‘Super’ or reactive phosphate rock?

Today ‘super’ is used to describe a range of chemical fertilisers that contain phosphorus, be they SSP, DAP, MAP or NPK. Nearly all phosphorus fertilisers start by reacting the P in phosphate rock. A strong acid like sulphuric acid, phosphoric acid or nitric acid is used to convert the phosphate bound in phosphate rock into a water soluble form. Depending on the process and intended use fertilisers containing various strengths of P in combination with other nutrients such as nitrogen, calcium and sulphur create fertilisers that are water soluble and immediately available to plants and soil antagonists.

How is reactive phosphate rock different to super?

Like ‘super’, reactive phosphate rock (RPR) originates from natural deposits. In the case of RPR, however for truly reactive rocks sedimentary deposits, those formed in ancient seabeds, are required. Igneous (volcanic deposits) are not reactive, while guano’s are reactive they are limited in number and typically higher in heavy metals.

Importantly RPR does not go through any chemical processing the phosphate is not water soluble but becomes available through the breakdown or decomposition of the rock by organic acids produced in the soil. The result is that in the soil the release of P is more gradual.

The importance of solubility

Solubility is determined by laboratory testing using the citric acid test, hence the term “cit soluble”. Other tests determine critical element content. For example, BioAgPhos® has a high level of cit soluble P (>35%). We have also ensured our RPR is sourced from deposits with the low levels of cadmium.

Alongside laboratory analyses of RPR’s nutrient content, trials and demonstrations analyse performance in the field. Among these has been a CSIRO study showing reactive phosphate rock can out-perform traditional chemical fertilisers in pasture production.

The release of phosphorus

Importantly, primary producers are finding a key role for reactive phosphate rock because of the manner in which P is released. The BioAgPhos form of reactive phosphate rock, for instance, has a third of its P immediately available with the balance released over time.

BioAg and reactive phosphate rock

At BioAg we use high-grade Algerian reactive phosphate rock in our products. We know what works and what doesn’t. Find out more about reactive phosphate rock as an effective fertiliser.