• Bert Quin

Fertiliser management of peat/organic soils (b): The RPR alternative

1. Peat/organic soils DO NOT retain water-soluble P, form of P present in superphosphate, TSP or DAP.

2. Scientific research has demonstrated mass that P fertilisers only need to contain 0.10-0.15 mg/L (ppm) water soluble P in the soil solution to maintain maximum pasture production. The sustained-release of RPR over 2-3 years maintains this 0.1-0.15 ppm in the soil solution nicely, on all soil types under high-producing dairy pastures, while maintaining the soil pH.

3. Compare this to superphosphate, in which concentrations of water-soluble P in the soil solution surrounding individual granules reaches concentrations of hundreds of ppm, and a pH as low as 2.2-2.7!

4. Not surprisingly, scientific research has demonstrated massive reductions in P loss when RPR (reactive phosphate rock) is used instead of soluble superphosphate (Simmonds 2016).

5. And in fact, The NZ “National Series” of trials comparing RPR vs superphosphate on different soils on 19 sites throughout NZ , with all sites chosen to be at below optimum soil Olsen P status, demonstrated that RPR was every bit as effective as superphosphate per unit P (and cheaper!) in reaching and maintaining optimum pasture production.

6. In fact, even on quite P-deficient soils, the difference in pasture production was only 3% and 1.5% in favour of superphosphate in years 1 and 2 respectively, with RPR being as good or better in Years 3 onwards.

7. RPR blended with S90 (90% fine elemental sulphur prills) is highly cost-competitive on the cost/kg P applied, plus –

8. Simmonds (2016) found that the cost of P leached from an organic soil at a pH of 5.5 was 94% higher for superphosphate than for RPR, representing cost savings of $72/ha/pa, or $14,400 pa for a 200 ha farm.