What is superphoshate?
Superphosphate, ‘SSP’ or ‘super’ is a fertiliser processed using sulphuric acid to boost the 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.
How is reactive phosphate rock different to super?
Like superphosphate, reactive phosphate rock (RPR) originates from natural deposits. In the case of RPR, the source is more specific, namely deposits formed on the sea floor through gradual absorption of phosphate present in seawater into dead sea organisms.
Another difference is that RPR does not go through the same processing as superphosphate to boost water solubility. 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 “citrate soluble”. Other tests determine critical element content. For example, BioAgPhos has a high level of citrate soluble P (37%). We have also ensured our RPR is sourced from deposits with the lowest possible levels of cadmium.
Alongside laboratory analyses of RPR’s nutrient content, trials and demonstrations analyse performance. Among these has been a CSIRO study showing reactive phosphate rock can out-perform superphosphate for 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.