Nitrogen is the most limiting element in Malawi soils with respect to crop production. However, grain legumes that are associated with biological nitrogen fixation are commonly grown in the country. Results are reported of a study on evaluation of the effects of sole cropped and doubled-up legume systems (legume-legume intercrop) on biological nitrogen fixation on the Ultisols of Kasungu district, Central Malawi. The modified nitrogen difference method was used to estimate the amount of nitrogen fixed per hectare. The total amount of nitrogen biologically fixed in each cropping system (that is, for intercrops, this means adding together amount of nitrogen fixed by the component crops) showed significantly (p<0.05) higher mean nitrogen amount in the pigeon pea/groundnuts doubled-up cropping system (82.8 kg N ha-1) that was 33 and 35% more nitrogen than those of the sole groundnuts (55.8 kg N ha-1) and sole pigeon pea (54.1 kg N ha-1), respectively. However, the pigeon pea/soybean doubled-up (53.6 kg N ha-1) was only significantly higher than that for the sole soybean (35.8 kg N ha-1) but resulted in nitrogen amount similar to that for the sole pigeon pea which implies a large suppression on intercropped pigeon pea biological nitrogen fixation. This could be attributed to competition for light and nutrients presented to the pigeon pea in the pigeon pea/soybean intercrop in early stages of development by the fast growing bushy solitaire soybean.
Key words: Biological nitrogen fixation, doubled-up legume cropping system, nitrogen difference method, sole crop.
BNF, Biological nitrogen fixation; EPA, extension planning area; ICRISAT,International Crop Research Institute for Semi-Arid Tropics; MAI, Ministry of Agriculture and Irrigation; N, nitrogen.
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