RPR is a natural, slow-release mineral formed on the sea floor over hundreds of thousands of years. Deposits that have been raised above sea level by changes in sea level or earthquakes are cheaper to mine.
When RPR is applied to acid soils, the soil acid attacks the phosphate mineral, releasing plant-available P in a sustained fashion. True RPRs release P fast enough to easily maintain the growth of high-producing pastures, with only a few limitations.
The science of using RPR in limiting situations
The first is that the soil has to have some level of acidity; otherwise the RPR will remain undissolved (as it has for eons in the North African desert). Because all pastoral soils in NZ and virtually all in Australia are acidic anyway, this isn’t an issue. It was originally advised that RPR should not be used at soil pH levels above 6.0, which is still slightly acidic (neutral pH is 7.0). The only reason for this recommendation was that RPR did not perform too well on a site that had a soil pH of 6.4. But this site also happened to have a very low rainfall (550mm). On farms with higher rainfall or irrigation, especially where existing soil P levels are already at or close to recommended maintenance levels, RPR has been shown to work perfectly well at soil pH levels up to 6.4. Few if any pastoral soils have been limed to a pH higher than this; it is simply not necessary for optimum production and induces trace element deficiencies.
The slow-release nature of RPR makes it less suitable where a rapid increase in soil P levels is required, eg in dairy conversions. This is particularly the case on high P retention soils.
Here are my recommendations for using RPR in less than perfect situations –
- Capital applications in very low soil P levels (less than two-thirds of the bottom of the recommended range):
Use a mix of RPR and a low-gypsum (non-superphosphate) soluble P, eg TSP, DAP or TSP. The water-soluble component should make up at least one half of the total P applied, until soil P levels reach the recommended range.
- Very low rainfall (<700 mm, no irrigation):
Following the end of a drought, apply the first P application as TSP, DAP or MAP.
- Soil pH in the range 6.4-6.7:
Use a mix of RPR and TSP, DAP or MAP. The water-soluble component should make up at least one-half of the total P applied.
- Very high P retention soils (PR>96%).
Use a mix of RPR with TSP, DAP or MAP for the first 5 years. The water-soluble component should make up at least 33% of the total applied P.