Reactive phosphate rocks (RPRs) can be an alternative to superphosphate fertilisers when applied to acid soils in high rainfall areas.

RPRs are naturally occurring slow release forms of phosphorus (P) fertiliser. The phosphate in RPR is in a form not directly available to plants. It must dissolve in the soil to produce water soluble phosphate that the plant roots can absorb.

The major requirements for RPR to dissolve are soil acidity and soil moisture.

Pasture soils therefore need to be acidic (low pH) with a high rainfall so that the soil remains wet for extended periods.

The dissolved P from RPR must also remain available in the soil for plant uptake. High 'P fixing' red soils reduce the availability of dissolved P and are not suitable for RPR use.

RPRs have a sandy consistency, or are ground to achieve this consistency. They differ from other phosphate rocks by having some of their phosphate substituted by carbonate during their formation.

This allows RPRs to dissolve more readily in wet acidic soils, enabling them to be more effective as a P fertiliser than other ground phosphate rocks.

They receive no chemical treatment and are therefore potentially cheaper than traditional water soluble superphosphate fertilisers.

Pasture land where RPRs are likely to be effective can be identified using the following checklist.

1. Sulfur (S)

RPR contains very little Sulfur, an essential nutrient for pasture and animal production. Producers who use RPR on a regular basis are advised to apply S with RPR.

2. Performance in the first year of application

RPR will be effective in the first year (relative to superphosphate) on deep acidic sandy soils in high rainfall areas, where the soluble P from superphosphate can be leached from the root zone.

This can also occur with prolonged winter flooding of paddocks where water drains over the soil surface. RPR is also effective in the first year in the tropics with nitrogen fertilised grass pastures in very high rainfall areas (greater than 2000 mm).

3. Performance in years 3-5

Three to five years of RPR application are generally required at most sites where RPRs are effective - before RPR performance is equal to that of superphosphate. This delay is known as the 'lag phase'. If the soil is low in P then there will be a large production penalty with RPR in the first few years. Do not switch to RPRs on low P soils, except for situations listed in 2 above. Soil P status should be built up with superphosphate before RPRs are used.

4. Cool season effects

RPRs perform poorly (relative to superphosphate) at many P-responsive sites in southern Australia during the cooler autumn/winter period, but then improve in the spring. RPRs appear unable to supply dissolved P quickly enough to meet increased plant demand during the cool season. Winter feed is important in southern Australia and an early season lag in RPR performance can be a potential problem. This seasonal effect with RPR can persist for a number of years.

5. Large, year 1 RPR applications

At sites where RPR is effective, a large application of RPR in the first year can be be as effective as the same total amount of P applied in smaller annual applications (up to 4 years). An advantage in using large initial applications of RPR is that they can eliminate the lag phase and the poor autumn/winter performance of RPR.

6. The effect of RPR on soil pH

There is no consistent evidence to show that RPR is able to increase soil pH. It should also be noted that liming soil to increase soil pH will reduce RPR performance.

7. Combined (RPR + soluble P) fertilisers

Partially acidulated RPR, consisting of slow release RPR and water soluble P for immediate plant use, can be effective in many environments. This fertiliser will generally overcome any lag phase (3 above) or cool season effect (4 above). RPR-superphosphate blends are expected to be similar.

8. Assessing the costs of RPR and superphosphate

To work out the relative costs of RPR and superphosphate, you need to determine the total costs per kg of applied P (fertiliser + freight/handling + spreading + added S), to account for the differences in percentage P between the fertilisers. Producers should seek assistance from fertiliser companies or advisers in making these calculations. Extra costs associated with spreading RPR due to its sandy consistency can be offset to some extent by lower freight costs, due to its higher percentage P.

9. Reactivity

The reactivity of RPRs vary depending upon the degree of carbonate substitution. More reactive RPRs come from Sechura (Peru), Gasfa (Tunisia) or North Carolina (North Carolina RPR is high in the heavy metal cadmium). The information in this brochure refers to these more reactive phosphate rocks.