Soil pH and Texture Affect the Relationship Between 1 M HCl and Mehlich 3 Micronutrient Extractions: Implications for Soil-Test Conversion

Soil testing methods differ in their ability to extract micronutrients, making comparisons between historical and newly adopted analytical procedures difficult. This study evaluated the effects of soil pH and texture on the relationship between micronutrient concentrations extracted by 1 M HCl (HCL) and Mehlich 3 (M3) and assessed the possibility of converting results between the two method. A total of 3865 agricultural soils representing a wide range of pH and texture were analysed for B, Cu, Fe, Mn and Zn using both extractants. Relative extractability (M3/HCl), correlations between methods and regression models for converting HCl values to M3 were evaluated across soil pH and fine fraction classes. M3 extracted lower concentrations of all micronutrients than HCl. Relative extractability was approximately 50–60% for B, Cu, Mn and Zn but only 27% for Fe. Soil pH primarily affected the relative extractability of B and Fe, whereas soil texture had a stronger influence on all elements, particularly Fe. Correlations between methods varied among micronutrients and soil conditions. A single conversion equation was applicable for Zn across all soils (R2 = 0.63), whereas conversion of Cu, B and Mn was limited to specific soil groups. No reliable conversion model was obtained for Fe. The relationship between HCl and M3 depends on both the micronutrient analysed and soil properties. Soil texture has a greater influence than pH on differences between the two extraction methods. Universal conversion of historical HCl data to M3 values is feasible only for Zn, while other micronutrients require soil-specific approaches.