The availability of native and applied soil cobalt to ryegrass in relation to soil cobalt and manganese status and other soil properties

A glasshouse pot trial was conducted using 18 New Zealand grassland soils to assess the effects of soil manganese (Mn) and cobalt (Co) status, Co fertiliser and soil moisture conditions, on the plant availability of soil Co. The uptake of native and applied Co by ryegrass was measured, and attempts made to determine the main factors influencing plant Co concentrations. There were highly significant relationships between ryegrass Co concentrations and total soil Mn, and EDTA-extractable and CaCl2-extractable soil Mn; ryegrass Co concentrations decreasing in a curvilinear fashion with increasing soil Mn levels. Similar relationships were observed between ryegrass Co concentrations and Mn determined in individual soil fractions as determined using a sequential fractionation technique. It was therefore clearly demonstrated that soil Mn status plays a crucial role for soil Co availability. Soils with high Mn contents have a high probability of strong fixation of soil Co and showed negligible responses to Co fertiliser treatment. For the soils in this study, which had a large variation in Mn status, soil Co extractants were poor predictors of Co availability to plants. However, soil Co fractionation data suggests that Co in the organic-bound fraction is probably one of the important sources of supplying Co in soils for plant uptake