indigenous legume fallows have potential to stimulate maize productivity under some of the most nutrient depleted soils.