Comparison of soil variability in a detailed and a reconnaissance soil map in central Iran

Abstract

Quality of soil maps is a prerequisite for rational land use and soil management. To assess the accuracy of the existing reconnaissance soil map (1:50,000) of the Chaharmahal Bakhtiari province, Iran, a second-order soil survey was conducted. Using aerial photo interpretation, profile description, and field observations, a detailed soil map (1:20,000) was prepared. Then, 85 pedons were described and samples were taken for laboratory analyses. The location of pedons was determined with a Global Positioning System (GPS) and then marked on both soil maps using ILWIS software. Properties of the surface horizons including percentage of gravel, clay, sand, silt, CaCO 3 equivalent, organic matter, and the thickness of A horizon were determined. Although detailed soil map units were more homogenous, purity of map units at family and series levels was lower than those expected in American Soil Survey Manual. Taxonomic purity at series level was 18.75% to 33.3% and 0% to 1.3% for detailed and reconnaissance soil maps, respectively. With respect to similar soils, interpretive purity could improve to 75% in detailed soil map. For all the surface soil properties except organic matter, the detailed soil map had within-map unit variances that were smaller than the total variance, whereas reconnaissance soil map variance remained high. Soil classification in both the detailed and the reconnaissance mapping units accounted for 5% to 45% and 1.1% to 27% of the total variance for these properties. Sampling demand to predict mean values was much larger in the reconnaissance soil map for most of the topsoil properties. Laboratory studies showed that the detailed soil map units in similar physiographic units could have different soils and vice versa. It appears that the definition of mapping units based on guidelines by Soil Survey Manual is rather optimistic and does not have sufficient reliability for using in sustainable agriculture.