Quinfacts - RPR Series (6)
6. Important facts about superphosphate and RPR
Superphosphate is advertised as having 9.0% P. But how much of this 9% is actually plant available? An indication of this is how much of the total P is citric soluble. Currently, it is about 8% P. The difference is largely unreacted phosphate rock that did not get acidulated in the manufacturing process. Because superphosphate is geneally made from hard manufacturing rocks, rather than reactive phosphate rock (RPR) , the value of this difference is questionable.
Another indication that unreacted phosphate rock present is present in superphsphate is to look at what level of sulphate-sulphur is present. If the phosphate rock is fully acidulated (ie, converted to plant-available form), the sulphate-S content should be about 12%. Figures of 10.5 or 11% indicate unreacted manufacturing rock is present.
The actual plant-available P level in superphosphate has a big influence on the cost per kg of P, especially on a transported and spread basis because it is a relatively low P-content product in the first place. At say $310 per tonne, and say $120 per tonne transport and spreading, if it really has only 8% plant-usable P, this means the true cost per kg P is about $5.37 per kg P. So it is important to know exactly what is in it.
By comparison, all the P in RPR will become available in a sustained time-frame that is compatible with its use as a maintenance P fertiliser. At say $350/tonne for an RPR/S mix containing 11.0% P and 8% S (the agronomically correct ratio), with $120/t transport and spreading this gives a cost of $4.27. This is a difference of about $10,000 on a 300 ha hill-country farm.
On the subject of RPR, Quinfert Algerian RPR is rated as a highly reactive phosphate rock suitable for direct application by the International Fertilizer Development Center (IFDC) in Muscle Shoals, Alabama, USA, the world authority on direct use of phosphate rocks as fertilisers. The naturally-occuring presence in Algerian RPR of 3-7% natural phosphatic dolomite (average 5%) can mean that the very shor (30 minute) citric solubility can be below 30% in some high-dolomite samples, although the eposit average is 30-30.5%. This is simply because the dolomite consumes a lot of the citric acid , leaving less to dissolve the RPR. It is a laboratory artefact: it is extremely important to note that this has no adverse effect whatsoever on its excellent field performance. I have a great deal of experience and a proven track record in RPR. The important question is whether the ‘RPR’ you are being offered contains all the P in the form of an identified RPR, or whether it is an RPR mixed with a non-RPR phosphate rock after arrival in New Zealand.
I am very sure that if I advertised a product as ‘super’ that was actually a mix of super and RPR, there would be hell to pay. Shouldn’t honest advertising work for everybody?
The comments and questions below from Robin Boom, agronomic Advisor, and the reply from Dr Quin, have been copied over from Dr Quin’s LinkedIn page (Bert F. Quin):
Comments and questions from Robin Boom, Business owner at Agronomic Advisory Services, 8/9/2018. Bert, I am of the understanding that Ravensdown are blending Moroccan BG4 with their Sechura to get the cadmium levels below 280 ppm/kg P whereas Ballance are mixing an Algerian based rock they call PB3 with their Sechura. With the new Biogro standards of not having RPRs with Cd levels above 150 ppm/kg P the ratio of the lower cit sol products increases. Your argument however about the dolomitic content of your Algerian rock lowering its 30 min cit sol, would also apply to Chatham Rise with its high CaCO3 content? Can you point us to the trial work with your Algerian rock? Is it the same product that Anton in Australia and Steve at Hawkes Bay and Shane have been marketing here in NZ for years? I have seen some of their claimed trial work done in Australia and it all looked a little dubious to me. For my RPR clients I am currently only comfortable recommending Ron Webby’s Sechura (45% Cit Sol) or Dickie Direct’s granular RPR (mid 30’s Cit Sol). Guano is fine but way too expensive. Don’t expect Ballance or Ravensdown to respond to your critique of their superphosphate products and the available P and S content.
Response from Bert F. Quin Managing Director at Quin Environmentals (NZ) Ltd, importer and distributor of QUINFERT Algerian RPR:
Yes, it is my understanding like you that they are blending beneficiated Sechura RPR (which contains about 12.7% P but averages up to 280mg Cd per kg P) with Moroccan BG4 grade phosphate rock from the Gantour region. This contains 13.0% P and low Cd, but it is NOT an RPR, and is not recognised by the industry-independent International Fertilizer Development Centre (IFDC) in Alabama, USA. Even the Moroccan’s themselves refer to it only as a DAPR.
I have been advised in writing by Ballance Agri-Nutrients Ltd that the product they label as ‘High-P RPR’ is a mix of Sechura RPR which has been beneficiated to ‘over 12% P’ , and a phosphate rock from Morocco known in the mining industry known as ‘PB3’, which contains approximately 14% P. My understanding from enquiries I have now been able to make from other sources is that ‘PB’ stands for PhosBoucraa, which is the mining entity in the disputed Western Sahara which mines the Boucraa phosphate rock. the existence of any field trial data from trials done with PB3.
he deposit is predominantly a deep (30m) layer which has a thin 1-3m vein of phosphatic dolomite running through it. The RPR contains 13-20 ppm Cd, average 18, or 140 mg Cd/kg P. Depending on the position of the phosphatic dolomite layer during a day’s mining (50-100,000t), the dolomite content can range from 3-7% of the total. There are very few other impurities present, unlike most other RPRs. The level of dolomite only alters the P content of RPR by +/-0.2%P (range 12-5-12.9, average 12.7%P); it is a high-P dolomite anyway. The presence of natural lime or dolomite in RPRs has long been known – in NZ and elsewhere – to give an artificially low reading in NZ’s very short (30 min) citric test. The citric acid preferentially attacks the more soluble dolomite, leaving far less citric acid to dissolve the RPR. Any chemist knows this. The pH of the citric acid solution when testing Algerian RPR increases in the first minute from 2.3 to 3.3. That may not seem to much to some, but you have to realise that the pH is a logarithmic (log for short) scale. At pH 3.3, there is only 10% or so as much citric acid to dissolve the RPR as there are at 2.3. So, depending how much phosphatic dolomite is present in a given sample, the citric solubility in the short 30 minute test can range from 28-37% , with a mean of 30-30.5%. The actual number has no effect on the effectiveness of the RPR as a fertiliser. These type of effects are detailed in several independent in scientific papers which anyone can find on google. An extreme case of this effect is found with the Chatham Rise Phosphorite (CRP) nodules (which Chris Castle hopes to mine). CRP contains 30% calcite (lime), which reduces the 30-min citric P to only 15%! That’s way lower than most phosphate rocks that be used for manufacturing only! Nevertheless, in every single field trial, CRP performed at least as well as any other RPR, right from the start. So you get my point hopefully. The 30-minute citric P test is simply not up to the job. As recommended by Dr Alec McKay (and myself for that matter) a far better test is do 5 sequential extractions. All the P will be extracted from an RPR, regardless of the dolomite or lime present, but not from a non-RPR. Finally, the initial high citric solubility of Sechura is itself a known chemical artefact. It does not mean it performs better in the field than other RPRs; it doesn’t, except briefly in some very low soil P situations.. Most unfortunately, this high solubility is abused by some players to get better citric solubilities for mixtures of it with unreactive rocks. The current 30-minute citric test was certainly better than nothing in the somewhat less devious 1970s, but is way, way past its use-by date now. A combination of the 5-sequential citric and the IFDC a-axis determination test need to be adopted by Fertmark without further delay.
History of Algerian RPR in NZ and Australia. I had been trying to get access to Algerian RPR since the earliest days of Summit-Quinphos. But Sumitomo, who owned about 80% of S-Q, would not let me go there because of the risk of kidnapping of visiting businessmen at the time. But I made sure I kept contact with the Algerian RPR producers (Ferphos’s subsidiary Somiphos) at IFA fertiliser industry conferences and by email. Ballance bought into SQ in 2000; I left in 2005. Sumitomo sold the company to Ballance about 2009. They first changed the name to ‘Altum’ , then completely merged the operation into Ballance, and gradually removed any advisory promotion of RPR.
About 2011 Anton Barton of BioAg Australia rang me for advice; he had been importing the very dusty Egyptian Red Sea Coast Kosseir RPR into Geelong, but it had been banned by the Port because of the dust. I set him up with the international trading company who have the agency for Algerian RPR in Australia, and he has been importing a bulk shipment of it into Geelong every 12 or 18 months since. I bought a few containers of it off him into NZ, as have Fert Wholesale Direct Ltd and one or two other companies. However, this is barely economic given the very high Tasman container rates. In 2017 I was approached (at the request of the Algerians) by the international shipping and trading company Agrifields, based in Dubai, who now have the shipping agency for Algerian RPR for all of south-east Asia and New Zealand. They asked me to market Algerian RPR in NZ. We agreed on terms, and I started importing containers direct into NZ in autumn. Somiphos make 2 grades; one specifically for direct application called 63-65, and another coarser grade for manufacturing (after very fine grinding). Provided the coarsest fraction is removed (and added back in after grinding if wished), this grade is also a perfectly good RPR, as it comes from the same deposit. Interestingly, a few hundred tonnes into NZ for superphosphate manufacturing trials about 2015, but it would have had a high sulphuric acid consumption because of its high liming equivalent (which as I mentioned is added to by the 3-7% dolomite content). It may well have been hoped it would be a good manufacturing rock that could also be pressed into service as an RPR if another RPR importer came along. The Algerian’s probably saw what was going on and asked me to come back into the market with bulk Algerian RPR, so I have!