Phosphate Rock: Re-activity and Solubilization

Traditionally RPR (Reactive Phosphate Rock) has been imported into New Zealand as an alternative to highly water soluble Standard Super Phosphate.

For a phosphate rock to be classified as RPR in New Zealand it needs to be 30% citric soluble. This is a measure of the solubility of the phosphate portion of a particular phosphate rock when exposed to a weak acid. Internationally this is usually expressed as the % of the whole rock not the phosphate portion alone, consequently phosphate rock citric solubility needs to be expressed in the same format to be relevant, often citric solubility appears much lower in standard international tests because it is expressed as a % of the whole rock rather than just the phosphate content.

The most important factors affecting phosphate solubility are:

  1. Phosphate Rock solubility expression defined by different testing methods ( 2% citric solubility test being one of these)
  2. Mineralogical composition
  3. Free carbonate (calcite dolomite) effect, high Ca content in sedimentary phosphate rocks will distort acid testing.
  4. Effect of apatite crystallinity and cementing of apatite with silica
  5. Particle size of phosphate rock applied
  6. Climatic conditions rainfall and temperature
  7. Soil type and pH

There are tests other than the 2% citric solubility test recognised internationally that are as accurate or more accurate for giving an indication of phosphate rock re-activity they are;

  1. neutral ammonium citrate (US mainly)
  2. 2% citric acid (NZ, Brazil)
  3. 2% formic acid (EU)
  4. absolute citrate solubility
  5. ammonium citrate
  6. The x-ray diffraction measurement of the a-axis of the crystal lattice

Bert Quin the New Zealand soil scientist heavily envolved in research regarding RPRs in New Zealand states;

“By far the single best test to define whether an RPR is actually an RPR is the x-ray diffraction measurement of the a-axis of the crystal lattice. Sounds complicated, but it’s not if you have the gear, which unfortunately doesn’t come cheap. They have one at the reknown International Fertiliser Development Centre in Alabama, USA, which I have visited many times. This instrument demonstrated that the Algerian, Tunisian and North Carolina RPRs all have identical a-axis dimensions.The IFDC have published comparisons of plant responses of different RPRs, showing that, provided the a-axis is the same, they perform the same. End of argument. The one and only exception to this Sechura RPR from Peru. As mentioned before, Sechura RPR is a bit different chemically, because it has hydroxide replacing some carbonate in the crystal lattice. This changes its solubility in citric acid, and its a-axis, but it still performs in the field exactly as do the others. The smaller the a-axis, the more resistant a phosphate rock is in the soil, to the point where it will not be capable of releasing sufficient P annually to maintain vigorous growth, and is therefore not worthy of the ‘RPR’ title. The IFDC have stated all this, and they know what they are talking about.”

The International Fertiliser Association states “the solubility of the Phosphate Rock in the different extraction media aids in classifying its potential effectiveness, but should not be used to evaluate the amount of plant-available P and its agronomic effectiveness as these depend on a range of factors”

A  citric solubility test alone does not express all the variables that effect phosphate rock re-activity a major flaw in the 2% citric solubility test is the failure to produce a realistic solubility rating in phosphate rocks with calcite content rather than quartz content.

In conclusion it needs to be understood that citric solubility is a useful measure of phosphate rock re-activity but that this is not the only factor that determines the effectiveness of phosphate rock as a fertiliser other tests and the factors listed also need to be given consideration.



Phosphate rock is dissolved or solubilized by the action of weak acids on the parent material. The reaction can be instigated in two ways.

  1. By the naturally acidic nature of low pH soil such as the tropical soils in temperate zones, or in the case of New Zealand lower pH soils.
  2. By the action of biological (solubilizing biology) agents within the soil.

Many people do not recognise the importance of solubilising bacteria and fungi and their role in making phosphorus available in a soluble form for plant uptake, there are many strains of fungi and bacteria that are capable of solubilising phosphate rock and re-releasing phosphorus locked up after excess delivery of soluble phosphorus in Superphosphate,  the most commonly recognised are;

  1. VAM or Vesicular-Arbuscular Mycorrhizae fungi
  2. PSM Phosphate Solubilizing Micro-organisms such as Bradyrhizobium and Rhizobium

Many people believe that soil acidity is the only contributing factor making phosphorus available to plant life and completely disregard the importance of biological agents, because of this they believe a soil that is pH neutral or alkaline is not capable of solubilising phosphorus research shows this is untrue in fact a pH of 7.2 is considered the optimum for solubilizing bacteria and fungi to perform at their full potential.

It needs to be understood that the phosphorus cycle still takes place in pH neutral or alkaline soils, the mineralisation of phosphorus from unavailable iron and aluminium cation combinations has always taken place through the agents of biology rather than purely as a result of acidic soil conditions.

As well as the biological catalysts that are capable of releasing phosphorus into the soil solution there are also chemical elements that enhance the process due to their acidic nature and inter-relation with soil biology that is not fully understood. The chemicals of particular interest are elemental sulphur and humate acids Fulvic Humic and Humin,



Jono Trolove

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