MODELING THE IMPACT OF EROSION ON SOIL PRODUCTIVITY - A COMPARATIVE-EVALUATION OF APPROACHES ON DATA FROM SOUTHERN BRAZIL

Erosion changes soil properties, removes nutrients and alters crop yie lds. A knowledge of these impacts on soil productivity is needed for e conomic analyses of erosion and conservation. Based on a United Nation s Food and Agriculture Organization experimental design to monitor the se changes, results are reported from four research sites in southern Brazil on Ferralsols and Cambisols, enabling the construction of erosi on-yield-time and nutrient loss relationships. Plot experiments ran fo r up to seven years of natural erosion, followed by one or two years o f maize cropping. A remarkably consistent composite erosion-yield rela tionship in logarithmic form was found, showing a sharp yield decline with initial soil loss. Soil 'resilience' was identified through erosi on-time relationships, 'sensitivity' through erosion-yield equations. As erosion progressed, losses of nutrients, especially of organic carb on and calcium, were significant. In situ changes in soil properties w ere far less marked. Together with measured yield reductions caused by cumulative erosion, these results enabled the modelling of changes in soil productivity over time with respect to both soil quality and imp act on yields. A production 'half-life' of between one and 39 years ac cording to soil type and level of erosion was also identified.