Effect of combinations of phosphate rock and superphosphate on dissolution of phosphate rock in water

Abstract

Most of the agricultural lands in the tropics are highly weathered and depleted of plant nutrients, particularly phosphorus and nitrogen and this has resulted in a decline in crop production. Phosphate rock can be used to correct the phosphorus levels but its effectiveness is generally low compared to water-soluble phosphate fertilisers such as single superphosphate or triple super phosphate due to its low solubility in soils. Incubation studies have shown that physically mixing phosphate rock and water-soluble phosphate fertiliser can significantly improve its solubility. This has been attributed to phosphoric acid produced fi'om the hydrolysis reaction in the soil of monocalcium phosphate in superphosphate dissolving the phosphate rock. However, the phosphoric acid may also have dissolved iron and manganese minerals in soil which subsequently lowers the dissolved phosphorus levels from phosphate rock by precipitation reactions, thus the need to carry out further experiments in other systems like water to investigate as to whether the increased phosphorus levels in the mixtures is due to dissolution of phosphate rock by phosphoric acid or increased phosphorus levels arising from increased amounts of superphosphate. Gafsa phosphate rock material and a commercially available phosphate feniliser (single superphosphate) were ground before digestion and total P determined. The effect on pH when 10 ppm of phosphoric acid, iron and manganese were singly applied and detennined as well as the extractable calcium and phosphorus contents by colourimetric methods and Atomic Absorption Spectroscopy respectively. Gafsa PR, single superphosphate (SSP) and monocalcium phosphate (MCP) were combined in ratios of 1:1, 1:4, and 4:1 respectively in 25 mL and 100 mL distilled water. The results demonstrated a positive effect of the superphosphate in increasing the extractable calcium and phosphorus in mixtures of Gafsa PR and SSP with acid. Iron and manganese had an effect of lowering pH while reducing the amount of both extractable calcium and phosphorus. From the findings, it is recommended using a ratio of 1:4 for Gafsa PR-SSP and Gafsa PR-MCP mixtures would provide plant-available P at a rate that is adequate for crop growth. This will be beneficial to farmers and extension officers in the field and also lower toxicity of manganese to plants through precipitation as manganese phosphate under acidic soil conditions.