Rock phosphate 28%

Rock Phosphate is used to produce soluble Phosphate fertilizers, but some is used for direct application to soil. While phosphate rock can be a valuable source of Phosphate for plants, it’s not always appropriate for direct application. Its suitability depends partly on naturally occurring mineral impurities, such as clay, carbonate, iron and aluminum. Labs estimate the effectiveness of phosphate rock for direct application by dissolving rock in a solution containing a dilute acid to simulate soil conditions. Sources classified as “highly reactive” are the most suitable for direct soil application.

Direct use of phosphate rock avoids the extra processing associated with converting apatite to a soluble form. The minimal processing may result in a lower-cost nutrient source and make it acceptable for organic crop production systems.

When a water-soluble P fertilizer is added to soil, it quickly dissolves and reacts to form low solubility compounds. When phosphate rock is added to soil, it slowly dissolves to gradually release nutrients, but the rate of dissolution may be too slow to support healthy plant growth in some soils. To optimize the effectiveness of phosphate rock, consider these factors:

Soil ph. Phosphate rock requires acid soil conditions to effectively nourish crops. Use of phosphate rock is not usually recommended when the soil pH exceeds 5.5. Adding lime to raise soil pH and decrease aluminum toxicity may slow phosphate rock dissolution.

Soil P-fixing capacity. The dissolution of phosphate rock increases with a greater P-fixing capacity of soil (such as high clay content).

Soil properties. Low calcium and high organic matter in the soil tend to speed phosphate rock dissolution.

Placement. Broadcasting phosphate rock and incorporating it with tillage speeds the reaction of the soil.

Species. Some plant species can better utilize phosphate rock because they excrete organic acids from the roots into the surrounding soil.