Modelling of soil nutrient budgets: an assessment of agricultural sustainability in Nepal

Sustaining soil fertility under agricultural intensification and expansion onto marginal lands is a significant challenge in the Nepalese Middle Mount ains. In a detailed watershed study it was shown that the overall soil fert ility is poor, forest soils display the poorest conditions as a result of b iomass removal, and sustaining agriculture is questionable due to the trans formation from traditional to multiple cropping systems. Parent material is a significant factor influencing low phosphorus status while insufficient inputs create deficiencies in total carbon, nitrogen and bases. A nutrient budget model was developed to assess inputs, redistribution and losses rela tive to soil fertility Yield, input and management data obtained from farm interviews, and soil analysis data were used in the calculation of nutrient budgets. Results from modelling indicate declining soil fertility under ra infed agriculture, forest and rangelands, and marginal conditions under irr igated agriculture subject to intensive cultivation. Nutrient deficits were relatively low for irrigated rice-wheat systems, which benefit from nutrie nt inputs via sediments and irrigation waters, but the introduction of trip le cropping showed greater deficits. Nutrient balances were most critical u nder rainfed maize production where 94% of the farms were in deficit. Curre nt shortages of organic matter make elimination of nutrient deficits proble matic but improvement of composting, biological N-fixation and fertilizer e fficiency and reducing erosion were found to be potential options.