Accelerated soil erosion, a process basically induced by human activities contributes greatly to the degradation of arable land quality throughout the world, and is the main non-point source of surface water resource pollution. Considering the effective demand for developing indicators to evaluate the impact of soil erosion on soil quality in agriculture production systems, the objective of this study was to develop an index with a predictive value to be applied as a planning tool for interpreting soil loss tolerance in agricultural areas. The "Soil Useful Life Index" was developed and applied to diagnose erosion in an area predominantly used for sugarcane cultivation in the Piracicaba county, São Paulo State, Brazil. Geotechnology and geostatistics analysis methods were used in this study and the data processed and analyzed in a geographic matrix information system. Average annual soil loss rates had been estimated in a previous study using the universal soil loss equation (USLE), adjusting the model factors to the local study conditions. A 0.2 mm year-1 renewal rate was assumed in the soil useful life index calculation, and two depths considered minimal for agricultural purposes (50 and 100 cm) were analyzed. The evaluation of soil thickness revealed that shallower soils are predominant in the study area, with average depths of 78 cm in areas occupied by sugarcane and 72 cm in areas used for other purposes. The application of the soil useful life index revealed that at the critical depth of 50 cm, the useful life of soils occupied by sugarcane is 178 years. By maintaining the current soil loss expectations, little more than 70 years will be sufficient to degrade 50 % of the area cultivated with sugarcane (soil half-life). The situation worsens at critical depths of 100 cm. Average soil useful life in areas occupied by sugarcane falls to only 102 years, and soil half-life is only 42 years. The method also permitted an estimation of the proportions of areas with sugarcane where the impact is already permanent (zero soil useful life) at 19 and 74 %, that is, where soil loss rates exceed the soil renewal rate, and soil thickness is already less than the critical depths considered, 50 and 100 cm, respectively. Furthermore, under the current use and management conditions, the resources conservation, that is the soil renewal rate is higher than the estimated soil loss rates by erosion, can be characterized in only 7.6 ha, or less than 1 % of the area under sugarcane. Therefore, in more than 99 % of the area occupied by sugarcane the estimated rates of soil loss by erosion exceed the soil renewal rate (p > r), characterizing a situation of resource degradation. The proposed index proved to be a promising tool for interpreting soil loss tolerance in the planning of sustainable agriculture.
IDRISI software; GIS; soil half-life; USLE
