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Investigations were undertaken on an agricultural farm (52 ha) located at the Wielkopolska region, Western Poland. Soil samples (294) were collected at 0–30 cm depth from 6 fields after harvesting of winter triticale, winter wheat, winter barley, grass mixtures, winter oilseed rape and sugar beet. The following properties were determined: silt and clay, organic carbon (C org ), pH (in 0.01 moles CaCl 2 dm -3 ), cation exchange capacity (CEC). Copper (Cu), zinc (Zn), iron (Fe) and manganese (Mn) were extracted by aqua regia test (total content) and 0.005 moles diethylene triamine pentaacetic acid (DTPA), pH 7.3 (active and potentially available forms). Potential availability and soil physical as well as chemical changes of these micronutrients were evaluated by using availability factors and thermodynamic parameters (Gibbs free energy change, ∆G°). Data show that 83% of all tested soil samples were characterised by C org 7.8), ca 1/6 of investigated soils (16%) were acid, but on the other hand, 82% exhibited pH > 5.5. The mean CEC value amounted to 12.0 cmol (+) kg -1 , with 71% of soils representing CEC Cu > Fe > Mn, with respective indices 22.9 > 19.8 > 5.5 > 0.42. Next, linear relationships established for the pairs C org vs DTPA Cu,Zn,Fe,Mn resulted in the following correlation coefficients (r): C org vs DTPA Cu , r = 0.67**; C org vs DTPA Zn , r = 0.60**; C org vs DTPA Fe , r = 0.56*; C org vs DTPA Mn , r = 0.50*. It appeared that in the case of increasing or decreasing pH, the reactions of Cu, Mn and Fe in the soil solution should be similar, i.e. progressive unavailability or progressive availability, respectively. Thermodynamic changes (ΔG° values) calculated for Zn, Cu, Fe and Mn were the basis for establishing the following operational ranges: −5.0 ≤ ∆G°, readily available (for Zn); −5.0 ≥ ∆G° ≥ −10.0, moderately available (for Cu); −10.0 ≥ ∆G° ≥ −15.0, slightly available (for Fe); ∆G° ≤ −15.0, hardly available (for Mn). The ∆G°-based values showed that micronutrient availability criteria should not be restricted to quantitative evaluation only.

Site-specific evaluation of Cu, Zn, Fe and Mn availability in arable soils