• Bert Quin

Quinfacts - RPR Series (1)

1. Economic and environmental benefits of reactive phosphate rock (RPR)


Both the economic and environmental benefits of using RPR as the source of phosphate on New Zealand’s soils have been deliberately understated by the fertiliser manufacturing industry in the past decade. With no significant independent importation and scientific promotion of RPR, farmers who want to keep using it have found it increasingly difficult to access true RPR at a reasonable cost.

Three years ago, the industry was caught out selling substandard material as RPR, and now describe those products as DAPR (Direct Application Phosphate Rock) instead. The only true RPR that is being imported by the superphosphate manufacturers  – Sechura RPR from Peru – has an undesirably high cadmium (Cd) content. And there are increasing reports that the unacceptable practice of blending this with cheap, lower-Cd manufacturing rock is continuing.

Why is this happening? Why are not greater efforts being made to supply true RPR to NZ farmers? The industry itself accepts that superphosphate manufacturing in New Zealand is on a slow but inevitable decline into oblivion, as they import more and more high analysis DAP and now MAP and even TSP to meet demand from their farmer shareholders. More and more of the sulphuric acid used to convert manufacturing phosphate rock into superphosphate is being imported directly rather than being made from imported elemental sulphur as traditionally done.

Even the argument that superphosphate has a ‘smaller carbon footprint’ than high analysis manufactured fertilisers does not stand up to scrutiny of the environmental emissions from superphosphate manufacturing, and the much higher  costs, per unit P, of internal storage, freight and spreading costs with superphosphate. High analysis fertilisers containing 20-22% P require less than 40% of the tonnes to be stored, transported and spread and New Zealand.

So, given the increasing importation of high analysis fertiliser into NZ, why has the importation of true RPR with low Cd content virtually stopped? The reasons given – mainly poor supply – simply do not stack up. As an example, the industry says they will not import  Algerian RPR – similar to Tunisian RPR but with lower cadmium – because it ‘does not meet NZ’s citric acid test’. This is incorrect; it demonstrates either a surprising lack of understanding of the deposit, or a deliberate attempt to confuse the farmer. More about this in a future Quinfacts. Other RPRs that could be imported are the Israeli Arad and the lower-cadmium end of the Tunisian deposit.

RPR has by far the lowest carbon footprint of any form of phosphate fertiliser there will ever be; it is simply dug out of the ground, crushed or washed and sieved to get rid of most of the accompanying clay and other minerals, and shipped here, to be mixed with elemental S and other nutrients and applied. Superphosphate served its purpose for 100 years building up available P levels in New Zealand soils, but they simply do not need to be increased anymore, just maintained where they are. RPR does that perfectly, as many, many trials have shown.

The science of the benefits of RPR

RPR has been researched extremely thoroughly in New Zealand. It will maintain pasture production and soil phosphate reserves at least as well as superphosphate in any situation except a combination of very low rainfall and higher than optimum pH. If annual fertiliser application has to be withheld, for example for farm income reasons, RPR maintains production far better than superphosphate. Sulphur requirements can be easily maintained by adding typically 7 kg elemental sulphur (either as fine S or sulphur-bentonite dispersable prills) for each 10 kg P required. Where soil sulphur levels have been allowed to run down to deficient levels (typically <5 ppm), the possibility of deficiency in early spring is easily avoided by the incorporation with the RPR/elemental S of any number of sources of sulphate-S, eg gypsum, sodium sulphate, sodium thiosulphate, calcium thiosulphate, ammonium sulphate and potassium sulphate, at about 10 kg S/ha.

RPR is a sandy mineral formed completely naturally by the decomposition of fish skeletons and shellfish on the sea floor, and their adsorption of phosphate from sea water, over hundreds of thousands of years. For cost reasons, almost all of the deposits being commercially mined are ones that have been raised above sea level by earthquakes or drops in sea level at some point in time. No chemicals are added to the product; beneficiation simply involves crushing and/or washing to remove impurities such as clay and organic matter. This increases the P content; this is further increased by drying to remove moisture. RPR is therefore a truly natural or ‘organic’ product, and is approved for use by organic farmers across the globe, as are gypsum and elemental S.

All true RPRs have an inherent liming effect of about 500kg lime per tonne of RPR applied. This is due to the fact that when RPR is applied to acid soil (as are all soils in NZ), the soil acid reacts with the RPR to release plant-available water-soluble phosphate. The soil acid consumed in the process reduces how much lime needs to be applied in the future to maintain the soil pH. This is particularly important in hill country where application costs of lime are very high. A typical soil in NZ needs about 250kg/ha lime annually to maintain the soil pH (this is typically done by applying 1 tonne every few years; this practice is simply to reduce the higher spreading costs of more frequent application of smaller amounts. The addition of elemental S reduces but does not eliminate the liming benefit of RPR. Long-term lime requirements are typically reduced by 30% where RPR/S is used to replace straight super, or 50% where it is used to replace sulphur-super.

On medium-high rainfall situations on low to medium P-retention soils, sulphur-super needs to be used instead of straight super. This is simply because the sulphate content in it is being leached too quickly to be of much use. As well as being economically wasteful (the farmer has paid for this sulphate), as it is leached from the soil it automatically takes with it ionically-equivalent quantities if calcium, magnesium, potassium and sodium. All of these unnecessarily leached nutrients ultimately need to be replaced, if production is to be maintained.

And finally, there is P run-off. A host of field studies in New Zealand and many other countries around the world have invariably shown that soluble P fertilisers such as superphosphate are far more prone to P run-off after rainfall events or irrigation than is RPR. This effect can remain for many weeks after application. The P lost in run-off enters streams, rivers and lakes where it is, alongside nitrate-nitrogen leached from cow urine patches, the biggest cause of eutrophication of waterbodies in New Zealand. Some recent industry-funded studies have argued that reducing ‘hotspots’ such as gateways and excessive soil P levels are more important than the form of P fertiliser used. However in my view, these studies are largely industry-backed smokescreens to try and hide the fact that if RPR was being used, these other loss mechanisms would simply not occur in the first place, or at least be greatly reduced. These hotspots are essentially caused by excess uptake of soluble P by pasture in the months after application. This excess uptake of P does not happen with RPR. More about this in a later Quinfacts.