Worldwide, most soils and crops require phosphorus (P) additions to improve fertility and production. Directly applying unprocessed phosphate rock to soil may provide a valuable source of plant nutrients in specific conditions, but growers must consider several complicating factors and limitations.
Phosphate rock comes from geologic deposits located around the world. Its main constituent is apatite, a calcium phosphate mineral primarily extracted from sedimentary marine deposits, with a small amount obtained from igneous sources. Most phosphate rock is recovered through surface mining, although some is extracted from underground mines. The ore is first screened and some of the impurities removed near the mine site.
Most phosphate rock is used to produce soluble P fertilizers, but some is used for direct application to soil. While phosphate rock can be a valuable source of P 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.
Timing. The time required for the dissolution of phosphate rock necessitates its application in advance of the plant demand.
Not all sources of unprocessed phosphate rock suit direct application to soil. Likewise, many soils don’t suit phosphate rock use. The total P content of a material isn’t a good predictor of the potential reactivity in the soil. For example, many igneous phosphate rock sources are high in total P, but are of low reactivity and provide minimal plant nutrition because they dissolve so slowly. However, mycorrhizal fungi may aid in the acquisition of P from low-solubility materials in some environments.
Over 90 percent of phosphate rock is converted into soluble P fertilizer through reaction with acid. This is similar to the chemical reaction that phosphate rock undergoes when it reacts with soil acidity. The agronomic and economic effectiveness of phosphate rock can be equivalent to water-soluble P fertilizers in some circumstances, but growers should consider the specific conditions when making this choice.
Source: Nutrient Source Specifics (No. 19), International Plant Nutrition Institute.