Determination of Soil Micronutrient Critical Values Using Mehlich 3 Extractant for Principal Field Crops

Reliable soil critical values (SCVs) for micronutrients are essential for accurate fertilizer recommendations. This study established crop-specific SCVs for boron (B), copper (Cu), iron (Fe), manganese (Mn), and zinc (Zn) extracted with Mehlich-3 under Polish soil and climatic conditions. Extensive paired soil–plant datasets were collected for wheat (n = 1921), oilseed rape (n = 1944), and maize (n = 916) across all provinces. Micronutrients were determined in all soil and plant samples, with soil extractions performed using the Mehlich-3 method. Two plant-based calibration approaches were applied: (i) regression models linking the bioaccumulation factor (plant-to-soil concentration ratio) to soil properties, and (ii) the highyield method, defining SCVs as the lower quintile of micronutrient levels in soils from high-yielding fields. Both approaches yielded comparable results. Soil pH, organic carbon, available phosphorus, and texture were the key variables influencing SCVs, which differed among crops and elements: B and Mn were pH-dependent, Cu correlated with organic carbon, while Fe and Zn were associated with phosphorus or texture. Final SCVs ranged from B 0.10–0.90, Cu 1.0–2.2, Fe 160–280, Mn 30–75, and Zn 2.5–7.0 mg kg−1, depending on crop and soil class. These empirically derived, crop-specific Mehlich-3 SCVs provide a robust basis for micronutrient diagnostics and fertilizer management in temperate agricultural soils.