Abstract: In order to prevent from the deterioration of soil fertility and the decrease of land productivity, the factors, such as rate of soil forming, the depth of soil layer and the deterioration of soil fertility, are mainly considered in previous definitions of soil loss tolerance (T). In general, the soil loss tolerance is determined on the rate of soil loss which should be less than the rate of soil forming, and under such soil loss tolerance the soil resources could be continuously renewed. However, soil erosion not only lies on human disturbance but also on natural factors. Besides the loss of soil nutrients and the decrease of soil productivity in localities where erosion occurs, a series of environmental problems may be developed far from the \{erosion\}. If the factors for improving soil quality and land productivity are merely considered while determining the soil loss tolerance, it does not suit the need of social evolvement and sustainable development of environment. Protecting and improving productive and existent environment must be regarded as important measures. Therefore, a new concept of soil loss tolerance proposed in the paper is based on the previously mentioned cognition. To determin the soil loss tolerance, ecological and environmental impacts should be taken into account. When sediment, which carried nutrients such as total nitrogen (TN) and total phosphorus (TP), enters into water bodies, the content of TN and TP increase in water and the water is polluted. Based on the contribution of nutrients loss due to soil loss to total input pollutant entering Taihu Lake the method for determining soil loss tolerance is set up under the water quality restriction. Then a general method for determining soil loss tolerance is further discussed.

Key words: soil erosion, impact on ecological environment, soil loss tolerance, water quality restriction