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

Abstract

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 (Corg), pH (in 0.01 moles CaCl2dm-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, ΔG°). Data show that 83% of all tested soil samples were characterised by Corg< 25 g kg-1. Moreover, the variability in pH was very high (4.7 < pHCaCl2> 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 < 12.0 cmol(+)kg-1. In fact, the range 12.0 ≤ CEC ≥ 25.0 cmol(+)kg-1, designating moderate buffering properties, shared ca 55%. The DTPA-based availability factor (AF) followed the order Zn > Cu > Fe > Mn, with respective indices 22.9 > 19.8 > 5.5 > 0.42. Next, linear relationships established for the pairs Corgvs DTPACu,Zn,Fe,Mnresulted in the following correlation coefficients (r): Corgvs DTPACu, r = 0.67**; Corgvs DTPAZn, r = 0.60**; Corgvs DTPAFe, r = 0.56*; Corgvs DTPAMn, 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 ≤ ΔG°, readily available (for Zn); -5.0 ≥ ΔG° ≥ -10.0, moderately available (for Cu); -10.0 ≥ ΔG° ≥ -15.0, slightly available (for Fe); ΔG° ≤ -15.0, hardly available (for Mn). The ΔG°-based values showed that micronutrient availability criteria should not be restricted to quantitative evaluation only.