Amounts and forms of micronutrient cations in a group of loessial grassland soils of New Zealand

The status of micronutrient cations Fe, Mn, Zn, Cu and Co was examined in a group of soils from beneath undisturbed grassland sites. The soils were selected such that factors of soil formation besides loess accumulation and rainfall remained relatively constant. In a previous study these two factors were found to have a large effect on the K status of the soils, but results from this study showed that there had been little effect on micronutrient content. With decreasing loess accumulation the total and HCl- and EDTA-extractable micronutrient content of soils tended to decrease, but rainfall had no consistent effect. Extractable levels of Cu and Co in A horizons tended to decrease from high to potentially deficient values with decreasing loess accumulation.HCl- and EDTA-extractable Fe, Mn, Zn and Cu were clearly higher in A than B horizons although total contents tended to be lower in A horizons. A chemical fractionation procedure demonstrated that there were greater quantities of pyrophosphate- and oxalate-extractable Zn and Cu and acetic acid-extractable Zn in the A than B horizons.With the exception of HCl-extractable Cu, extractable micronutrients were significantly correlated with the organic matter content of the HCl or EDTA extracts. There were no significant correlations between extractable micronutrients and soil pH or oxalate-extractable Al or Fe.Acetic acid-extractable Co was not significantly correlated with any measured soil property. It is concluded that the cycling of nutrients and the build-up of organic matter, which characteristically occur under grassland conditions in New Zealand, were the major factors influencing the micronutrient status of the soils.