Using Bio-Organic Acids to Improve Agronomic Efficiency of Unreactive Phosphate Rock.

The principal mechanism of mineral P solubilization by P solubilizing bacteria (PSB) strains is associated with the release of low molecular weight organic acids (LMWOAs), of which gluconic and 2-ketogluconic acids seem to be the most frequent agent of mineral P solubilization. Gluconic acid and other LMWOAs are synthesized from a wide range of bacteria and fungi.  These organic acids can dissolve poorly soluble calcium phosphates such as hydroxyapatite and phosphate rock (PR). The overall objective of this study was to evaluate the effectiveness of bio-organic acids in solubilizing PR for plant use. In a preliminary study, the effectiveness of carboxylic acids, including maleic acid, citric acid and gluconic acid as additives to enhance the solubilization of P from two PRs (Idaho and North Florida PRs) of different reactivity was evaluated, using wheat as a test crop in a greenhouse. Results from the preliminary study showed that gluconic acid-amended PR was as effective as TSP. In follow-up studies, we amended four different PRs (Namphos, Gafsa, Egyptian and Sechura) and compared their effectiveness as P sources with TSP for two continuous seasons in a greenhouse, using soybean, wheat and canola as test crops. The crops were grown on a highly weathered, high P-fixing soil, and the crops were grown to maturity. Biomass and grain yields and P uptake were measured, and agronomic effectiveness of the gluconic acid-amended PRs were calculated. The combined results indicate that amending the PRs with gluconic acid greatly enhanced the relative agronomic effectiveness (RAE) of the PRs with RAE values ranging from 0.85 to 1. Thus, a low input technology of adding a modest amount (~1% a.i.) of gluconic acid to non-reactive PRs could make the PRs >85% as efficient as TSP. We hypothesize that bioprocessing of PR with organic acids to improve P availability could be an energy efficient, environmentally desirable alternative to current technology for industrial P fertilizer production.