Reactive Phosphate Rocks (RPR's), which include those from Tunisia (Gafsa), North Carolina and Peru, are suitable for direct application in their raw form without any chemical processing. Their mineralogy is such, that the natural acidity of soils with a pH of less than 6.5, will be sufficient to release the phosphorus (P) from the rock in a form which is available to plants and will not be locked up in the soil. This process is entirely natural and is not acidifying to the soil.

Conversely, the mineralogy of non-reactive phosphate rocks is such that they require to be treated with strong acids (sulphuric and phosphoric) to render the P into a water-soluble form for use as a fertiliser. The use of strong acids makes water-soluble P fertilisers very acidifying to the soil which, in turn, results in the P becoming locked up in the soil within a matter of weeks after application.

These differing scenarios are depicted below:

RPR's are often referred to in the fertiliser industry as being 'insoluble' but, as the above diagram demonstrates, this is misleading and agronomically incorrect. RPR is soluble in the natural acidity of soils with a pH of 6.5 or less.

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