Soil fertility evaluation of slope farmland in karst area：A case study of slope farmland in Zhongliang mountain in Chongqing

Slope farmland is one of the main land-use types in southwest China, and also an important farm land resource in China. However, rocky desertification and soil erosion problems are serious in slope farmland areas, leading to soil fertility degradation. Taking Zhongliang mountain in Chongqing as a case, the soil fertility of slope farmland was evaluated on a small scale by the fuzzy comprehensive index method. According to statistical analysis of sample number rationality, the regional soil fertility evaluation based on 25 soil samples was sufficient to meet the required accuracy of 10 %, 15 % and 20 % relative error at a 90 % confidence level. The minimum data set (MDS) components including soil organic matter, total nitrogen, total phosphorus, total potassium, available nitrogen, soil depth and silt/clay were selected by calculating the norm values based on principal components analysis (PCA). Correlation coefficient test between the indicators inside MDS showed that MDS represented maximally all the soil properties with minimal loss of soil fertility information. Membership function was used to transform the indicators into dimensionless data and a weighted summation model was used to calculate the integrated fertility index (IFI). The membership values and IFI reflect the single factor fertility and the integral fertility respectively. This indicated that soil organic matter and total phosphorus were the limiting factors for the soil fertility in the study area, and soil integral fertility was at a medium level with integrated fertility index (IFI) ranging from 0.40 to 0.60.