‘The potential of RPR (reactive phosphate rock) for New Zealand’
I have been involved with RPR research and promotion in New Zealand for 40 years. I spent 17 years as a soil fertility research scientist with the Agricultural Research Division of the Ministry of Agriculture – what became AgResearch – including 3 years as Chief Scientist (Soil Fertility) at Ruakura. I designed and coordinated the ‘National Series’ of RPR trials, which ran over a period of up to 8 years on 19 farms throughout New Zealand in the 1980s. The trial sites were all deliberately selected to have below-optimum soil Olsen P levels, to more clearly show any differences in performance. Despite this, differences in pasture production between RPR and super were minimal – 0-3% on average – and had totally disappeared by Year 3 on low and medium P-retention (ASC) soils and by Year 5 on the highest P soils. The only exception was a low-rainfall (700mm), non-irrigated site at Winchmore in mid-Canterbury which
had also been over-limed to a soil pH of 6.4.
The simple solution to any initial ‘lag-phase’ with RPR was proven to be to use a blend of RPR and high-analysis P fertilisers such as TSP, DAP or MAP (and added S as required) for the first few years. Note that mixtures of RPR and superphosphate were shown to be not as effective for this purpose. Subsequent research demonstrated that where Olsen P levels were at or above optimum – as is the case on about 98% of dairy farms and about 75% of hill country farms in New Zealand – there were no measurable differences in pasture production right from the start of using RPR, meaning a a soluble P component is unnecessary.
If the fertiliser industry in NZ was not dominated by the superphosphate manufacturing duopoly, I am convinced that RPR – mixed with sulphur and other nutrients as required for individual farms – would (i) already be the main source of P used in NZ, and (ii) we would have far less polluted waterways and lakes as a result. RPR is proven to result in far less run-off of water-soluble P than superphosphate. Imported high-analysis would be blended in where required.
Also, because RPR particles dissolve in the soil steadily over time, releasing P for direct uptake by plants, there is far less P existing as soluble P adsorbed onto the surface of soil particles than is the case with superphosphate. Most of what is described as ‘particulate P’ lost from soil in run-off and erosion is actually present in the form of
soluble fertiliser P that has become adsorbed (to use the technical term) onto soil particles near the soil surface. When these particles end up in a waterway or lake through soil erosion, as much as 25% of this adsorbed P can easily be desorbed back into water-soluble form, (as demonstrated in an excellent soil chemistry paper by Australians Barrow & Shaw in 1975). This ‘particulate P’ form of loss is also greatly reduced with RPR, but you need multi-year constant-treatment trials to clearly demonstrate this.
Unfortunately, this vital area of water-quality research has received no funding in NZ, largely because most of the government research funding on fertiliser and the environment is – completely inappropriately – channelled through the duopoly, who have no wish to encourage this research. New Zealand simply does not need to be taking the manufacturing-grade phosphate rock from the Moroccan-occupied deposit in the Boucraa area of the Western Sahara to make into superphosphate. Certainly, superphosphate has played a very important part in developing NZ’s low-P soils. However, virtually all our agricultural soils have long been developed to the point where they can now be maintained very easily with slow-release form of P, containing up to 30% soluble P where needed, to ensure that our
waterways are protected.
If we continue to allow the industry to be dominated by two management groups who refuse to accept what is happening to our environment, we will only have ourselves to blame as we progressively lose our hard-earned reputation as a ‘clean and green’ country. Farmers must consider the question ‘Which P fertiliser should I use, and why?’ far more seriously than they have in the past.
Algerian RPR is easily the match of any other RPR agronomically, and contains a low cadmium level of 18ppm, which represents only 140mg Cd per kg P, well under the Biogro’s and Demeter’s organic farming limits, and only half the limit that the industry allows itself. It performed even better than North Carolina RPR in trials run by the
International Fertilizer Development Center, Alabama, USA. The Managing Director and the Senior Scientist of the IFDC released the following statement in 1999: “Unground Djebel Onk (Algerian) phosphate rock is classified as a
highly reactive phosphate rock for direct application to acid soils”.
All NZ soils are acid. As it happens, all RPRs are also liming agents in their own right, automatically reducing and in some cases going close to eliminating the need for maintenance lime applications. Algerian RPR has the highest lime equivalent (58%) of all RPRs, helped a bit by the small amount of naturally-occuring dolomite running through the
deposit (3-7% by weight). Note that the naturally-occuring 3-7% phosphatic dolomite in Algerian RPR can reduce its citric acid solubility in NZ’s current but obsolete 30-min test. This is an artefact only, and has no effect whatsoever on the excellent field performance of Algerian RPR. It also contains among the lowest levels of Cd, mercury (Hg) and uranium (U) of all IFDC-recognised RPRs.
Because of all these positive attributes, some industry players have tried to put farmers off using Quinfert Algerian RPR by playing the ‘it is not soluble enough in NZ’s test’ game. So we also offer the product with some of the dolomite screened out, to ensure it reaches 30% citric solubility in the current test. Both the normal (V1) and ‘low-Mg’ (V2) versions perform the same as each other in the field as fertilisers, i.e., exceptionally well. One just has a bit more dolomite than the other!
Finally, there are several other low-Cd RPRs available for blending from around the world as well, so there is absolutely no reason for anyone to resort to reducing the high cadmium level in Sechura RPR by mixing it with a low-Cd manufacturing -grade (non-RPR) phosphate rock, which may be as little as 20% as effective as an RPR.
A 50/50 blend of Sechura with Boucraa slimes (PB3) or Moroccan rock may be only 60% as effective agronomically as 100% RPR. And take note, the Khouribga rock from Morocco, commonly used in the manufacture of fertilisers, can contain large amounts of uranium, up to 566 ppm (FAO, 2004). This is so high (10 times higher than Algerian RPR for example), it can be economically mined to produce uranium! Ask for an updated declaration of the heavy metals in your superphosphate.
The ball is in your court. Please phone or email me if you have any questions.
Dr Bert Quin
021 427 572
Quinfacts 9. Quinfert Algerian now available in two grades.
Solely due to games being played by the industry, Quin Environmentals (NZ) Ltd has decided to make 2 very slightly different grades available to farmers.
V1 or ‘HRPR’ was field-tested by the IFDC, and rated by them as a Highly Reactive Phosphate Rock. It performed even better than North Carolina RPR in published response trials conducted by the IFDC. It contains 12.7% P, 35% Ca, 0.8 % Mg (as free dolomite present in the deposit) and 1.3% S.
Note however that the small amount of free dolomite present naturally in V1 (5-7%) slightly reduces its solubility in the obsolete 30-min citric P test (citsol test) used in NZ (and now nowhere else in the world, to my knowledge). This small amount of dolomite has NO EFFECT on its excellent field performance. Ironically, the very same people who mix Sechura RPR with Moroccan rock and call the mix an RPR are arguing that V.1 should not be called an RPR. Go figure.
V2. Is Algerian RPR from the very same deposit, with very slightly more P (12.75% total P) due to some of the dolomite being removed (magnesium content reduced t0 under 5%). This product meets the obsolete NZ citric solubility test.
If you wish, you can choose whether you want V1 or V2. Same price. They perform the same as one another. You get 0.2% more Mg in V.I or 0.05% more P in V.2. Your choice. Let us know. It’s all a bit trivial, don’t you think? I think so. But then, I’ve only been researching RPRs for 41 years…
Quinfacts 8: The time has come for New Zealand to completely stop making superphosphate
18 September 2018
The National Series of RPR vs superphosphate trials, ran for 3 to 7 years in the 1980s on 19 sites throughout New Zealand. The sites were deliberately chosen to have below-maintenance levels of soil Olsen P. This was done to make it easier to assess if RPR performed better or worse than superphosphate. There were tiny differences (typically 1-3%) on some sites in the first 1-3 years.
Where soil P levels are at or above maintenance (as is the case on 98% of dairy farms), no differences occur. Where soil pH is 5.6 or below, as occurs on over 80% of hill country, no differences occurred, regardless of the Olsen P.
The only situation where RPR had not fully caught up with super by year 4 was a non-irrigated, over-limed (pH 6.4) site with a very low Olsen P in dryland Canterbury (average annual rainfall 750 mm).
In any situation where a small difference in production may occur initially (called the ‘lag-effect’) there is a very simple, proven solution. This is, to use a mixture of RPR and a soluble form of P such as triple super, DAP or MAP, in a ratio that gives 30% of the total P in quick-release form, for the first few years.
So OK, it’s proven that RPR-based alternatives are just as effective. But why should we change from using super? There are a number of important reasons. I will just deal with some of the most important here.
Perhaps the very most important is the fact that fertiliser P losses in surface run-off after rainfall are far greater with super; there is virtually none with RPR. This P run-off loss is the prime cause of eutrophication of waterways and lakes. Some people who should know better have said things like ‘there’s no point; you still get what is called particulate P losses regardless of what fertiliser you are using’. But particulate P is largely soluble P that has become ‘adsorbed’ on soil particles. Using RPR long term, production can be maintained with much lower levels of ‘adsorbed’ P. The RPR stays present as RPR particles that slowly get dissolved by soil acid and used by the plant. These particles are far, far denser than superphosphate and therefore much less prone to being lost in run-off. Short-term trials on land that has a history of superphosphate use cannot be expected to show this effect. But not a cent of the tens of millions of taxpayers money that goes to the superphosphate industry for ‘research’ via so-called ‘Public Good’ funding gets spent on this. No prizes for guessing why.
Then there is leaching. Up until 15 years ago, most NZ researchers dismissed the idea of P being leached through soils. We now know that enormous quantities of P applied as superphosphate (up to 40% of it) can be leached right through the very low P-retention soils of Northland and the West Coast, and that significant P leaching occurs on all soils with P retention values below 45%; in other words about half of New Zealand. RPR by contrast does not leach.
And then there is the fixed amount of sulphur (S), present as sulphate-S, in superphosphate. Super normally contains about 9% P and 12% S if correctly made (ie fully acidulated). This an S:P ratio of 1.3 to one, almost double the 0.7 to one S:P ratio that pasture actually needs. The excess sulphate-S gets leached from the soil, taking with it (for electrical charge reasons) a mixture of the cations calcium, magnesium, potassium and sodium. These losses all have to be replaced over time to maintain production, representing on an annual basis a totally unnecessary cost of about $30/ha annually. Under high rainfall, almost all the sulphate is leached (taking even more cations with it), so the farmer has to use super that is fortified with elemental S, a form of S which doesn’t leach. All of this expensive, completely avoidable nonsense could be avoided by adding the precise amount of fine elemental S to RPR for each farm. No wastage, less cost, less leaching of cations.
And what about storage and manufacturing costs? RPR has a much higher P content than super (12.7% P for Algerian RPR) compared to 9% P (probably a maximum 8% P available). Combined with the much higher bulk density of RPR (1.65 compared to 1.1 for super), that means a given amount of P (or P plus S) as RPR or RPR/S requires only 40-45% as much storage size as does super. Only 65% as many tonnes need to be transported and spread, greatly reducing costs to the farmer.
Then there is the liming value of RPR. Every tonne of Algerian RPR applied has the liming action of 580 kg/ha of pure lime. This is offset by the elemental S added, which produces acidity as it is converted to sulphate-S by soil bacteria, but RPR/S containing the agronomically correct S:P ratio of 0.7 to 1 still has a liming value of 300 kg/ha. Therefore RPR/S being applied at say 275 kg/ha is providing, for free, about 85 kg/ha of pure lime effect. Most soils in NZ need 150-250 kg/ha lime expressed on an annual basis, but about half of this is a function of sulphate leaching from super. Put simply, farms using RPR/S will only need half the amount of lime to maintain a given soil pH than where super is being used. And the full liming effect of RPR of gypsum (calcium sulphate) is used as the source of sulphur instead of elemental S. The important thing is to only use the amount of sulphur that is needed.
Superphosphate did a good job helping to develop the productivity of New Zealand’s soils, but put quite simply, it is way, way past its use-by date.
Quinfacts 7: The facts about serpentine superphosphate
17 September 2018
- Serpentine is a natural magnesium-calcium silicate. It is widely used as a slow release magnesium (Mg) fertiliser in finely ground form. In the case of the product called ‘serp-super’, about 25% by weight of finely ground serpentine is added to the freshly-made superphosphate, before the sulphuric acid has all been utilised in reacting with the phosphate rock to produce soluble phosphate. This means that some of the Mg in the serpentine (maybe 20%) is converted to water soluble form. This may be a slight advantage in the short term, if soil Mg levels are particularly low.
- The big disadvantage of doing this however, is that the more acid utilised attacking the serpentine rock, the more manufacturing-grade phosphate rock, which is essentially useless agronomically, is remains in the final product. We saw in Quinfacts 6 how even in ordinary super, up to 15% of the total P can be present as unreacted manufacturing rock. In serp-super this can be 25%.
- So a serp-super advertised as having say 6.8% P, 8% S and 5% Mg, may in fact have only 5% usable P. No mention of this is made in information provided by the superphosphate manufacturers. In my view, this must constitute misleading advertising. The manufacturers cannot have it both ways. They cannot criticise reactive phosphate rock (RPR), which typically dissolves in the soil up to ten times faster than does a manufacturing phosphate rock like Boucraa (from the disputed area adjoining Morocco), yet very clearly imply that all the manufacturing-grade phosphate still present in the super you buy, and even more in serp-super, is plant-available. But this is exactly what they do.
- Finally, the question of asbestos. Asbestos is actually one of the many crystalline forms of serpentine that can exist in a deposit. Some serpentine in some serpentine deposits is present as asbestos; some fortunately contain none. When a serpentine deposit containing asbestos is mined and crushed, much of the deadly-to-lungs fine asbestos can be released. Much serpentine used to make serp-super in the past was made with asbestos-containing serpentine. More of the dust got released during spreading. It is now illegal to mine serpentine containing more than trace amounts of asbestos.
Quinfacts 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!
Note: For real-time information on product information, email Bert Quin at email@example.com or phone him on 021 427 572
There was a time (in the 1990s and early 2000s) when farmers were spoilt for choice with RPR. Summit-Quinphos offered Gafsa RPR from Tunisia and Kosseir RPR from the Red Sea Coast of Egypt; other companies offered Sechura from Peru. The superphosphate industry sold medium-reactivity (at best) phosphate rock from Morocco as an RPR until they got caught out two years ago and changed to calling it a ‘DAPR’ (direct application phosphate rock), and even got the buyer so sign a declaration to say that they knew it wasn’t the real McCoy. Unbelievable!
So what is available on the New Zealand and Australian market today? I have been having a look at what is going on and am not impressed! In New Zealand, the message and efforts of a few small private companies is being swamped by what I believe is deliberate disinformation and misinformation coming from the Big 2, who I think are, with some justification, afraid of the implications for superphosphate manufacture in New Zealand if true RPRs become widely available and better promoted, as they were up until a decade ago. The environmental advantages of switching to RPR for maintenance are huge.
As a result, I have decided to get back into true RPR importing – at the wholesaler level – and provide reputable companies with a product that has my full confidence and support. This RPR I have selected comes from the ‘Djelbel Onk’ area of the Algerian RPR deposit. This is a truly massive RPR deposit – quite possibly the biggest in the world – and has the ideal combination of characteristics. It has a good total P level of 12.7%, 30-30.5% average citric solubility, and only 18 ppm Cd (which equates to only 140 micrograms Cd per kg of P, only half what the NZ industry regards as an acceptable limit.
NB: Note that because the Algerian RPR has a vein (1-3m thick) of phosphatic dolomite running through it, depending on where a particular sample of RPR was taken, the dolomite can range from 3-7%. At its minimum, the citsol is about 35%, at its maximum, the citsol can be only 28%. Either way, this has NO EFFECT on its effectiveness as an RPR fertiliser (apart from supplying extra Mg for free at the top end). The effectiveness comes from the geological and geochemical makeup of the RPR itself, NOT from some outdated, el-cheapo, easy-to-manipulate 30-minute citric acid test.
I would like to fill in a bit of the background story here, so bear with me!
The Tunisian RPR (which Summit-Quinphos used to import, along with Egyptian Kosseir) is actually an extension of the Algerian deposit. Unfortunately for the Tunisians, on their side of the border, the deposit increases rapidly in cadmium (Cd) content as it gets further into Tunisia, from about 20ppm at the border with Algeria to about 80ppm at the eastern end of the deposit. In those days, I insisted that we received product from close to the Algerian border, with a maximum of 25ppm, equivalent to under 200 micrograms of Cd per kg of P, a bit higher than the Egyptian’s 12-15 ppm but still much lower than the industry’s self-imposed limit.
At the time, Sumotomo Corporation – who owned most of Summit-Quinphos – would not let me go to Algeria, because it was considered too risky for kidnapping of overseas business executives. Well, what a change a decade makes! Algeria sailed through the ‘Arab spring’ of a few years ago with little or no disturbance, and it has a relatively stable government and steadily improving economy, phosphate and other exports and overseas investment. I used to meet with their senior fertiliser executives at international conferences, and told them that one day I would find a way to start exporting their product. So finally, the opportunity to represent the Algerian RPR in NZ has come about!
Unlike some self-appointed ‘experts’ on RPR, I make very sure I visit all the deposits themselves, so that I truly understand not just the geology and geochemistry of the deposit, and just as importantly, the mining operation and beneficiation capabilities and options.
It continues to amaze me how many soil fertility scientists still refer to all RPR deposits by country alone, as though somehow all the RPR or even phosphate rock in general within a given country is the same! It illustrates the arrogance of ignorance, and a total lack of respect for other scientific disciplines outside their own, particularly geochemistry and geology. I consider myself fortunate that I was able to include some of both in my university studies.
It has recently been determined that the Algerian RPR deposit near Djebel Onk is probably the biggest, and one of the most homogeneous, RPR deposits known. It exists of one very deep layer of 25-30m covering a large area. Its variation is very small, being affected only by a 2m-thick layer of dolomitic phosphate that runs through it at variable depth. This dolomite content can be as high as 10% in some depths of the deposit, but averages about 6-8% in a typical days production of 10,000 tonnes. Since the installation of a new mineface and beneficiation plant 2 years ago (following an earthquake), the amount of dolomite present can be varied a bit to a during mining the beneficiation process. This is important, because although dolomite is beneficial agronomically for NZ farmers and many others, it reduces the P content slightly by dilution (only slightly, because the dolomite has P in it as well.
Another significant but purely semantic point is that the presence of dolomite artificially reduces the citric solubility as measured in the standard citric acid solubility test. At the maximum 10% dolomite content, the dolomite in the RPR quickly consumes most of the citric acid in the test, increasing the pH of the citric acid solution from 2.2 to 3.2, and thereby reducing the measured ‘citsol’ from 31% to about 27%. This does not adversely affect its agronomic performance in the field at all, but I decided to go with a bit less less dolomite. Funnily enough, Summit-Quinphos came across the same issue with Egyptian Red Sea RPRs way back in the early 1990s. The Hamrawein deposit contained free dolomite, but the Kossier did not, so we switched to the Kossier. I think the Hamrawein and Algerian deposits are the only two that contain significant dolomite mixed with the RPR. Another well-established example is NZ’s own under-sea deposit, the Chatham Rise phosphorite nodules. These come with 30% calcite (lime) in the nodules, reducing the total P content to only 9%, and reducing the initial citric solubility test to only 15%. Despite this, in all comparisons, it has performed as well as other RPRs. So has Algerian RPR.
The Algerian phosphate mining company (Somiphos, a subsiduary of Ferphos) make two particle size grades; a finer grade for use as an RPR, and a coarser grade sold for the manufacture of phosphoric acid and high analysis fertilisers. In these, the manufacturer is grinding the rock to a very fine powder before treatment with concentrated sulphuric acid, so there is no need for Somiphos to grind it as finely themselves.
That is the end of my digression, so let’s get back to other RPRs (and psuedo RPRs) available in NZ.
My biggest concern by far is what has become the common practice in NZ of misusing the atypically high citric solubility of Sechura RPR (40%) by mixing it say 50/50 or 70/30 with much lower quality phosphate rocks such as those from Morocco, knowing the citric solubility of the mix will still be 30% or above. It might still have 13%P, but its agronomic peformance will be be greatly reduced. As my father Frank Quin used to say, you can’t put wings on a chicken and expect it to fly like a eagle! It is abusing both farmers and the Sechura RPR itself to do this.
The practice of blending Sechura RPR with other phosphate rocks is also done to get a lower cadmium (Cd) analysis by blending it with a lower Cd content phosphate rock, again usually Moroccan. The Cd content of the 8 layers that make up the Sechura deposit range from 20 up to 50 ppm. I know this because of extensive consultancy work I did there for an international mining company. Unfortunately, the low Cd layers are at the bottom, and it is totally uneconomic to selectively mine them. The best you can achieve with the mining and beneficiation available in Peru, without calcining the RPR at high temperature to drive off some of the Cd into the atmosphere, is about 38 ppm, or 290 mg Cd/kg P. With costly calcination you can get the Cd down to 33 ppm (250 mg Cd/kg P), but the reactivity is greatly reduced in the process. This is fine if you are using the phosphate rock to manufacture soluble fertiliser, but for direct use as an RPR that’s a no-no. One manufacturer is believed to have made this rather basic mistake quite recently.
So, if you are buying what you are told is Sechura RPR, it should have at least 40% citric solubility, and it will have about 38ppm Cd. If either of these figures are lower than this, it is not true Sechura RPR, so beware. The only exception to this is the small quantities of completely unbeneficiated Sechura RPR brought in by one or two companies from time to time. Because it still contains the clay and silt, it contains only 9-10% P, not 13%, so beware even more. The Cd will be about 25-30 ppm Cd (260-300 mg Cd/kg P), depending on the Cd content of the clay and silt. Some of the this clay contains just as much Cd as the RPR itself!
You should be asking how on earth we can avoid these shenanigins. Well the good news is that it can be achieved by Fertmark imposing two, preferably 3, quite simple requirements. Firstly, the importer and retailer must both submit declarations stating the exact mining origins and percentages of all significant mineral components of any product they sell that is claimed to be an RPR. Secondly, samples need to be analysed for their crystal a-axis measurement by the International Fertilizer Development Centre in Alabama, USA. This test is very difficult to fake, which is I suspect why there is resistance to adopting it from some quarters in NZ. Thirdly, and most importantly, dump the current 30-min citric acid test, which is as said is far, far too easy to manipulate and therefore way beyond its use-by date, and replace it with the far better 4-stage or 5-stage sequential citric solubility test. It’s time for Fertmark to stand up and refuse to let the reputation of true RPR be undermined. It is simply too important to NZ farming and the environment not to do this.
This article aims to present an overview of how knowledge of and attitudes to phosphate fertiliser efficiency have developed within New Zealand, to clarify areas of confusion that exist, to discuss possibilities for improving phosphate efficiency in the future, and for reducing losses to the aquatic environment. There is far more detailed information on a range of sub-topics to be found elsewhere, but no recent attempt has been made to present an overview that addresses major areas of disagreement and uncertainty regarding phosphate fertiliser.
Origins of recommendations for phosphorus (P) use in New Zealand
The importance of phosphorus in pastoral agriculture is totally accepted in New Zealand. This is because the development of introduced ryegrass and clover pasture, which quickly came to be the backbone of the New Zealand economy, could not have happened on the previously acidic and nutrient-poor soils without lime, phosphorus (P) and sulphur (S). Clover establishment in particular, even with inoculation with Rhizobia, was very difficult without these vital inputs.
A large number of field trials conducted the Department and then Ministry of Agriculture (MAF) throughout the country enabled P requirements for establishment and maintenance to be fairly well understood by the 1950s. Summaries were made by Peter During in his excellent book ‘Fertilisers and Soils in New Zealand Farming’ in the 1960s, and updated and refined by MAF’s Ian Cornforth and Allan Sinclair into the nutrient balance-sheet driven Computerized Fertiliser Recommendation Scheme (CFAS) of 1972. This in turn has been succeeded by the dynamic-system driven ‘Overseer’, developed by AgResearch scientists, particularly Alister Metherell and Dave Wheeler.
Natural sources of phosphorus
The presence of P is inextricably interwoven with that of calcium (Ca) in nature. Ca is continually being washed from the land into the sea in rivers and precipitating with soluble carbonate on the ocean floor. The sedimentary phosphate ‘rocks’, many of which are now above ground and mined for their P content, were formed in places where the presence of unusually high concentrations of phosphate in sea water lead to replacement of carbonate in limestone over time to make tricalcium phosphate, or ‘hard’ phosphate ‘rock’, a far more stable compound, which has to be treated with strong acids to make phosphate fertilisers. ( I have put the word rock in inverted commas as it is an unfortunately misleading misnomer – most phosphate deposits actually exist as sands, and only become more ‘rock-like’ if they have become compressed by the weight of other material deposited over them over huge periods of time). The strength and durability of tricalcium phosphate is why it is nature’s building block in skeletons.
Where the conversion of limestone to tricalcium phosphate on the sea floor was not complete before earth movements or dropping sea levels left it above sea level, the crystal lattice contains both carbonate and phosphate bonded to Ca, and we have what are called ‘reactive phosphate rocks’ or RPRs, which can be used directly as a fertiliser on acid soils. New Zealand’s own underwater Chatham Rise phosphorite deposit is an RPR in the making.
Many other types of calcium phosphate compounds are formed by igneous or volcanic acivity. Many of these incorporate iron and aluminium as well, which reduces their usability for fertiliser, directly or through manufacturing. Many of these compounds have been changed further by weathering and deposition of bird or bat excreta, which in isolated island or cave conditions can result in the natural fertiliser known as guano.
Manufactured phosphate fertilisers
The earliest recorded use of ground limestone to improve ‘sour’ soil is by the Romans. Gypsum has been used to improve infiltration and drainage for a long time as well. Both ground bones and calcium phosphate minerals were widely used as P fertilisers in the 1800s. Then it became known that treating either with sulphuric acid to make what Lawes in England called ‘superphosphate’ was far more effective in most cases. The sulphuric acid converted the tricalcium phosphate to water soluble (and hence immediately, if only temorarally) available to plants. The Ca that was liberated by this forms gypsum with the sulphate from the sulphuric acid. “Super’ made from pure tricalcium phosphate is by weight 45% monocalcium phosphate and 55% gypsum. The key point is that all the Ca from the phosphate rock remains in the superphosphate. “Super’ became by far the most widely used fertiliser in New Zealand.
With the development in the USA in the 1920s of technology to separate the intermediary phosphoric acid from the gypsum during the superphosphate manufacturing process, everything changed, in most countries. First, triple superphosphate (20.5% P but only 9% Ca and 1% S) could be made, and phosphoric acid, containing no Ca, could be reacted with ammonia gas to make MAP and DAP. The latter has become increasingly widely used in NZ in the last 10-15 years. We will come back to this.
It is important to note that all phosphate rocks have some liming effect. This is due to the fact that soil acidity slowly – albeit much more slowly than in a factory – dissolves the tricalcium phosphate, and acidity is consumed in the process. Liming effects of phosphate rocks range from about 25% for apatite and other hard phosphate rocks, to over 50% for RPRs, which contain destabilising carbonate in the lattice.
Natural P in New Zealand soils
European settlers discovered we had a very good climate for growing pasture, but we needed to reduce the acidity, and put on lots of P. Surprisingly, our soil Ca levels themselves were reasonably good, given the climate. This is because the parent sedimentary material was high in the tricalcium phosphate (often called apatite) we talked about. However, the rate of dissolution of this phosphate into plant-available form, at about 1-2 kg P/ha annually, was far too slow to meet the demands of high-producing introduced pastures.
Most New Zealand soils have quite a high cation exchange capacity (CEC), so the ground lime applied to increase the soil pH meant that its Ca content was not leached too quickly. The use of ‘single’ superphosphate or ‘super’ as the main fertiliser to supply P and S also helped maintain soil Ca levels, although the gypsum component is present as a very soluble form from which the sulphate is easily leached from most soils, taking cations with it.
What factors drive the need to continue to apply P?
No-one disputes the vital importance P plays in all living organisms. All farm produce taken from the farm contains the P used to produce it. This is generally the largest or second-largest type of P removal or loss that must be replenished, the other being fixation in the soil. Other smaller losses include direct deposition of excreta into waterways, surface run-off into waterways, leaching through the soil, deposition as excreta on unproductive areas of the farm, and uneven redistribution of excreta, for example heavy deposition on camp sites. The latter mode of loss in particular can be exacerbated by the use of soluble P fertilisers, if they result in short-term excessive plant uptake and animal intake.
The original native vegetation evolved to survive on very low P inputs from slow weathering of P minerals present in the parent material, plus inputs in rainfall, minus losses in leaching and run-off. This system was necessarily very ‘tight’, with very little net loss.
With the introduction of high-producing ryegrass-clover pastures and various crops, removals increased dramatically. After a capital application phase to get concentrations of P and other nutrients in the soil up to levels where the growth of these plants are not unduly restricted, ongoing losses from the system have to be replaced with ‘maintenance’ applications, typically annually (sheep and beef farms) or twice a year on dairy farms.
As both real the cost of fertiliser and the adverse effects of losses of nutrients to the environment increase, increasing attention is being paid to finding out where P is being lost, and how to reduce the losses, ie, tighten the cycle. Lets look at how these losses occur and what could – or already can – be done about them.
Animal-induced losses of P from the production cycle
Direct deposition of excreta into waterways
This source of P loss represents an economic loss to the farmer, and is a substantial contributor to environmental pollution in the form of algal and weed growth in waterways and lakes. The campaign by the dairy industry in recent years to fence off drains and ditches to stock has substantially reduced this mode of P loss, and much remains to be done.
Direct entry of dairy shed effluent into waterways
Likewise, industry action and policing by regional councils has lead to more efforts being made to ensure effluent is irrigated onto land efficiently, and reducing the risk of accidental splillage. In addition, the nutrient content of effluent is incorporated into ‘Overseer’ fertiliser nutrient recommendations for the treated paddocks. As a generalisation, effluent needs to be spread over a much greater percentage of the farm than the current 5-15% typical range, as this leads to accumulation of excess soil K levels on the ‘effluent block’, and increases the risk of pugging and nutrient run-off.
Uneven deposition of excreta
While not achieving the uniformity of effluent irrigation, strip-grazing on dairy farms achieves quite high evenness of distribution. This is not possible on hill country farms, and the flatter camping sites receive disproportionately large amounts of excreta. Increasing subdivision reduces this, as does using sustained-release P sources such as RPR. This maintains a more consistent level of P in the herbage, whereas luxury uptake in the first few weeks after application of soluble P fertilisers can result in very elevated herbage P levels, much of which will end up being excreted in camp sites.
Dairy cows can however also excrete significant quantities in small unproductive areas, for example while waiting at gates to walk to the milking shed, and on the walking lanes to and from milking. Ideally, these lanes should have shallow culverts to collect runoff during wet weather.
Pugging of heavily-grazed areas either in wet weather or under irrigation can greatly reduce soil infiltration and drainage rates, resulting in far higher losses of nutrients, especially P, in runoff. The use of standing pads between grazing, from which effluent can be collected and irrigated onto land, markedly reduces such losses.
‘Fixation’ losses of P in the soil
While all the above factors are important, it is in reducing losses (of N as well as P ) into and from the soil is where the potential to really clean up New Zealand’s waterways and lakes, and reduce fertiliser costs, really lies.
‘Fixation’ losses of P in the soil relate to the accumulation of P in the soil that is too tightly bound to soil clays and/or organic matter to be available for uptake by plant roots, or has been precipitated as insoluble P compounds, or is incorporated in very stable (resistant to microbial breakdown) soil organic matter.
The worst ‘fixation’ offenders in New Zealand by far are the volcanic ash soils of the North Island and Southland, which contain a very highly P fixing amorphous aluminium silicate clay called allophane. These soils require typically half as much again to double the amount of P to maintain a given level of production as non-allophanic soils – simple but undeniable proof that 30-50% of the P applied to these soils is being effectively ‘lost’ in terms of availability to the plant. As it happens, because they mostly occur in areas with reliable rainfall, have good drainage characteristics and are capable of sustaining high levels of production if topdressed with appropriate amounts of nutrients, they tend to be largely devoted to intensive dairy farming, which further increases the need for P application, and with it the potential for loss in runoff.
Traditionally, the ability of soils to fix P was determined by a laboratory test called the ‘P Retention’ test, which rated this ability on an arbitrary 0-100% scale. Allophanic soils typically lie in the range of 90-100, with sedimentary soils typically ranking at a much lower 20-30%.
Unfortunately, this test has been hijacked by both ends of the ‘fixation’ schools of thought spectrum. Those seeking to claim that P requirements can be reduced by as much as 90% by applying P as a spray for foliar uptake use it as ‘proof’ that 90% of the P applied in conventional fertiliser is permanently lost through fixation. This is a nonsense on two grounds. Firstly, the test does not claim to determine permanently ‘fixed’ P but rather just the ability of the soil to strip it out of solution and hang on to it rather tightly. This is why it was called the ‘P Retention’ rather than the ‘P Fixation’ test. Secondly, in a grazed pasture, once the herbage has been consumed in the next grazing, and converted into meat, wool or milk, or excreted, what then? The pasture is back to being reliant on soil P. The economics of pastoral farming can not cover the cost of multiple split applications of foliar P fertilisers per year.
Partly as a reaction to this misuse of the P Retention test, and probably partly because the traditional fertiliser industry wished to see its products as being efficient, the test was some years ago renamed the ‘Anion Storage Capacity’ test, or ASC. This is just as misleading, because, as any dictionary will tell you, a store is somewhere you put something until you need it, and from where it can be retrieved at any time you want to. This flies in the face of scientific knowledge on allophanic soils, which many decades after development, still require much higher maintenance P applications than most non-allophanic soils. It isn’t ‘storage’ in the soil, it is in effect ‘theft’ by it.
Like in most things in life, there are equilibria involved. If one stops applying P to these soils, P will continually become available from the surface of clays, but at far, far too slow a rate to maintain a vigorous pasture once readily available P levels in the soil have been depleted, which typically takes 1-5 years. Thereafter, production inevitably falls.
So what actually can be done to reduce this loss of applied P as through fixation?
Reducing P fixation on allophanic soils
On allophanic soils, most of the fixed P is either attached to clay particles via aluminium ions that have themselves been fixed, or precipitated as (more or less) permanently unavailable aluminium and iron-aluminium phosphate compounds. Lime application doesn’t reduce this process very much, basically because the strength of the aluminium-phosphate bond is much stronger than the calcium-phosphate bond. So the already-fixed P is as I say, pretty much stolen for good, not ‘stored’.
There is however real scope to make maintenance fertiliser P applications more efficient, and therefore reduce the amount of P that is required to maintain a given level of production.
There are several approaches to this being investigated. One is to coat granular soluble P fertilisers in a slowly-dissolving coating of some sort, which allows the plant uptake to keep up with the rate of release from the granule, so there is less opportunity for the P to be fixed. Theoretically, if a granule dissolved slowly over 6 months say, the plant might get to use far more of it than at present. There is some evidence that coating fully granular soluble P fertilisers such as DAP with fine elemental S or some types of polymers does improve efficiency, perhaps by 20-40%, but at significant extra cost per unit P.
The second approach is to simply use a slow-release natural form of P such as RPR. The problem with using RPR as the sole source of P on allophanic soils is that because the P is being released from each particle very slowly – typically over 3-5 years – the concentration of soluble P at the surface of the particle is often not high enough to ensure that the majority of P gets taken up by the plant rather than fixed. The answer to this problem, as demonstrated by Dr Sunder Rajan and others, was to combine fine RPR with water-soluble P in the same particles. This can be achieved by either granulating the two components together, or by applying them in a sufficiently damp or wet manner (such as a fluid) than ensures that each particle as it lands will contain both ingredients.
What then happens is that root growth is stimulated in the vicinity of each granule (dry application) or particle (fluid application), which in turn stimulates the utilisation of the RPR component through excretion of RPR-dissolving acidity by the plant roots. Dr Rajan demonstrated a minimum of 30% of the total P needed to be in water-soluble form, and a form of P that (unlike single superphosphate) which not contain gypsum, which inhibits the dissolution of RPR. In my estimation, such a product reduces long-term maintenance P applications on allophanic soils by about 15%. Costs per unit P are comparable with straight soluble sources, and environmental losses are greatly reduced as well.
The third option is to effectively use the maintenance fertiliser applications as a convenient way of getting soil additives delivered to the site of the fertiliser granules or particle. These additives can be anything that will help the applied P get into the plant. For example, some polymers will bind with P to make a compound that the plant can still take up, but is less susceptible to fixation in the soil, at least until the polymer itself is decomposed by soil bacteria. Other additives reduce the levels of toxic soil aluminium, iron or manganese in the soil by precipitating them. Soluble aluminium stunts plant roots, blocks the synthesis of nucleic acid if it enters the roots, and blocks P uptake. These type of additives have I believe enormous potential to improve P efficiency on both allophanic soils and acid, low-pH hill-country soils.
The pumice soils of the central North Island are also of volcanic origin, and also fix P, although not quite to the same degree as the allophanic soils. They need the soil P status raised to a high level, but once there, maintenance needs slowly reduce because of lower net fixation.
Reducing P fixation on acid hill soils
On the yellow-grey and yellow-brown earth sedimentary soils that cover much of New Zealand, much less P is fixed onto clays in plant-unavailable form than is the case on allophanic soils. A much larger source of in-soil loss on these soils is the accumulation of P adsorbed onto, and incorporated into, soil organic matter, specifically has inosital phosphate compounds.
However, unlike the P fixed by allophone, this organic-fixed P can be released by the use of adequate lime. Excessive lime releases it but it then reprecipitates it as calcium phosphate. Infrequent heavy applications of lime, as traditionally practiced in New Zealand, can exacerbate this cycle. He answer is to move to regular applications of reasonably fine maintenance lime with the maintenance fertiliser. Fluidisation represents a cost-effective way of achieving this without risking uneven distribution of differently sized dry components, or aircraft safety.
Another benefit of this approach is to eliminate aluminium toxicity in the soil, which is an insidious and, I believe, increasing problem on hill country farms. Early work by Doug Edmeades and others demonstrated that soil aluminium levels were likely to reduce both ryegrass and clover growth at soil pH levels below about 5 (measured in water). However, researchers in other countries had demonstrated that at soil pH levels right up to at least 5.8, there can be sufficient soluble aluminium in the soil to inhibit the establishment and function of rhizobia in clover root nodules. Some clover plants will still grow, especially in locations where urine has been deposited, supplying very high levels of N and K, but the overall pasture growth will be severely limited by N deficiency. Losses from the system are simply not being matched by fixation of atmospheric N, if rhizobia is inhibiting N fixation.
Although catch-up research was eventually carried out in New Zealand and published a decade later, in the 1990s, the contemporaneous disbanding of the Farm Advisory Division of the MAF meant that to this day, many farmers remain totally unaware of the threat posed by aluminium.
P losses off and through the soil
Reducing P losses in surface run-off
It took many years for traditionally-taught agricultural scientists in New Zealand to accept that any P at all could be lost in surface run-off following rainfall, such was the belief in the ability of the soil to ‘retain’ P. Likewise, when this was eventually established, it took more research by Alan Gillingham, and NIWA research by Long Nguyen funded by Summit-Quinphos, to demonstrate that (i) much of this run-off occurred in the first few weeks after application, and (ii) using sustained-release RPR as the P source reduced P run-off by over 50%, especially soluble P. Losses of P in soil particles will continue to happen through erosion events of course, but the entry of this P into waterways is far less eutrofying. Subsequent work by McDowell and others confirmed lower losses using RPR, and lead to this reduced loss being incorporated into the Overseer program, albeit in a watered-down form, if one can excuse the pun.
These runoff P losses are typically only a few percent of the applied fertiliser, and therefore do not represent a major direct economic loss to farmers, but they represent by far the biggest contributor to the eutrophication of our lakes and waterways. Making RPR the required form of P (or at least 70% of the total applied) in sensitive attachments would single-handedly reduce P run-off by over 50% over a period of 2 or 3 years, to levels less than 0.5 kg/ha annually, which receiving waters could cope with without suffering excess weed growth and algal blooms. Further reductions, to nearer 75%, could be obtained by applying fertiliser in fluidised form, because of its evenness of application, accuracy of placement and consequent ability to avoid direct entry into waterways and onto unproductive areas.
However, the reality is that the superphosphate industry in New Zealand, a duopoly of two giant farmer cooperatives, aided by some ‘friendly’ researchers, has so far represented far too powerful a lobbying and marketing force for acceptance of what must be done to take root politically, nationally or at regional council level. Instead, we have witnessed the absolute charade of many millions of shareholders funds being wasted on a meaningless fight between Ravensdown and Ballance over patents over the use of nitrification inhibitors to reduce N losses from urine patches. Like fiddling while Rome burns, we are playing games while our waterways are ruined.
Reducing P leaching through the soil
Even harder for many scientists to accept was the fact that not only could fertiliser P be washed off the surface of the soil into waterways during rainfall-induced runoff events, but fertiliser and recycled P could be washed straight through the soil into the groundwater. On extremely weathered podzols of the West coast and parts of Northland, fully 30% of the applied P can be leached through the soil beyond the root zone.
On other soils, higher P retention reduces this percentage, but it can still be very significant on non-allophanic soils. The leached P is not just recently applied fertiliser P, but inorganic and organic P ions that have become desorbed from soil clay particles. Because recently-applied fertiliser P is not the only major culprit with leached as opposed to runoff P, changing to slow-release fertiliser will certainly help, but not guarantee that waterbodies will be protected. Another part of the answer here is carefully managing the amount of P in the system, and using plants that are efficient foragers of P.
What needs to be done first?
In the past, fertiliser and lime were treated as essentially separate issues. This was partly because lime in New Zealand was regarded purely as a soil ameliorant, for reducing acidity and with it aluminium toxicity, and not as a source of Ca as a nutrient. Partly because of this attitude, and partly because it suited the spreading industry to deal with dusty lime in fewer, larger applications, the accepted wisdom of maintaining soil available P and S levels, and K on dairy farms, with annual ( and biannual in the case of dairy farms) was never applied to lime itself. The assumption, if considered at all, was that the input of Ca in super would take care of that.
However, two big changes have been occurring in New Zealand for over a decade, one each in hill country farming and dairy farming, that mean that this assumption is no longer valid, even if it used to be.
On hill country farms, several things are causing the regular application of lime, particularly sufficiently fine lime, to diminish. Until 2010, low meat and wool prices meant that often, when the 5-yearly application of lime was due, cash-flow problems meant it didn’t happen. The fertiliser industry ( and the banks ) pressured farmers to put on P as a priority, when in increasingly many cases lime should have been the priority. Now, with a bit more money to spend, farmers are often finding that (a) applications of the increasingly coarser ‘aerial’ lime aren’t giving the same benefit as before, and (b) because of increasingly common aluminium toxicity, they aren’t getting the response to P they used to. In my personal experience, 70-75% of long-term hill country farmers would be of this view. In desperation, many are being sucked into ‘muck and mystery’ wonder products that promise everything, but deliver just a massive hole in the bank account.
What must be done is to increase the fineness of lime considerably. Where aluminium toxicity is possible, then the first option is to apply a capital application of whatever quantity is required to get the soil pH to at least 5.6 (typically 1-2 tonnes/ha), and then maintain it at this level with annual applications mixed with the maintenance fertiliser. The annual maintenance lime requirement will vary with type of fertiliser used, soil type, stocking rate and rainfall, but will typically be around 100 kg/ha. Doug Edmeades and I agree that the new fineness standard for aglime should be 100% minus 0.5mm and 50% minus 0.2mm. To that I would add 25% minus 0.1mm. The actual weight of lime applied must be adjusted according to the CaCO3 content of the actual product used, as commercial lime can range from 80-98% CaCO3.
Where farm budgets simply do not permit capital application of lime, the alternative is to incorporate Quin Environmentals’ “PORTAL” lime-sparer additive with the annual maintenance fertiliser and lime application. PORTAL precipitates soluble aluminium and manganese present in the soil, reducing the need for capital lime and improving P uptake. Where Al and Mn toxicity is not a problem, ryegrass and clover will perform well at soil pH levels of 5.2 and below. In addition, it takes less lime to maintain a soil pH of 5.2-5.5 than one of 5.6-5.8.
Lime as fine as I have specified may be a problem for some contractors, who will be concerned about the risk of bridging in fixed-wing aircraft hoppers and helicopter buckets. Options include the use of flow agents, attaching vibrators to hoppers and buckets, mini-granulating the lime, or fluidising the lime/fertiliser mix. I have come to view the latter as the way forward.
As far as P is concerned, sustained-release, non-leaching reactive phosphate rock (RPR) should be used wherever soil and climate conditions permit, augmented if required by a proportion of soluble P (plus other nutrients and lime as required of course).
On dairy farms, and on the more intensive sheep and beef farms, the use of ‘super’ is giving way to DAP, due to increased fertiliser N usage and relative costs of P, especially on an applied basis. I believe that overall, this is a good thing, as efficient elemental S can be added to exactly meet S requirements. Super on the other hand contains a fixed 1.4 to 1 ratio of S (all as sulphate) to P, double the actual agronomically required S to P ratio. On many soils the sulphate is so easily leached (taking valuable cations with it), farmers have to use elemental S-fortified ‘sulphur-super’ instead. On the volcanic ash soils that do retain the sulphate in plant-available form reasonably well, the excess sulphate builds up over years to the point where the soil will not hold any more, and then the excess gets leached, taking cations with it.
Where the N content of DAP relative to P is excess to requirements, RPR should be used instead of soluble P fertiliser.
As DAP contains no Ca, it is becoming increasingly important for dairy farmers to maintain soil and pasture Ca levels by ensuring that adequate quantities, again of a sufficiently fine lime, are applied mixed with the maintenance fertiliser. If P is being applied twice a year, each application should include typically 100kg/ha of lime.
Optimising soil and herbage Ca levels is particularly important on dairy farms, where high applications of potash are practiced to maximise pasture growth, but which increase the risk of mineral imbalances in the animal if other cations are marginal.
Given the massive advantages in fertiliser N efficiency that are achieved by applying urea in fluidised form containing a urease inhibitor, and the increasing frequency with which N is being applied to dairy farms, it is to me a no-brainer that fluidised application of all fertiliser and lime is the way forward on intensive farms. With increasing improvements in fluidisation technology, total costs per unit ‘plant-effective nutrient’ or PEN are now fully competitive with conventional application, and the benefits and convenience (including being able to mix anything with anything, solid or liquid) are immense.
Sufficient knowledge already exists to reduce P losses from agriculture to levels that would reduce eutrophication of our waterways and lakes to very satisfactory levels within 5 years. The barriers are a conservative fertiliser supply and application industry in which no one wants to take the risk of being the first to act, and regional councils who lack the courage to lead. Instead a ‘bit-by-bit’ approach is being taken, by setting caps on fertiliser application and dairy farming in the most environmentally-sensitive catchments. While this action is helping water quality in these areas, it is not providing the necessary impetus for the introduction of real initiatives in farm nutrient supply and management, with support rather than resentment from farmers and farming organisations.
What exactly are you getting for your money?
Dicalcic (aka lime-reverted superphosphate) has been around a long time. When properly made, the water-soluble P component in super, called monocalcium phosphate or MCP, is fully converted after crushing by chemical reaction with lime, to a different form of P called dicalcium phosphate or DCP (hence the name ‘dicalcic’).
A well made straight superphosphate should contain at least 90-95% of its total P in the form of water-soluble MCP. The difference between this – say 8.0% P and the total P content of say 9.0%, is comprised of agronomically very ineffective, unreacted manufacturing phosphate rock and various complex iron-aluminium-calcium phosphates of very dubious value to plants. Interestingly, since the demise of the Fertilisers Act, superphosphate manufacturers now clearly imply, in the way they and their field staff use the total P rather than the water-soluble P content of super to calculate how much super to apply per hectare, that this non water-soluble P component is all plant-available.
Chemically, it should only be necessary to thoroughly mix in 30-35% of lime by weight to achieve full conversion of the MCP to DCP over a couple of days. For a variety of reasons, the main one I think being reducing the amount of mixing effort required, much higher proportions of lime – typically 50% – are used. This means that the total P of the product has been diluted from 9.0% P to only 4.5%, and what was water-soluble MCP component of say 8.0% P, to a mere 4.0% P of DCP.
It is extremely important to ask and try to answer the question “is it worthwhile for the farmer to use dicalcic and why?”
Manufacturers and suppliers claim that (1) the DCP form of P is a far more efficient per unit P than is water-soluble MCP, and (2) the lime content helps the plant to utilise all soil nutrients. DCP is not water soluble, but unlike the very unavailable forms of non water-soluble P in straight super, is very easily converted to plant-available water-soluble P in the soil. Probably the main specific advantage of applying P as DCP instead of MCP is that, by being initially non water-soluble, it avoids much of the severe run-off risk that the water-soluble P in straight super faces, when a rainfall or irrigation-induced surface water run-off event in the weeks after application can easily result in a kg or more per hectare of P ending up in the nearest waterway.
This is the direct cause of much of the increasing eutrophication of our lakes and rivers – nitrate leaching from urea and cow urine patches being the other major contributors.
As deeply serious as this is environmentally, it typically represents less than 10% of the P that is applied. So how does this fit with the claims that dicalcic super only needs to be applied at the same rate per hectare as straight super – in other words, at only half the rate of P, clearly implying that it is somehow twice as effective per unit P as straight super.
Rather than totally rubbish this claim, as some of my ex fellow scientists have done and continue to do, we need to dig a bit deeper, if you will excuse the pun. In my experience, farmer observation, despite the fact that it may not be based on the results of scientifically-conducted, fully replicated and statistically analysed trials, are not to be ignored. Time is a great leveler. I believe it is no coincidence that the deepest farmer support for dicalcic super is in areas of yellow grey and yellow brown soils, particularly where there is ready access to cheap lime, and family continuity in farm ownership, meaning long-term observation of cause and effect, and the passing on of these observations.
It so happens that the yellow grey earth soils in particular are known to be (a) capable of accumulating huge amounts of P in very inert organic form, and (b) capable of having this organic P mobilised if lime is applied. I observed this for myself on the ultra-long term superphosphate trial at the MAF’s Winchmore Irrigation Research Station where I was stationed from 1974-82. When the whole trial was limed for the first time for 20 years in 1975, there was a massive mobilisation of soil organic P into plant available form, as measured by Olsen P soil tests and plant uptake.
The question within the question then, is whether applying dicalcic super annually is any more effective than applying straight super annually, and is there a need for a heavy rate of lime separately every 5 years or so, even on these soils? Most agricultural scientists would say no. I would say probably yes, with some provisos, and certainly not to the extent of being able to halve the rate of application of P long term. My estimate is that a 25-30% reduction is possible.
The remainder of the benefit in improved efficiency that cannot be explained by the 10% improvement resulting from reduced P run-off comes, I believe, from the intimate contact between P – now, remember, in the alkaline DCP form – and lime in dicalcic super provides a stimulation of soil microbiological activity, resulting in improved turnover of all nutrients held largely in organic form, meaning not just P, but N and S and trace elements as well. Faster turnover of nutrients provides more opportunity for root growth and nutrient uptake.
Other major soils, especially the allophonic ash soils whose depth and drainage, combined with reliable rainfall, have made them the mainstay of dairy farming in this country, store proportionately far more of their P in inorganic form fixed to clay particles. They have a naturally higher rate of biological activity and this, combined with a natural high pH buffering capacity, means that there is not the same extent of P-release benefit from lime. The real challenge on the ash soils is finding out how to prevent the P being fixed by allophone clay before the plant can use it. But that’s another story.
RPR, S and (reasonably) fine lime
So, I’ve said that dicalcic is possibly 25-30% more efficient per unit P than straight super and lime applied separately on the yellow grey earth and possibly yellow brown earth soils, but not much elsewhere. It is also better for the environment because of reduced run-off. But it is about 40-50% more expensive on an applied basis. So is there a better way? I believe there is – blends of RPR, S and reasonably fine lime. Let me explain why.
If we go back to dicalcic for a minute, it is made by reacting straight super with lime. Manufacturers of single superphosphate (SSP) have gone to the trouble of acidulating a non-recative phosphate rock with concentrated sulphuric acid, to convert the phosphate into a plant-available form, and then semi-granulating it so it can be handled more easily. This costs money, and the product has no liming value. So where is the logic in taking that, crushing it up again, mixing it with lime to change the type of phosphate into a non water-soluble form again, all to give the final product a liming equivalent of 50% of the same weight of lime? No wonder it costs 40% more per unit P than SSP!
So all in all, an expensive process, which also results in high transport and spreading costs per unit P because of the low P content. Lets say you want to put on 15 kg of available P per hectare, typically enough to maintain 10 SU/ha. At 4.0% P available P in dicalcic, that means 375 kg/ha is required. This will also provide the liming equivalent of about 135 kg/ha of lime – the typical annual requirement on a reasonably high-producing non-ash soil. However, the reality is that few users of dicalcic put on this amount, because of the high transport and spreading costs, and therefore soil P levels continue to slowly decline, while soil pH tends to be more or less maintained.
Lets look now at RPR and lime. RPR, or reactive phosphate rock, typically contains 12.5-13% P. (Warning 2015: recently, some suppliers, including at least one of the large companies, have been importing completely unbeneficiated RPR ore from Peru containing about 40% inert clay, reducing the P content to 7-8%. Often, it is claimed to be much higher, such as 11.5%P. To do this is theft. Sometimes, it is blended with a non-RPR to increase the overall citric solubility. This is also theft. If you are in any doubt whatsoever about the P content of what you are buying, tell the supplier you well be taking a representative sample from the truck when it arrives at the farm, and getting it analysed at an independent laboratory such as Hills).
The definition of an RPR as used in New Zealand is a natural phosphate rock which, because of the way it was formed, will dissolve fast enough per year to supply sufficient P to maintain the growth of a high-producing pasture at soil pH levels of up to 6.0, which is all you need.
RPR automatically contains some of its high calcium content in carbonate form, which is what lime is, and it gives each tonne of RPR the liming ability of 0.5 tonne of high-quality lime. This is actually what makes RPR more ‘reactive’ – meaning more quickly released into plant-available form in the soil – than ordinary phosphate rock, which has to be acidulated with sulphuric or other acids. It also consumes soil acidity during this process.
So, lets say you wanted to put on 15 kg P/ha as RPR (120 kg RPR/ha). Add in 10 kg/ha of fine elemental S, which is all you need because it doesn’t leach unlike the sulphate-S in super and dicalcic, and you still have a net liming effect of about 30 kg/ha. To maintain soil pH in the 5.6-5.7 range typically requires an application of 150 kg lime/ha annually. This 150 minus the 30 from the RPR/S leaves 120 kg/ha to be added in as lime, giving a total application of 240 kg/ha of this 50/50 mixture.
Note that the lime does not need to be expensive $200/tonne superfine minus 20 micron lime. Minus 100 microns is fine enough, and doesn’t cost the earth (typically $70-80/t). To that the components of the mix (RPR, lime and elemental S) are all very evenly distributed as they land on the ground, we need to apply the mix in a slightly damp form. A dry mix will be subject to segregation and drift.
A damp mix may not discharge evenly from the truck or aircraft, and in fact can be very dangerous in the latter case. A high water-content suspension will be very expensive to apply because of the cost of transporting and spreading the weight of water. In addition, the likelihood of settling out of high water-content, so-called ‘suspensions’ in aircraft tanks means application rates can be variable.
A high-solids fluid – typically only 15-25% water – actually stays far more uniform, because the water content is far more tightly held. Further, each droplet as it lands contains the same proportion of the components. But the cost of making the fluids, keeping them mixed in the aircraft tank during application is just too high (see 2011 costings).
The simple solution is to spray the dry mix with about 5% water as it leaves the aircraft. Instead of more than doubling the spreading cost as with suspensions and fluids, it adds a mere 5-10%.
Dicalcic Super vs Wetted RPR/S/Fine Lime – Cost Comparison Example
Objective – to apply hill country maintenance P (15 kg/ha) and S (12 kg/ha as sulphate or 10 kg/ha as fine elemental S, plus sufficient lime, in fine enough form, to stimulate soil microbial activity and maintain soil pH.
|dicalcic super (dry) |
|RPR/S/fine lime (wetted)
|Required fertiliser application||360 kg/ha||244 kg/ha|
|Cartage to farm||$20/t||$40/t|
|Cartage (per ha basis)||$7.20 /ha||$9.75/ha|
|Spreading cost/ha (air)||$28/ha||$29/hac|
|Saving/ha/yr with RPR/S/lime||$28.50 (23%)|
|Annual saving (300 ha farm)||$8550.00d|
Notes: prices ex Ballance price list
(a) ex Wairoa
(b) ex Hastings, assuming $383 for RPR/S and $100/t for finer lime and blending – 50/50 blend gives $241.50/t
(c) $4/ha lower rate for lower fert weight/ha, plus 10% for water spray
(d) the $8550 will pay for a trip to Disneyland for the family, or a new quad
This demonstrates the large savings in maintaining hill-country P, S and liming.
Lime (and the calcium it brings with it) should be treated no differently than the fertiliser nutrients. That is, we should be striving to keep pH levels as near to the optimum hill country of 5.6-5.7 continuously, rather than suffering a widely varying soil pH during a 5 year cycle.
Final Comments: Other options such as fixed-wing application of annual superphosphate plus capital lime (say 1 tonne/ha every 5 years), or RPR/S plus reduced capital lime are typically slightly cheaper averaged over 5 years. However, what often happens is that, if cash-flow is not good when the capital lime is due, the pH can fall to a level where aluminium toxicity becomes a real problem, reducing pasture growth and N fixation by clover, which in turn will have a negative effect on soil biological activity.
Farmers must be alert to the existence of unscrupulous ‘muck and mystery’ merchants who use the mask of attractively sign-written helicopters and very slick marketing to promote the use of very costly low-nutrient dilute sprays which are claimed to contain components that will free up nutrients locked up in the soil. It would be good if these components could be shown to achieve something, and unfortunately this has not been the case. By the time you realise this, in 2 or 3 years, your production will have fallen 10%. Sad but true.
What actually is RPR?
The expression ‘RPR’, or reactive phosphate rock describes a phosphate rock that – with certain provisos like soil pH of 6 or less, and an annual minimum of 800mm rainfall and/or irrigation – will adequately supply the maintenance P requirements of (in New Zealand’s case) a vigorous pasture. Given the knowledge that exists in 2010, to state that ‘not all RPRs are the same’, as some self-appointed experts do, could be interpreted by as a deliberate attempt to confuse farmers.
Various laboratory tests have been developed to predict whether a particular phosphate rock is soluble or ‘reactive’ enough to maintain vigorous pasture growth. The intention of these tests is to help avoid avoid the lengthy time, trouble and very high costs of doing scientific, replicated field trials. Solubility in citric acid is one. If the solubility of the P in a phosphate rock in the citric acid laboratory test is at least 30% of the total, such a product is labeled an ‘RPR’, and it is highly likely that it will maintain a vigorous pasture, at least from the third year onwards. In the first 2 years, the use of a blend of soluble P and RPR is usually recommended.
RPRs were all formed on the sea floor originally, by the gradual absorption of phosphate present in sea water into dead sea organisms, which are largely calcium carbonate. The level of carbonate remaining in the crystal lattice largely determines the solubility of the product. The deposits that are economic to mine are those that are now above sea level, due to earthquakes or sea-level changes.
Some countries in which RPR exists contain phosphate rock deposits varying in reactivity from medium to very high. Egypt is one example. Others contain only one, such as North Carolina, USA. In other cases, the same deposit is spread over what are now two or more countries. The Tunisian and Algerian deposits are essentially one and the same, but with the cadmium (Cd) content rising from very low in Algeria(10 ppm) to very high (80ppm) at the eastern extremes in Tunisia.
Many deposits consist of more than one layer, often with clay, limestone or dolomite in between, that were laid down at different geological periods. The Sechura deposit contains one layer that is far less ‘reactive’ than the others, and there is also a wide variation in Cd levels.
North Carolina is no longer sold as an RPR for direct application, partly because its cadmium level of 48ppm is considered too high by most fertiliser retailers, and partly because its owners regard using to produce phosphoric acid (from which most of the Cd can be stripped) to be more profitable. The phosphoric acid is used to make DAP, MAP and TSP.
Risks in relying solely on laboratory tests to assess ‘reactivity’
A problem with the citric acid test is that you can generate an artificially high test if you grind some semi-reactive phosphate rocks very finely…
A problem with the citric acid test is that you can generate a ‘satisfactory’ test if you grind some semi-reactive phosphate rocks (like Moroccan BG4) very finely, or test just the fine fraction, or blend them with a highly citric-soluble RPR such as Sechura . Sechura has, for chemical reasons (the presence of hydroxide in the crystal lattice), an atypically high solubility in citric and the similar formic acid tests. While it is unquestionably an effective RPR, it generally performs no better in field trials than the other RPRs such as Tunisian, Algerian and North Carolina. One of New Zealand’s largest fertiliser companies was recently accused of doing this.
On the other hand, naturally-occuring free lime or dolomite mixed with a true RPR can artificially reduce the solubility in the test a bit, even though it doesn’t adversely affect its field performance. Some true RPRs from Egypt can show this effect, as the deposits were laid down with dolomite.
So, neither citric acid or formic acid solubility tests are perfect!
The unbeatable test; the a-axis measurement
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. This can be done quite easily with modern x-ray diffraction equipment , but the equipment is very expensive. They have one at the renown International Fertiliser Development Centre in Alabama, USA, visited many times by the author. 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 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 is the 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 the same in the field as other RPRs. 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.
It is concerning that a phosphate rock such as Moroccan BG4 can continue for years to be advertised as an RPR, in the complete absence of scientifically robust field trial data, or the independent measurement of the a-axis dimension.
The ‘National Series’ of RPR Trials
During his time (1981-84) as Technical Advisor to the Directorate of MAF’s Agricultural Research Division (which subsequently became the AgResearch Crown Research Institute), and subsequently as Chief Scientist for Soil Fertility at the Ruakura Research Center in Hamilton (1984-87), the author had the responsibility for designing and overseeing the New Zealand-wide series of field trials comparing RPR with superphosphate, otherwise known as the ‘National Series’ of RPR trials.
This series of trials was conducted on 19 sites for 6 years, from 1981-86. A few of the sites continued for a further 2 years, purely looking at residual effects in the absence of fertiliser applications.
Before leaving the MAF in late 1987,the author presented a summary of the trial results to a farmer conference at Ruakura, including a simple predictive model for predicting how RPR would compare to superphosphate in various situations.
Subsequently, the late Dr Allan Sinclair, who had been closely involved with the trials throughout, wrote up the results for publication in scientific journals, with the assistance of several of the other scientists involved.
So, what exactly did this series of trials establish? Here are the key points, all of which assume that the RPR has had blended with it sufficient fine elemental sulphur to meet maintenance S requirements-
Provided that (a) the initial Olsen P in the soil was not well below the recommended range for a high-producing pasture on a given soil, and (b) that the soil pH was not much higher than 6.0, and (c) that the average annual rainfall (or rainfall plus irrigation) was at least 800mm, then RPR would usually equal the performance, or in the worst case come to equal the performance over a 3 year period, of superphosphate.
With the same provisos as above, the average pasture production with RPR was 3% less (range 0-5%) in year 1, 1% less (range 0-3%) in year 2, and no different in year 3 onwards. This ‘lag-effect’ is caused by the time required for annual applications of RPR to build up a sufficiently large P reserve in the soil so that each year, sufficient RPR was being dissolved into plant-available form by soil acidity to provide sufficient plant-available P to maintain the growth of a vigorous ryegrass-clover pasture.
The explanation is very simple. Essentially, any one application of RPR took 2-5 years depending on soil type, soil pH and rainfall (average 3years) for most of it to be dissolved into plant-available form. So, after 3 years of application, you reached an equilibrium situation where a year’s maintenance requirement became available in each year. It’s not rocket science.
Where the initial soil Olsen P was high enough on a given soil to mean that there would have been no response to superphosphate for a year or more if withheld, there was no difference in pasture production between RPR and superphosphate right from the start. This is simply because the reserve of P in the soil avoided any drop in production from the ‘lag-effect’ from RPR.
Where the soil Olsen P was in the low or medium (‘maintenance’) range at the start, any lag-effect in pasture production from RPR could be totally avoided by either of two practices-
(a) Using a 70-30 blend (by P) of RPR and a low gypsum-content form of water-soluble P (this excludes superphosphate) for the frst 2 years, before switching to straight RPR. These blends of P could be either a physical blend of RPR and TSP, DAP or MAP, or what is called a partially-phosphoric acidulated reactive phosphate rock, or PAPR. Blends of RPR and superphosphate do not work as well initially. This is believed to be due to the calcium ‘common-ion’ effect, whereby the Ca dissolving from the soluble gypsum of superphosphate (55% by weight), inhibits the dissolution of the Ca from the RPR, and therefore the release of the P into plant-available form.
(b) Where cash-flow permits, an application of RPR equivalent to 3 years maintenance, in one initial application at the start, and then reverting to maintenance application in year 4 onwards.
All RPRs performed the same. Although only Sechura RPR was included at multiple rates to produce a full ‘response curve’, North Carolina RPR and Chatham Rise phosphorite were included at 0.75x maintenance rates in virtually all the trials. These invariably performed no better, and no worse, than Sechura RPR. Other work has shown than if the RPR is not sufficiently fine, or has too much free lime or dolomite occurring with it, initial production can be lower.
Other trials have shown that if a less reactive phosphate rock is used, most accurately determined by measuring its crystal a-axis dimension, it is likely that it will never be able to maintain the same level of production as a true RPR or superphosphate, because it simply cannot release enough P each year, unless the soil is extremely acid, and soil aluminium toxicity is not present.
There is anecdotal evidence over the long period of time (25 years plus) that some farmers have been using RPR that eventually, maintenance P requirements start to drop significantly (by up to 10 kgP/ha/yr) with RPR. This is likely to be partly because of the scientifically proven reduction of P run-off into waterways when RPR is used, and partly because of reduced fixation onto soil clay particles.
A review of all New Zealand RPR trials has been published in a series of scientific articles and conference papers by Dr Quin, named RPR Revisited (1)-(4), 2013/14.
And read the new Quinfacts series of articles on this website!