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A partial static nutrient balance for improved and natural fallow was constructed using data from the literature and trials conducted in the Mangwende communal area, Zimbabwe, from 1994-97. The objectives were to determine the effect of improved fallows of Sesbania sesban and natural fallows in sustaining and/or maintaining the nutrient resource and to identify improved management options for depleted lands. Net positive nutrient balances for nitrogen ranging from +36.6 to +120 kg ha-1 year-1 nitrogen (N) were obtained in maize following S. sesban after 1 or 2 years, in fertilised and unfertilised maize plots in the first year of cultivation. The nutrient balance of phosphorus (P) was negative in unfertilised plots following S. sesban in contrast to fertilised plots. Negative nutrient balances, ranging from –2.6 to –11.4 kg ha-1 year-1 for N and –0.4 to –1.6 kg ha-1 year-1 for P, were obtained from continuously unfertilised maize plots during the three seasons. The results suggest that nutrient depletion is taking place but the magnitude was low because of small nutrient losses due to the low yields of maize. Although there was a positive nutrient balance for N and P in a fertilised maize crop after natural fallows, the magnitude was much lower than when S. sesban was incorporated into the system for two years. The maize yield improved with S. sesban fallows resulting in a high nutrient off-take, which was offset by biological nitrogen fixation. A combination of improved fallows and inorganic fertilizers seems to be needed to maintain positive nutrient balances for N and P.
Transactions of the Zimbabwe Scientific Association Volume 72 (Supplement) 1998, pp. 88-92