Assessing the potential benefits of organic and mineral fertiliser combinations on legume productivity under smallholder management in Zimbabwe

Abstract

Productivity of grain legumes on sandy soils of southern Africa is critically limited by marginal fertilisation. Effects of co-applying phosphorus (P)-based mineral fertilisers and organic nutrient resources to cowpea (Vigna unguiculata (L.) Walp.) and soyabean (Glycine max L.) were investigated on smallholder farms in eastern Zimbabwe over two years. Over 70% of the surveyed farmers grew cowpea without fertilisation. Fertilisation of legumes with one or more nutrient resources increased shoot biomass productivity by between 20% and 300% relative to the non-fertilised control. Fertilised soyabean and cowpea yielded 2.2 t grain ha⁻¹ and 2.5 t grain ha⁻¹, respectively, translating to more than double the yields of unfertilised controls. In contrast, sole application of either mineral P-containing fertilisers or organic nutrient resources yielded less than 1 t ha⁻¹ legume grain. The effects of combined organic and mineral fertilisation were also reflected in increased CO₂-carbon evolution from soils following growth of the legumes. Under the same soils, net nitrogen (N) mineralisation was highest where cattle manure was co-applied with an NP-containing fertiliser, with at least 85 mg N kg⁻¹ soil released within six weeks. Co-application of organic and NP-containing fertilisers significantly enhance legume grain yields and residual soil N availability, but most smallholder farmers do not currently use this fertilisation strategy.

Keywords: carbon mineralisation, locally available organic resources, nitrogen mineralisation, nutrient-depleted fields, P-fertilisation