Agrochemical Properties of Arable Soils in the Kostanay Region (Northern Kazakhstan) and Their Control on the Mobile Forms of Micronutrients and Heavy Metals

The mobile, not the total, content of trace elements governs micronutrient supply and heavy-metal risk in arable soils. We related the mobile forms of 18 elements to soil pH, organic matter (OM) and available N, P, K and S in four districts of the Kostanay Region, Northern Kazakhstan. Samples (n = 1050) from 67 fields were extracted with ammonium acetate buffer (pH 4.8), measured by ICP-OES and analysed at the field level by non-parametric and multivariate statistics. Two opposing groups emerged: mobile Co, Ni, Mn, Cr and Fe rose with OM and fell with pH, whereas Ca, Mg, K, Na, S, Cd, Pb and Zn behaved oppositely. pH and OM dominated (mobile Co R2 ≈ 0.75); N–P–K–S added little for Co, and more for Zn and Cd, which OM most suppressed. Mobile Co exceeded its hygienic threshold (5.0 mg kg−1) in 41% of samples, almost exclusively in low-pH, high-OM northern districts (all p < 0.001). The labile pool is organised by a single pH–OM axis; soil organic matter and reaction are therefore the principal candidate targets for managing trace-element mobility, with the qualification that mobile cobalt calls for site-specific rather than unconditional reduction, pending soil–plant validation.