Superphosphate, ‘SSP’ or “super” is a fertiliser processed using sulphuric acid to boost water solubility of phosphorus. Various strengths of P plus additions of nitrogen make up a range of products that feature a high proportion of nutrients in a form that is readily available for uptake by plants.
Like the familiar “super”, reactive phosphate rock (RPR) originates from natural deposits. The source is more specific, however, namely deposits formed on the sea floor through gradual absorption of phosphate present in seawater into dead sea organisms.
Another difference is that it does not go through the same processing as superphosphate to boost water solubility and the release of P is more gradual.
Solubility is determined by laboratory testing using the citric acid test, hence the term “citrate soluble”, and other tests determine critical element content. For example, BioAgPhos has a high level of citrate soluble P (37%) and sourcing Algerian product ensures the lowest possible levels of cadmium.
Apart from laboratory analyses, trials clarify performance. Among these has been a CSIRO study showing reactive phosphate rock can out-perform superphosphate for pasture production.
Importantly, primary producers are finding a key role for reactive phosphate rock because of the manner of release of P. The BioAgPhos form of reactive phosphate rock, for instance, has a third of its P immediately available with the balance released over time.