Effects of organic-acid treatment of phosphate rocks on the phosphorus availability to Italian ryegrass

Abstract

The solubility of six phosphate rocks (PRs) in organic acids was studied in order to increase phosphorus (P) availability of low-rcactive PRs to crops. One gram of PR was incubated with 25 mL of 100 mM solution of ten low molecular-weight aliphatic organic acids and sulfuric acid for 3 days at 28°C. Tartaric, sulfuric and oxalic acid solubilized 31–34%. 30–37% and 18–26% of total P. respectively, but other acids dissolved less than 13% of the total P. Ca concentration in the solution at equilibrium was highest in citric acid and lowest in tartaric acid, indicating that Ca exclusion from the solution accelerated P release from the PRs.Based on the above-mentioned results, one gram of each PR was treated with 2.5 mmole (2.5 mL of 1M solution) of oxalic or tartaric acid at 28°C for three days. The resulting materials dried easily under room conditions within a week and contained high amounts of water soluble P (14–41 mg P g−1 treated PR). They were applied to a Typic Udipsamment (sandy siliceous) at 200 mg P pot−1 where Italian ryegrass (Lolium multiflorum Lam.) was grown for 56 days. Both dry matter yield and P uptake were increased by the acid-treated PRs 3–4 times over the untreated PRs and highly correlated with the water-soluble P content of the PRs. Since acidulation of PRs with the organic acids is a simple and effective way to improve P availability of PRs to crops, it will provide practical means for efficient utilization of low-reactive PRs if the organic acids are available.