A fuzzy logic slope-form system for predictive soil mapping of a landscape-scale area with strong relief conditions

We studied an improved slope form system using a fuzzy logic method to assess and map soil fertility of a mountain region in northern Vietnam that has strong relief conditions. The lack of good soil mapping techniques in Vietnam has brought about insufficient soil information, which often leads to false recommendations for land use and crop planning. The reviewed literature describes soil-mapping techniques using fuzzy logic method, but all of them are applied for mapping areas that have gentle relief conditions that are unlikely to be applicable to the mountainous soils in our study area. In this paper, we introduce a detailed slope form system that significantly describes the complexity of terrain characteristics of the area to be mapped, and provides more detail about the variability of the soil fertility of the area. Nine basic slopeforms were used to characterize for each of upper-, middle-, and foot slope positions, making the list 27 slopeforms. Together with crest and valley, the total unit number is 29. We investigated soils of the area and classified them into the major soil groups and calculated soil property indices for all of them. We identified four major environmental parameters affecting soil formation and soil quality: geology, elevation, slope inclination and land use. The findings indicated that soil fertility differs at slope positions. Soils located at upper slope positions, where agricultural activity only started recently, are more fertile than those found at middle slope positions. Soils located at foot slope positions, where eroded sediments accumulate, also have high levels of fertility compared to those on the middle slope. The improved slope form system then became an important additional environmental parameter for this soil mapping work. At a same comparable category, i.e. slope position, geology, soil group, elevation, slope gradient, straight slopeforms are an indicator for better soil fertility compared to convex and concave forms. Although the findings could not specify soil fertility variability for all 29 slopeforms, they did emphasize the major differences in soil fertility and soil formation based on three major forms of convex, straight and concave, with other factors taken into account, such as slope inclination, geology and elevation. We expect our results to be used by scientists and local authorities in deriving more effective land use and crop options for land use management strategies for the northern Vietnam's mountain regions.