• Bert Quin

Quinfacts - RPR Series (15)

Industry finally dumping the Fertmark 30-minute citric P test for RPR


Great to see that a Ravensdown-authored paper on revisiting the late Dr John Watkinson's dissolution rate test for RPR will be presented at Massey University's FLRC conference next month. Credit where credit is due, I believe.


I was very proud to have John in my team at Ruakura during my time as Chief Scientist (Soil Fertility) in the mid-1980s. John was a truly gifted scientist; using his deep mathematical knowledge to better understand soil nutrient behavior, especially sulphur, selenium and RPRs.


I discussed John's proposal to develop the Dissolution Test with him way back in 1984, and made the decision to fund it. It combined a 3-hour physical dissolution in an artificial soil water solution, coupled with some high-level mathematical calculations. A big step forward; it should have replaced the Fertmark citric test (used nowhere else in the world) 30 years ago!


I did mention to John that the result produced was particularly susceptible to abuse by entering incorrectly fine particle size distributions of non-RPRs into the mathematical calculation. We no longer have an independent Govt lab in NZ, so how do we police this? Let's see what Ravensdown suggest.


I was busy hosting the Commercial VP for Dubai-based Agrifields DMCC last week. As well as having the shipping agency for the Algerian RPR to many countries including NZ, Agrifields are in the fertiliser business bigtime in India and the Philippines.


But I did attend the last day of the FLRC Conference, so was fortunate to hear the presentation by Henrick Ventner (of Ravensdown-owned ARL) on analyses of recognised RPRs and non-RPRs with the proposed simplified Watkinson Dissolution ('WD') test.


The WD test is vastly superior to the obsolete Fertmark 30-min citric solubility test, which was originally designed to analyse manufactured fertilisers - it was found to have major weaknesses when used to assess RPRs; in particular, it gave artificially low results for RPRs which contained free limestone; eg NZ's Chatham Rise Phosphorite (which exists 400m below sea level); or dolomite, like Quinfert Algerian RPR. Lime and dolomite neutralise the citric acid used in the test.


The 'WD' test involves 3 hour's stirring of the sample in an artificial soil solution. The concentration of P maintained in the soil solution is measured. Algerian RPR performs very well in this test (as it does in overseas tests). Finally, the truth is out!

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