Slow‐release cobalt fertilisers: What release rate is required?

Abstract

Slow-release cobalt (Co) fertilisers have been suggested as a way of avoiding the pattern (often observed with soluble Co fertilisers) of immediate elevation of herbage Co concentration to high levels followed by a rapid decline to deficient levels. However, the Co-release rate required to maintain adequate pasture Co concentrations is not known. Our work aimed to determine what Co-release rate is required for such a material to be effective. Cobalt-containing phosphate glasses with varying rates of dissolution in water were prepared and their rates of cobalt release measured in the laboratory. Materials were selected to cover a wide range of cobalt release rates (5-61% of total cobalt in 24 h during laboratory leaching with water) and evaluated in field experiments at Rotorua and Raglan, North Island, New Zealand. Measurements were aimed at testing the ability of the glasses to extend the period of elevation of herbage Co concentrations compared with cobalt sulphate. The fastest leaching glass was found to elevate herbage Co concentrations above control values for 2 or 3 months longer than CoSO 4 . Periods of elevated Co concentration in herbage could be increased by using material with a slightly slower release rate. A mixture of different release rates, determined by particle size and/or composition, could also be considered. Results of these field experiments will assist interpretation of laboratory leaching results for evaluating any material being considered as a slow release cobalt fertiliser. A similar approach could be used with other trace elements of interest, such as copper.