The phosphate rock used in agriculture is known by a number of names. Reactive phosphate rock (RPR), direct application phosphate rock (DAPR), mineral phosphate, soft rock, hard rock. Variants on all of these are names or brands used across the fertiliser industry.

What is reactive phosphate rock?

Phosphate rock is a natural mineral. The phosphate component of the mineral is present in the form of apatite – a mineral lattice.

There are two main forms of phosphate rock:

  • igneous phosphate rock – makes up about 5% of known global resource and is formed through volcanic activity
  • sedimentary rock- makes up the other 95% of known global phosphate resource and is formed from marine life deposits in ancient seabeds.

Phosphate rock formed from guano deposits (such as the rock mined at Nauru) have declined and are small in output.

Can Phosphate Rock be used as a fertiliser?

Being a mineral, phosphate rocks vary from deposit to deposit. Variations can also occur within a single deposit.

Variations may occur in:

  • the amount of elements including trace elements, heavy metals, radioactive isotopes, calcium, carbonate, fluorine and silica
  • how well the phosphate is bound within the mineral lattice (apatite).

These variations impact the effectiveness of phosphate rocks as a fertiliser. Being a natural mineral, phosphate rock is commonly sold and promoted as an organic product. While this is commonly true “organic” does not guarantee the effectiveness of phosphate rock as a source of nutrient, i.e. a fertiliser.

Of the two main forms of phosphate rock , igneous rock phosphate is typically not suitable as a fertiliser because the phosphate is tightly bound and is not liberated.

Sedimentary phosphate rocks vary in their effectiveness as a fertiliser and need to be evaluated on their ability to release nutrient, either by laboratory analysis or in field trials.

How we determine phosphate rock’s effectiveness as a fertiliser

To measure the effectiveness of rock phosphate as a fertiliser, industry bodies and governments across the world have tried to define the chemical properties of phosphate rocks (typically RPR) used as fertilisers.

In New Zealand Reactive Phosphate Rock (RPR) is the commonly known and used phosphate rock fertiliser. To be able to sell phosphate rock as RPR in New Zealand it must meet a number of parameters, including a minimum P content of 10% with a minimum of 30% of the total phosphorous soluble in a defined citric acid test.

There is no standard in Australia that phosphate rock must meet when being used as a fertiliser. However, since 2001 BioAg has been evaluating the phosphate rocks available to us. Our selection of rock is based on a number of criteria but most importantly is the reactivity of the rock (as measured in a laboratory) and its successful performance in the field, both in Australia and around the world.

The availability of phosphorus determines effectiveness

While phosphate rock is commonly defined by the total amount of phosphorus or P, it is the availability of the phosphorous that will determine the products efficiency as a fertiliser.

When assessing phosphorus availability the most important aspect is to ensure you use a laboratory that is capable and known for analysing phosphate rocks. Soil and mineral laboratories may not use the correct methods. Comparing analysis results of different RPR products from the same laboratory is always best.

The common laboratory tests performed to determine phosphorous release are:

  • solubility in dilute citric acid; and
  • solubility in dilute formic acid.

The results can be provided as %P or as a % of Total P.

In addition there is a plant availability test which will analyse for the amount of phosphorous that will become plant available over time.

How is phosphate content in RPR measured?

The three common ways to measure phosphate content in phosphate rocks are:
• phosphorous (%P)
• phosphate (%P2O5)
• %BPL (% Boned Phosphate of Lime)

Conversion:
• 1% P = 2.2915% P2O5
• 1% P2O5 = 2.1852% BPL

Example: A product that contains 12.6% P has the same amount of phosphorus as one with 28.9% P2O5

What about BioAg’s products?

Given the difference in performance of phosphate rocks used as a direct application fertiliser, it is imperative that an appropriate rock is used.

BioAg’s products are manufactured using Algerian rock phosphate. Algerian phosphate rock is a known as a highly reactive rock that is suitable for use as a fertiliser in the right soil and climates. Over 800,000mT per annum of Algerian phosphate rock is used as direct application fertiliser in markets such as Brazil, Europe and Argentina. 

BioAgPhos bulk pile and conveyor
Processing BioAgPhos at our Geelong facility

BioAg produces its BioAgPhos product by adding our phosphate digester, which contains microbes and microbial food. BioAgPhos composts for between 14 to 20 days after which it is ready for sale.

BioAgPhos has been used in a number of replicated trials to prove its value as a fertiliser.

Growers

Want to know if a BioAg program including BioAgPhos will give your crops or pasture the P it craves? Talk to one of our qualified and experienced area managers. They’d be delighted to assist.

Further reading

Fertiliser Standards Australia National Code of Practice for fertiliser labelling and warnings

Peir K. Pufahl, Lee A. Groat Sedimentary and Igneous Phosphate Deposits: Formation and Exploration: An Invited Paper Economic Geology (2017) 112 (3): 483-516.
Read the abstract

Phosphate Rock Factsheet on the NSW Government Resources & Geoscience website

A report on varying rocks at the New Zealand Journal of Agricultural Research
A. G. Sinclair , P. D. Johnstone , J. H. Watkinson , L. C. Smith , J. Morton & A. Judge (1998) Comparison of six phosphate rocks and single superphosphate as phosphate fertilisers for clover‐based pasture, New Zealand Journal of Agricultural Research, 41:3, 415-420, 
Read the abstract

Németh, T & Magyar, Marianna & Csathó, Péter & Osztoics, E & Baczó, G & Holló, S & Németh, I. (2002). ‘Long-term field evaluation of phosphate rock and superphosphate use strategies in acid soils of Hungary: Two comparative field trials.’ Nutrient Cycling in Agroecosystems. 63. 81-89. 10.1023/A:1020529001629.
Read the abstract

Read more about the role of reactive phosphate rock fertilisers in Australia

BioAg products using reactive phosphate rock

BioAgPhos

12.7% P | 35% Ca | 1% S
41% citric soluble, 73% formic soluble
Highly reactive, high-grade reactive phosphate rock (RPR) combined with BioAg’s microbial digesting agent.

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BioAgPhos S10

P 11.4% | S 10% | C 32%

Provides elemental sulphur in the form of sulphur bentonite; a sustained release form of sulphur for the entire growing season. Ideal for higher rainfall and higher leaching environments.

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BioAg Superb

P 8.4% | S 6.8% | Ca 31%

Highly reactive, high-grade reactive phosphate rock (RPR) combined with BioAg’s microbial digesting agent and gypsum. Ideal for soils that need sulphur, phosphorus and calcium.
Suited to pastures/grazing, and crops needing extra sulphur (such as canola).

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MagPhos

P 6% | S 5% | C 28% | Mg 4.8%
Highly reactive, high-grade reactive phosphate rock (RPR) combined with BioAg’s microbial digesting agent and magnesite. Ideal for soils low in magnesium and phosphorus where risk of grass tetany is present.

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Pasture Primo

P 3.7% | S 0.3% | Ca 37%
Highly reactive, high-grade reactive phosphate rock (RPR) combined with BioAg’s microbial digesting agent. A source of calcium through the increase of the lime component in the blend. Designed for soils lacking calcium and phosphorus.

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PotPhos

Available with Muriate of Potash (MoP) or Sulphate of Potash (SoP) Highly reactive, high-grade reactive phosphate rock (RPR) combined with BioAg’s microbial digesting agent and potassium sulphate (sulphate of potash) in a ratio of 3:1. Suited to all grazing systems where potassium (K) inputs are needed.

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Read stories of growers who have reaped the benefits of using BioAgPhos

Rely on BioAg’s RPR knowledge and experience

At BioAg we understand Reactive Phosphate Rock. We know what works and what doesn’t.

Phosphate rocks have very different suitability for direct applications. If you want to use long lasting and non-polluting phosphates, talk to a BioAg agronomist, and consider BioAgPhos. Depending on your soil’s nutrient requirements, BioAgPhos can be blended with other nutrients, at source or on farm, to optimise the nutrient package for your soils and plants.

The RPR we use in our products performs reliably across a wide spectrum of soils and enterprise types. BioAgPhos is the long lasting P product that you can rely on, supported by considerable replicated trial based evidence.

Find your nearest BioAg area manager or specialist agronomist