Restrictions in the use of correlation coefficients in comparing methods for the determination of the micronutrients in soils

Fifty nine soil samples from the State of Sao Paulo were collected from the plow layer, with about half of these samples presenting low or medium concentrations of micronutrients. Micronutrients were extracted by one of the following solutions: DTPA at pH 7.3, DTPA at pH 5.3, Mehlich-1 BR, Mehlich-1 USA, Mehlich-3, and 0.01 mol L-1 calcium chloride (CaCl2), and then determined by inductively coupled plasma emission spectrometry (ICP-AES). Simple linear correlations were calculated for the results of all methods against the DTPA method at pH 7.3. Two sets of calculations were always considered: one including all 59 soil samples and another set including only those samples containing low and medium amounts for each element. Higher correlation coefficients were observed when samples with high contents were included. For example, the range of correlations (R2) obtained for zinc (Zn) varied from 0.41 to 0.98, if all samples were considered in the calculations, but the range decreased to 0.41 - 0.55 if samples with high contents of Zn were not considered. The correlation coefficients for CaCl2-extractable Zn were not significant. The correlation coefficient (R2) range for copper (Cu) from 0.83 to 0.94 for all samples, decreased to 0.66 - 0.72 with the exclusion of the samples with higher Cu contents. For manganese (Mn), the range of correlation coefficients (R2) varied from 0.74 to 0.92 for all samples but was reduced to 0.30 - 0.50 when the samples with high contents of Mn were excluded. In the case of iron (Fe), the DTPA method at pH 5.3 presented results close to DTPA at pH 7.3 (R2 = 0.92), while the CaCl2 procedure had the poorest correlation (R2 = 0.42). In conclusion, the substitution of an extraction procedure based only in the correlation coefficients between methods may be misleading if the interest is diagnoses in the deficiency range.