Comprehensive evaluation of soil fertility in karst area by GIS and improved Grey Relation Model：A case study in Fengdu county karst area, Chongqing

Soil fertility evaluation can provide the basis for rational fertilization and agricultural production structure adjustment. Under the technical support of GIS, we take Fengdu county karst area as research object and collected 316 soil sample of this county, according to topographical features, land use status and soil types. In this paper, we report organic matter, total nitrogen, available nitrogen, available phosphorus, available potassium, available sulfur, available cuprum, available iron, available manganese, available zinc, available boron content and pH for all soil samples. Based on data, we evaluated the soil fertility of the county by using Grey Relation Model improved by principal component analysis and membership function, and discussed the special distribution characteristics by using statistical methods. The study shows that the soil integrated fertility index (IFI) in the study area ranged from 0.352 to 0.881 with the average value of 0.579 in the study area. Geostatistical trend analysis of IFI presented a one-order (linear) global trend and semi-variance analysis of the residuals showed a weak degree of spatial correlation. Overall fertility quality in the study area is middle or better, the second soil fertility grade region accounted for 10.61%, the third soil fertility grade region accounted for 31.09%, and the fourth soil fertility grade region accounted for 35.94%. The contribution rates for integrated fertility of soil conventional nutrient indexes and microelement indexes were 76.77% and 23.23%, respectively. Therefore, bringing microelement indexes into the soil fertility evaluation index system is reasonable and practicable, the improved Grey Relation Model can quantitatively evaluate integrated soil fertility, and the geostatistical method can satisfactorily reveal spatial variation patterns in soil fertility.