Effect of cobalt application on the cobalt status of pastures 2. Pastures without previous cobalt application

Abstract

A field trial was carried out on a cobalt (Co)-deficient soil to determine the effect of repeated annual Co application on plant and soil Co concentrations. Initially, Co concentration in white clover and ryegrass increased immediately to high values in the month after Co application (350 g/ha CoSO₄) and then decreased quickly to become inadequate for sheep within 2–3 months. After six or seven applications, the pattern of increase and rapid decrease continued but plant Co remained above 0.08 mg/kg DM (adequate for sheep) for much of the season. A large initial application (3150 g/ha CoSO₄) maintained plant Co above 0.08 mg/kg DM for 1 year only and appeared to be less effective in the long term than repeated annual additions. Soil Co extracted with EDTA increased gradually with each annual application of Co but reached an equilibrium value after 7 years. It is concluded that Co needs to be applied continuously to Co-deficient soil for 7–10 years to grow pasture that is Co-sufficient for livestock. Then animal tissue and plant tests should be used as a guide to the need for further application.

Keywords:

• pasture cobalt
• EDTA soil cobalt
• sheep
• fertiliser application