Potential bias in cross-validation results
The evaluation metrics derived from cross-validation may not be unbiased estimators of the true numeric evaluation metrics due to non-uniform distribution of soil observations across the globe.
Simplistic approach to vertical soil variability
The vertical dimension of soil variability is addressed using depth of observation as a covariate, which may be too simple and lead to inconsistency over the predicted depth sequence, especially for datasets where short-range spatial variability is similar to vertical variability.
The study uses legacy soil data, which may not reflect current soil conditions due to changes in land use or management practices, particularly for dynamic soil properties like pH and soil organic matter content.
Lack of consideration for age of observations
The study doesn't address the age of observations, which could impact the accuracy for dynamic soil properties significantly affected by changes in land use or management.
Uncertainty quantification limitations
The uncertainty assessment uses quantile random forests, which may overestimate uncertainty for coarse fragments and underestimate it for sand. The uncertainty for silt and clay might be overestimated. The transformation method used to derive prediction intervals for texture components could contribute to these inaccuracies.
Limited use at local scale
The study acknowledges that SoilGrids250m is not intended for detailed, subnational, or local-scale use as more comprehensive local datasets are usually available for those applications.
Uncertainty visualization limitations
The visualization of uncertainty as maps using the interquartile range over the median ratio might not be fully accurate, particularly for properties with values near zero, like coarse fragments.
Limited scope of soil properties
The study only models some primary soil properties, and more work is needed to map other crucial information such as soil thickness, hydrologic soil properties, and complex properties like carbon stocks. These properties are important for modeling soil functions and Earth system modeling.