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Determinations of soil organic carbon using the Dumas method were in the range of 0.09-9.49% C with a mean of 2.37% C in the ILPP data.
For this case study, we chose to use data from ILPP and the DEDJTR Macleod quality control program (Table 2).
Results from the study of the ILPP data showed that there were indeed differences between these two popular methods as they were applied in different laboratories and that each method was applied with different levels of precision in the different laboratories.
This contrasts with the ILPP data where the application of the Dumas method was often more precise but its application was not more precise by a factor of the magnitude observed by Lettens et al.
Values for the ILPP robust CV (Table 3) were available for 10 attributes: [pH.sub.water], organic C measured with the Walkely-Black method, organic C measured with the furnace method, total N, exchangeable Ca, Mg, K, Na, total P measured with an x-ray fluorescence (XRF) method, and bicarbonate P measured with the Colwell method.
For all attributes together, the ILPP robust CV correlated strongly with the cross-validation SEP (Pearson correlation coefficient, p = 0.78), which measures the precision of the spectroscopic models (Fig.
Without the data for organic C measured using the Walkley-Black method, the correlation between the ILPP robust CV and the (absolute relative) bias of the spectroscopic models' cross validations increased (p = 0.36, Fig.
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