soil organic matter Soils 7 soil fertility 4 Biogeochemistry 3 Forestry 3 Land use 3 Organic carbon 3 Organic compounds 3 Soil fertility 3 Soil surveys 3 agricultural land 3 nitrogen 3 soil carbon 3 soil quality 3 Carbon 2 China 2 Decision trees 2 Indicator indicator 2 Interpolation 2 Machine learning 2 Minerals 2 Nitrogen 2 Random forest 2 Random forests 2 Soil organic matters 2 accuracy assessment 2 carbon sequestration 2 crop yield 2 fertilizer application 2 machine learning 2