Monoculture is common in sugarcane production throughout the world and leads to decline in yields and soil fertility and build up of pests and diseases. Legumes have been shown as potential crops that break the monoculture cycles in several crops. Farmers can reduce nitrogen fertilizer requirements to the subsequent sugarcane crop when soyabean (Glysine max (L.) Merr) is used as a fallow crop. Field studies were conducted from 2004 to 2005 on nitrogen depleted sandy loam soils at the Zimbabwe Sugar Experiment Station in the South East Lowveld of Zimbabwe. The objectives of the study were to determine nitrogen fixed by soyabeans at various growth stages, determine nitrogen in the foliar of subsequent cane and estimate the artificial nitrogen fertilizer reduction. The treatments used were (i) vegetable soyabeans followed by cane topdressed with 80 and 120 kg nitrogen ha-1, (ii) grain soyabean followed by cane topdressed with 80 and 120 kg nitrogen ha-1, (iii) monoculture cane topdressed with 120 kg nitrogen ha-1. Both grain and vegetable soyabeans fixed more nitrogen at flowering stage, 128 and 118 kg nitrogen ha-1, respectively. The results showed that farmers can save nitrogen fertilizer by using vegetable soyabeans. The nitrogen saved was estimated at 80 kg nitrogen ha-1 as shown by the number of tillers, biomass and nitrogen in leaves of cane.
Key words: Nitrogen fertilizer, vegetable, grain soybeans, tillers, CP72-2086.
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