Generalization estimates for the soil characterization data (including REE) based on the the three sets of data (bulk composition, pXRF, and color sensor).

Root mean squared errors (RMSE) and the coefficient of determination (R2) are provided for both the average and optimized test-train splits, calculating using a binned stratification resampling procedure to reduce potential bias from the train-test split in the Extremely Randomized Trees (ET) model. The values represent the results after 500 sampling iterations, illustrating both the expected (average RMSE) and best-case (optimized RMSE) model performance.