Manipulating geological phosphorus resources for improved production and environmental outcomes during plant establishment.

Phosphorus (P) fertilisers are under scrutiny due to resource constraints and environmental impacts. Simple rock phosphate (RP) modifications with acids and co-applied with microbial inoculum could offer sustainable alternative P fertiliser products. We evaluated the effects of acid-treated rock phosphate (RP) in combination with fungal inoculum on plant establishment, environmental impacts (nutrient leaching) and soil quality in a 5-month pot trial. The treatments were evaluated in a clayey Vertisol and a silty Acrisol using cotton (Gossypium hirsutum) as a model plant. The RP treatments - apart from the unmodified and HCl products - were effective in promoting plant establishment with two of the microbial formulations superior to conventional P fertilisers by an average factor of 2 in both soil types (p < 0.05). All RP products restricted P leaching compared with conventional P fertilisers (p < 0.05), by an average factor of 5 for diammonium phosphate (DAP) in both soil types and 3 for the triple superphosphate TSP (only in Acrisol). Nitrate leaching from all treatments was high although much lower from the RP treatments compared with the conventional fertilisers towards the end of the establishment trial, by an average factor of 1.5 (p < 0.05). Ranking analysis revealed that some RP treatments showed evidence for improved ongoing soil quality, including decreased P leaching and soil acidification risks. Microbial analysis showed complex interactions between treatment and soil type. Nonetheless, inoculum persistence at the end of the plant establishment phase was observed for all pots analysed. Our results demonstrate that relatively simple modifications to RP could pave the way for developing sustainable P fertilisers.