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Soil Phosphorus Fertility Degradation: A Geographic Information System-Based Assessment

Institute for Resources and Environment, Univ. of British Columbia, Rm. 436E, 2206 East Mall, Vancouver, BC, Canada V6T 1Z3.

P. B. Shah

International Centre for Integrated Mountain Development, Kathmandu, Nepal.

^* Corresponding author (sjbrown{at}interchange.ubc.ca).

ABSTRACT

Soil phosphorus is a critical macronutrient limiting agricultural productivity in many parts of Nepal. This study evaluates a geographic information system (GIS)-based approach to assess soil P degradation risk as influenced by site factors and human land use impacts. The status of P fertility in a Nepalese watershed was evaluated by stratifying the soil analysis by soil type, elevation, aspect, and land use (irrigated or rainfed agriculture, rangeland, and forests). Human impacts were shown to be significant, and with GIS overlay techniques it was possible to produce a soil P status map based on land use and soil type. Some 27% of the area was found to be deficient in P. Soil nutrient budgets displaying annual surplus or deficit conditions for the common crops were combined with the soil fertility map to derive a soil P degradation risk map. Low P conditions with high annual deficits posed the greatest degradation risk, while adequate P status with high annual deficits were of more long-term concern. Forty-eight percent of the area was considered at low risk because of adequate conditions and minimal deficits for P. In contrast, some 36% of the study area had a high short-term risk for degradation because of the low status and high annual deficit in P. Given the dynamics of soil fertility, the site factor approach used in combination with soil analysis, nutrient budget calculations based on farm interview data, and GIS overlay techniques provided a unique way of assessing long-term soil P degradation risks.