Relationships of soil properties with Mn and Zn distribution in acidic soils and their uptake by a barley crop

The extractability and solid-phase fractionation of manganese (Mn) and zinc (Zn) in acid-to-neutral agricultural soils from Central Spain was evaluated by sampling and analysing twenty-nine representative soils and by greenhouse cropping eleven of them with spring barley (Hordeum vulgare, L.). All soil samples were extracted with three chemical extractants commonly used for soil fertility evaluation (0.43 M HOAc, DTPA and Mehlich-3). The soil samples were also operationally determined in six steps with the following extractants: 1 M Mg(NO3)2 extractable (WSEX, water soluble plus exchangeable), 0.7 M NaOCl extractable (OC, organically complexed), 0.1 M NH2OH·HCl extractable (MnOX, Mn-oxide), 0.2 M (NH4)2C2O4 + 0.2 M H2C2O4 extractable (AFeOX, amorphous Fe-oxide), 0.2 M (NH4)2C2O4 + 0.2 M H2C2O4 + 0.1 M ascorbic acid extractable (CFeOX, crystalline Fe-oxide), and HCl, HNO3, and HF in mixture (RES, residual). Soil-extractable amounts for the three single extractants were highly correlated with each other for both metals. Distributions among metal fractions showed that Mn was mainly found in the MnOX fraction (30.9%, ranging from 13.0 to 51.2%), whereas Zn was predominantly found in the RES fraction (44.3%, ranging from 26.4 to 56.8%). The proportion of Mn fractions extracted from the soils was in the order as follows: CFeOX ≤ WSEX = OC ≤ AFeOX = RES < MnOX, whereas Zn was in the order: WSEX ≤ OC ≤ AFeOX < MnOX < CFeOX < RES. The soil properties that correlated best with the distribution of Mn and Zn forms in these soils were soil organic matter and pH. The “availability factor” values [AF = (WSEX + OC) 100 / total metal], were higher for Mn than for Zn in these soils. Plant metal concentrations (Y) and soil-extractable and sequential extracted fractions showed few significant correlations. However, it was possible to significantly predict the phytoavailability of Mn and Zn for barley using a series of empirical equations involving extractable metals, solid-phase fractions and soil properties as components. The R2 values of the best-fit regression models ranging from 0.50 [Y-Zn = 19.3 + 6.32 (WSEX + OC)-Zn] to 0.92 [Y-Zn = 57.3 + 0.23 P − 8.56 pH + 20.6 DTPA-Zn].