The U.S. soils data were obtained from the USDA STATSGO dataset (; verified 19 Mar. 2010). The China soils data were taken from a 1:4,000,000 soil map of China that was produced by the Ministry of Agriculture of China and the National Soil Survey office of China (Shi et al., 2004). The digital version of the original map was obtained from the Chinese Academy of Agricultural Sciences. The polygon feature data were then converted into a raster-gridded dataset at 2.5-min (~4 km) resolution. Each grid cell was assigned a pH, drainage, and salinity value and category based on the values of the polygon features in the original data.

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