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Sustainability of Leucaena leucocephala fallows in shifting cultivation on the Island of Mindoro, Philippines MacDicken, Kenneth G.


Increasing human populations, declining crop yields and reductions in per capita arable land area suggest that shifting cultivation as traditionally practiced in much of the humid tropics is no longer a sustainable farming practice. The use of nitrogen-fixing trees such as Leucaena leucocephala (Lam. de Wit) (leucaena) as a fallow species may be an important strategy to maintain or improve crop yields in shifting cultivation where natural resources are rapidly declining. A series of experiments comparing leucaena and non-leucaena fallows was conducted on sites with fallows established between 1977 and 1992 on the Island of Mindoro, Philippines to determine the impacts of leucaena fallows on rice yields, soil nutrient status and the sustainability of production in shifting cultivation in the study area. Rice yields and yield components were measured on 10 shifting cultivation fields. One burning experiment and a series of chronosequence studies in fallows of 1-4 years of age were conducted. Informal interviews with farmers were also conducted to determine perceptions and social impacts of leucaena as a fallow crop. Moisture content-adjusted grain yields (@0.14 g H20/g dry matter) were 3.8 t ha"1 following leucaena fallows and 2.71 ha"1 following non-leucaena fallows for an average yield response of 42%. This difference appears to relate most closely to soil N accumulated during the fallow period and N added through seedling biomass applied as a surface mulch during the cropping period. N-fixation of leucaena and possibly the inhibition of nitrification of ammonium ions as demonstrated by higher NH4 + concentrations in soils under leucaena were associated with (and may explain) these differences. Higher N levels appear to delay crop maturity, as evidenced by higher grain moisture contents in the leucaena treatment. Available phosphorus levels are low, but do not appear to differ between fallow types, although the size of phosphorus pools in the biomass and litter varied with fallow type. Ca and Mg were not limiting in either fallow type, due to the large soil pool of these nutrients. Organic phosphorus inputs to the rice crop are higher in the leucaena fallows when leucaena wood removals from the site are low. The greatest potential threat to sustainability of crop production following leucaena fallows is charcoal making and the potential losses of P from the system. There appears to be little disadvantage to burning leucaena fallow fields prior to planting. The most significant advantages of burning appear to be increased P availability and reduced weeding costs, although no difference in grain yield due to burning was detected. Leucaena can increase sustainability of shifting cultivation through higher N and P contributions that result in grain yield increases. Additional increases in yield are probably attainable with the use of a minimum fallow age of 3-4 years, timely weed control and use of high-yielding, traditional rice varieties.

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