Residual effects of organic acid-treated phosphate rocks on some soil properties and phosphate availability

The effects of the application of organic acid-treated phosphate rocks on the growth and nutrient uptake of Italian rye grass (Lolium multiflorum Lam. cv. Tachiwase) and some properties of the soil were evaluated in a greenhouse pot experiment. Phosphate rocks (PRs) collected from six countries; China, Florida (USA), Jordan, Sri Lanka, Togo, and Tanzania, were treated with 1 M oxalic or tartaric acid at the ratio of 2.5 mL g^-1 PR. The organic acid-treated PRs, containing 12–31% water soluble P, were applied to a granitic regosol (pH 5.8) at 200 mg P pot^-1 (4 kg soil). Untreated PRs and single superphosphate (SSP) were included in the treatments. Italian ryegrass was grown for 175 dafter planting (DAP) with ample supply of other nutrients and water. Shoots were harvested at 56, 119, and 175 DAP and the soils were analyzed for pH and Olsen-P after the experiment. Application of organic acid-treated PRs consistently increased the dry matter yield and P uptake of the plants compared with the application of untreated PRs at each harvest, but they were less effective than SSP. A larger amount of P (calculated per unit water-soluble P applied) was recovered from the organic acid-treated PRs than from SSP. The amount of residual extractable P in the soils with the organic acid-treated PRs was about the same as or significantly larger than that in the soil treated with SSP. Soil pH was also significantly higher than in the control and SSP soils. The results suggest that organic acids could be used to improve the P availability of PRs to plants with favorable residual effects in terms of available P and soil pH, without exerting any adverse effects on plant growth or nutrient acquisition.