A fertilizer management strategy for sequential production of maize and legumes in smallholder farming systems of Zimbabwe
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Grain legume-cereal rotations are long known to farmers in southern Africa, but their productivity and benefits to household food and income security is often compromised by inadequate fertilization. This study was conducted between 2008 and 2011 in Makoni and Hwedza districts, to determine the effects of co-applying basal P-containing fertilizers and locally available organic nutrient resources on grain legume productivity, legume - derived soil fertility benefits, subsequent maize yields and financial returns on smallholder farms. The study approach entailed a household and field survey, on-farm field experimentation, laboratory analyses and financial analyses. Amounts of N2 fixed by grain legumes were determined using the N-difference method while carbon and nitrogen mineralization patterns were quantified by a back titration and leaching tube method, respectively. Financial analyses were done through a gross margin and sensitivity analyses. About 30% of the farmers practiced disproportionate legume-maize sequencing, but more than 70% of these farmers did not apply fertilizer to the legume. Legume fertilization with either sole or combinations of cattle manure, woodland litter, Compound D (7% N, 14% P2O5, 7% K2O, 6% S) or PKS blend (0%N:32% P2O5:16% K2O:5% S) increased biomass productivity by between 20% and 300% compared to the non-fertilized control which yielded < 1 t biomass ha-1. Amounts of N2 fixed by grain legumes increased by at least 50% after fertilization ranging from 31 kg N ha-1 under basal PKS and 161 kg N ha-1 under organic and inorganic nutrient resource combinations. External nutrient application raised the amounts of N derived from N2 fixation by grain legumes from 50% to 94% of total N taken up by the plant. The increase in amount of mineral N released from soils sampled after legume fertilization, reaching up to 85mg N kg-1 soil, reflects significant differences in biological activity of soils under experimental treatments. Cumulative amounts of CO2-C evolved from soils following different fertilization regimes increased in the order: continuous fertilized maize < unfertilized grain legume < sole organic-fertilized grain legume sole inorganic- fertilized grain legume < grain legume under inorganic x organic nutrient resource combination. Co-application of an N – containing basal fertilizer with cattle manure onto maize produced grain yields of up to 5 t ha-1. Sole application of P-containing mineral fertilizers resulted in grain yields of ~ 2 t ha-1, which were 30% and 260% above sole organic nutrient treatments and the unfertilized control, respectively. When fertilizer combinations were applied to a preceding grain legume crop, the rotational maize yielded 2.9 to 4.2 t ha-1. Highest nutrient uptake was recorded after co-application of cattle manure or woodland litter and an NPK compound reaching up to 54.1 kg N ha-1 and 23.8 kg P ha-1. Recovery efficiencies after fertilized grain legumes ranged between 22% – 90% N and 18% – 53% P reflecting better nutrient capture under combined application of organic and inorganic nutrient resources. Co-application of N-containing fertilizer with manure gave the highest gross margin of US$ 1433 ha-1 for the legume – maize sequence. Targeting P- based fertilizers combined with low quality organics to the legume, even under low rainfall conditions, resulted in higher yields and profit margins. After sensitivity analyses, the fertilization strategies which yielded the highest stable profit was PKS + organic and half rate AN under a legume – maize sequence. It was concluded that low P status of soils coupled with non-application of external nutrients accounts for the low yields of < 0.5 t ha-1 in smallholder farming areas. Fertilization of legumes with both organic and P-containing inorganic nutrient resources could substantially increase grain productivity; amounts of N2 fixed by legumes together with rotational cereal yield benefits and associated financial returns.