Fractionation of cobalt and manganese in New Zealand soils

Abstract

Many New Zealand soils are known to produce pastures deficient in cobalt (Co) for grazing ruminants; however, the development of an effective soil diagnostic test for Co is hindered by the lack of knowledge of the forms and distribution of Co in New Zealand soils. A sequential fractionation technique was used to determine the forms and concentrations of native and/or fertiliser derived soil Co in 18 New Zealand grassland topsoils, and the distribution and interrelationships of Co between fractions compared with those for manganese (Mn). Previous studies in other countries have indicated a strong association between Co and Mn in soils. For both Co and Mn, very small proportions were present in soluble and exchangeable forms, and the highest proportions of Co and Mn (mean 45.8% and 35.8%, respectively) were found in the residual fraction. However, there was little similarity overall in the distribution of Co and Mn between fractions. Multiple regression models, and the presence of substantial amounts of Co and Mn in iron (Fe) oxide fractions, indicate that the Fe content of the soil is much more important than Mn in influencing the distribution of Co in the soils. Exchangeable Co and Mn also appear highly sensitive to soil pH. Soil organic matter did not appear to have a major effect on the distribution of Co in soils, apart from Pumice Soils that have low concentrations of Fe and Mn.