Abstract
This paper describes the technical development and accuracy assessment of the most recent and improved version of the SoilGrids system at 250m resolution (June 2016 update). SoilGrids provides global predictions for standard numeric soil properties (organic carbon, bulk density, Cation Exchange Capacity (CEC), pH, soil texture fractions and coarse fragments) at seven standard depths (0, 5, 15, 30, 60, 100 and 200 cm), in addition to predictions of depth to bedrock and distribution of soil classes based on the World Reference Base (WRB) and USDA classification systems (ca. 280 raster layers in total). Predictions were based on ca. 150,000 soil profiles used for training and a stack of 158 remote sensing-based soil covariates (primarily derived from MODIS land products, SRTM DEM derivatives, climatic images and global landform and lithology maps), which were used to fit an ensemble of machine learning methods—random forest and gradient boosting and/or multinomial logistic regression—as implemented in the R packages ranger, xgboost, nnet and caret. The results of 10–fold cross-validation show that the ensemble models explain between 56% (coarse fragments) and 83% (pH) of variation with an overall average of 61%. Improvements in the relative accuracy considering the amount of variation explained, in comparison to the previous version of SoilGrids at 1 km spatial resolution, range from 60 to 230%. Improvements can be attributed to: (1) the use of machine learning instead of linear regression, (2) to considerable investments in preparing finer resolution covariate layers and (3) to insertion of additional soil profiles. Further development of SoilGrids could include refinement of methods to incorporate input uncertainties and derivation of posterior probability distributions (per pixel), and further automation of spatial modeling so that soil maps can be generated for potentially hundreds of soil variables. Another area of future research is the development of methods for multiscale merging of SoilGrids predictions with local and/or national gridded soil products (e.g. up to 50 m spatial resolution) so that increasingly more accurate, complete and consistent global soil information can be produced. SoilGrids are available under the Open Data Base License.
Highlights
There is a growing demand for detailed soil information, especially for global estimation of soil organic carbon [1,2,3] and for modeling agricultural productivity [4, 5]
The first version of SoilGrids, was, at the time, a ‘proof of concept’ demonstrating that global compilations of soil profiles can be used in an automated framework to produce complete and consistent spatial predictions of soil properties and classes [9]
Mulder et al [10] observed, using more detailed soil profile data and maps, that SoilGrids likely overestimated all low values for organic carbon content in France
Summary
There is a growing demand for detailed soil information, especially for global estimation of soil organic carbon [1,2,3] and for modeling agricultural productivity [4, 5]. Spatial information about soil water parameters is likely to become increasingly critical in areas affected by climate change [6]. The first version of SoilGrids (predictions at 1 km spatial resolution released in 2014), was, at the time, a ‘proof of concept’ demonstrating that global compilations of soil profiles can be used in an automated framework to produce complete and consistent spatial predictions of soil properties and classes [9]. Mulder et al [10] observed, using more detailed soil profile data and maps, that SoilGrids likely overestimated all low values for organic carbon content in France. Some users have expressed concerns that the first version of SoilGrids did not provide predictions for arid and desert areas and can be considered an incomplete product [13]
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