Responses to controlled release potassium fertilisers in agriculture following phosphate mining

The transition from mining to agriculture is hampered by a range of abiotic challenges to crop growth, including nutritional issues and heavy metal stress. Building on our previous work showing that potassium (K) limits legume growth in post-phosphate mining substrates on tropical Christmas Island, Australia, we undertook two field trials. The first compared the efficacy of controlled release K fertilisers (CRFs: KCl 2-month release, K2SO4 3-month and K2SO4 9-month) with immediately available potassium sulfate (K2SO4) fertiliser, on the legume Lablab purpureus. The second trial tested responses of L. purpureus to different rates of K2SO4 9-month CRF, and a combination treatment (CRF and K2SO4). Both trials were undertaken to determine how CRFs compare with immediately available K2SO4 in terms of increasing biomass, reducing cadmium (Cd) concentrations, maximising plant K concentrations and maintaining K soil retention. The first trial revealed that K2SO4 3-month and 9-month CRFs were similar to the 160 kg/ha K2SO4 treatment in significantly increasing L. purpureus biomass. Plant Cd and other heavy metal concentrations were significantly lower as plant biomass increased with increasing K, including with CRFs. The second trial showed no difference between various rates of K2SO4 9-month CRF and immediately available 160 kg/ha K2SO4 to increase biomass, reduce Cd or increase K concentrations. We have shown that although post-phosphate mining substrates can limit legume growth, high biomass can be attained with some CRFs, or K2SO4 at 160 kg/ha. Optimising nutrient input in post-mining agriculture is critical for developing safe, sustainable crops.