Any phosphate rock will release some P into plant-available form when applied to acid soils. The question is, in the pastoral context, will it release P at a sufficiently fast rate to maintain the needs of a high production pasture? Most phosphate rocks do not. Even if very large quantities are applied they still do not, as the dissolution in the soil reaches equilibrium.
This is why, to be effective as a fertiliser, most phosphate rocks have to be chemically converted into a more soluble form of phosphate using a concentrated acid. The simplest process, to make single superphosphate, known as SSP or ‘super’, is to react the finely-ground phosphate rock with concentrated sulphuric acid. The acid reacts with the apatite tricalcium phosphate mineral which makes up the phosphate rock to form a mixture of water-soluble monocalcium phosphate (about 45% of the weight) and gypsum (about 55% by weight). Various greenhouse gases and others such as poisonous sulphur dioxide are emitted during this process. The product is then dried and granulated in most countries, or simply semi-granulated in the case of New Zealand, giving a product containing about 9% P. Typically a total of 8% P is plant available, with the rest being made up of unreacted phosphate rock and complex calcium-iron-aluminium phosphates. The gypsum provides about 11% S, all present as water-soluble sulphate-S.
If a wider ratio of sulphuric acid to phosphate rock is used, phosphoric acid is formed which can be separated from the gypsum. This phosphoric acid can then be reacted with more phosphate rock to form triple superphosphate (TSP, 20% P) or with ammonia to form diammonium phosphate (DAP, 18% N and 20% P) or monoammonium phosphate (MAP, 10% N and 22% P). These 3 ‘high-analysis’ fertilisers (and phosphoric acid itself), make up the vast majority of phosphate fertiliser traded internationally, because the high analysis greatly reduces transport, storage and spreading costs compared to SSP. SSP usage in NZ is slowly but surely giving way to imported TSP, DAP and MAP, amended with added elemental S to suit individual farm needs, rather than applying a blanket amount as with SSP.
It has however been known for nearly a century that some phosphate rocks can be used directly as a phosphate fertiliser on acid soils. All of New Zealand soils, and most of Australia’s agricultural soils, are acidic. The phosphate rocks that can be used with some level of effectiveness on acid soils are loosely defined as direct application phosphate rock or DAPR. With this number, there is a smaller group defined in New Zealand and Australia and an increasing number of other countries as reactive phosphate rock, or RPR. The fundamental definition of an RPR in the pastoral context is a phosphate rock that is reactive enough to release sufficient plant available P to maintain the growth of high-producing pastures. There are several laboratory methods used to help determine whether a DAPR is sufficiently reactive to be defined as an RPR. These methods are designed to avoid the need to do a whole series of new field trials whenever a new potential RPR is located or commercialised.
In the case of both New Zealand and Australia, nation-wide series of field trials comparing various DAPRs with superphosphate (SSP and TSP) were conducted in the 1980s and 1990s which ran for 4-6 years, followed in some cases by no-application for 1 or 2 years to measure the residual effects of the different fertilisers. Many other single-location trials and laboratory and glasshouse studies focusing on specific aspects were also conducted, and much research was conducted on predictive solubility tests.
The science of determining which phosphate rocks qualify as RPRs
In both New Zealand and Australia, it was determined that the simplest, reasonably accurate laboratory method to ensure that a particular DAPR would maintain pasture production as well as the same amount of P as soluble phosphate on acid soil, and therefore be described as an RPR, was whether or not at least 30% of the total P content of the DAPR would dissolve in a dilute organic acid (2% citric acid) in a short 30-minute extraction. RPRs that contained more than 30% ‘citric P’ or ‘citsol’ (as the test has come to be called) performed no better in this role as a maintenance P fertiliser, although they will give a slightly faster P response in very P-deficient situations. Although this citric P definition is not controlled legally, it is sufficiently strongly industry-recognised in both countries that using the term RPR to describe DAPRs that do not meet the 30% citric P test is unacceptable in the market, as both Ballance and Ravensdown have found to their cost in recent years.
When MAF analytical research chemists Mike Brown and myself first introduced the test back in the 1980s, it became rapidly accepted. A particular strength of the test was that, by doing the test on RPRs as sold, it provided an automatic control regarding whether the product as sold to farmers was fine enough. If the product was too coarse, the reduction in surface area meant that it did not dissolve fast enough – either to be fully effective as a fertiliser in the field, or to meet the 30% citric solubility requirement.
Over time however, two significant shortcomings in the citric acid test have become apparent. One of these produces an artificially low citric P for some true RPRs; the other allows one particular RPR (Sechura) to be blended with poor-quality phosphate rock and still produce a test of 30% or more. Let’s explain both problems.
- The effect of the presence of free lime or dolomite in RPRs. A small proportion of true RPRs contain a percentage of 10% or so free lime or dolomite mixed with the carbonoapatite mineral. Two examples are the small Hamrawein deposit on the Red Sea coast in Egypt and the massively larger Algerian RPR deposit. Much of the latter contains 10% free dolomite. This is even better from an agronomic point of view, as even less lime needs to be applied to maintain soil pH than with other RPRs. However, the presence of this dolomite selectively consumes about 20% of the citric acid in the laboratory test, leaving less of it to dissolve the P. So Algerian RPR, if mined where dolomite is present, can come up with a slightly lower citric P than 30%, even though it is just as good as a fertiliser. When applied to the soil, the individual grains of RPR and dolomite land in microscopically different locations, each consuming different micro-pools of soil acid. Fortunately, it is relatively easy to determine whether sufficient lime or dolomite is present to affect the citsol test.
- Sechura RPR has a different chemical composition in the crystal lattice to other RPRs, caused by the presence of hydroxide ion replacing some of the carbonate. This has no effect good or bad on its agronomic effectiveness – field trials have shown that it performs identically to other RPRs on pasture; but it does have a much higher citric solubility, ranging from 38-46%. Unfortunately, some unscrupulous companies have used this as a means to cheat farmers, by blending low-cost, low-solubility non-RPRs with Sechura, knowing that the average citsol can still meet 30%. Unfortunately for the farmer, the percentage of P present that is not an RPR will still be ineffective. One solution to this real problem is to insist that all companies selling RPR identify the source of their product, and sign a declaration that samples taken for Fertmark or other analysis contain only that RPR and no other source of P. It still requires honesty on the importer’s part however.
The a-axis measurement – unbeatable test for RPRs. I have been campaigning for 20 years for New Zealand and Australia to adopt an alternative RPR test, called the a-axis dimension test. It is a relatively simple test but requires a quite expensive piece of laboratory equipment – an x-ray diffractor or XRD.
The XRD measures the dimension in Angstroms of the a-axis of the carbonoapatite crystal lattice. RPRs have a smaller a-axis measurement due to at least 20% of the phosphate in the lattice being substituted by carbonate the in the crystal lattice (note this is totally different carbonate to the carbonate in the form of free lime or dolomite present in some RPR deposits). Having a smaller a-axis makes the crystal less chemically stable and therefore more susceptible to being attacked by soil acid. The a-axis measurement that is used internationally to define the most reactive phosphate rocks should be a maximum 3.329 A (angstroms). Phosphate rocks with a higher a-axis than this, such as these from Morocco, cannot maintain a sufficient concentration of P in the soil solution to ensure maximum growth of the vigorous ryegrass/clover pastures grown in New Zealand and Australia. The only known exception to this a-axis rule is Sechura RPR, which because it has more hydroxide (OH–) in the crystal lattice than other RPRs, has an a-axis of 3.334 A. Partly for this reason, it is vital that the location of origin, and proof of reactivity, of any phosphate rock being mixed with Sechura RPR be declared. Amazingly, this is not a requirement under Fertmark in New Zealand.