Sustainable maize production through leguminous tree and shrub fallows in Eastern Zambia

Nitrogen is the major nutrient limiting maize production in Zambia and Southern Africa. Removal of subsidies on manufactured fertilizers made them very expensive and most farmers cannot afford them. Short duration planted fallows using a wide range of leguminous trees have been found to replenish soil fertility and increase subsequent maize yields. Species such as Sesbania sesban, Tephrosia vogelii and Cajanus cajan have been found to be well suited for planted fallow technology. These improved fallow crop rotations are being adopted by small-scale farmers in Eastern Zambia. Since the seminal paper of Kwesiga and Coe, research has been carried out to understand how the planted tree fallows replenish soil fertility and improve maize yields. A wide range of species has been screened as alternatives to Sesbania fallows to overcome limitations of Sesbania such as susceptibility to nematodes and insect pests. Species such as Gliricidia sepium and Leucaena leucocephala have maintained maize yields of 3 t ha1 over 10 years of cropping when Sesbania fallow yields declined to 1.1 t ha1 after 3 years of cropping. The selection criteria for good fallow species are high biomass production and litterfall. Maize yields after fallows were highly correlated to biomass and litterfall yields. High quality biomass, which is low in lignin and polyphenols and high in N, is needed for higher maize yields. Mixing of Gliricidia and Sesbania fallows resulted in higher maize yields compared with single species fallows (3.0 vs. 1.8 t ha1). Mechanisms contributing to the efficacy of mixed fallows will be discussed. Pre-season inorganic-N (NO3- + NH4+) was highly correlated with maize yield (r2 = 0.62) and this could be used to select fallow species and management practices. Nutrient budgets of N, P and K over 8 years showed that a positive balance of N and P was maintained for coppicing fallows, while a negative balance of K started from the fourth year onwards on fertilized maize, Gliricidia, Leucaena and Sesbania fallows, emphasising the need to use P and K fertilizers to supplement the N supply from leguminous fallows. Improved fallows increased infiltration, reduced runoff, increased water storage, and reduced soil loss. The biophysical limits of most fallow species and other emerging issues such as pests and diseases, the need to inoculate with rhizobium, the amount of N fixed by different species and provenances and soil acidification under improved fallows are the subjects of further research. Biomass transfer technology using biomass from leguminous trees was evaluated on maize and vegetable production in the dambos (wetlands). Maize and vegetable yields were significantly increased by application of high quality biomass from Gliricidia and Leucaena. However, financial analysis showed that it is not viable to apply biomass on a low value crop like maize, but biomass transfer was economically viable on high value crops such a vegetables.

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