It was good to see an admission from Ravensdown CEO Greg Campbell on Stuff a few days ago (Gerard Hutching, Feb 05) that too much nitrogen fertiliser is being used on many farms, and that more efficient products are needed and under development. Credit where credit is due. Take note Doug Edmeades. It will be interesting to see the cost premium put on these. Meanwhile, ONEsystem (wetted, urease-inhibitor treated prilled urea, twice as efficient as granular urea in Canterbury trials), is already here, greatly reducing environmental losses and farmer N requirements and REDUCING COSTS at the same time. What’s not to like? It would be good to discuss options for the the wider uptake of this technology with Ravensdown. I’ll bet a Speights or 3 that ONEsystem will beat any product they are developing hands down on cost-effectiveness to the farmer and on environmental benefits. I always said when I developed SustaiN in 2002 (rubbished for 10 years by the industry and now Ballance’s biggest-selling N fert by far, forcing Greg’s bunch to follow with copycat N-Protect), that it was a good start, but only a start. ONEsystem is the end-game. Trust me.
News and Opinion
‘News and Opinion’ is just that. It contains posts of important and interesting news relating to agriculture and climate change, water use and quality, soil fertility and fertiliser, and our opinion on these posts. We welcome your opinion on our opinions!
Rich McDowell used to be very precise and no-nonsense with scientific facts and cause and effect before he became a sort of unofficial roving ambassador for the superphosphate industry a few years ago. Now, outside of peat soils, the form of P fertiliser used apparently doesn’t matter. The fact that the average concentrations of filterable reactive phosphorus (ie dissolved P, more or less) at testing sites over the period 1994-2013 has decreased on more sites than have increased is implied to be a major achievement (interview of Rich by Tim Fulton, Farmers Weekly online, 5 Feb). To me, this is nonsense. Firstly, where is the data on the actual changes in concentrations? Is the mean and average up or down or sideways? Etc, etc. Certainly, fencing off waterways, riperian strips, planting susceptible areas in native bush have helped reduce P run-off. But as Rich knows, the single biggest improvement by far will be made by changing from soluble P to RPR, at no cost – in fact a saving – to the farmer. It is misleading to say “The work found little evidence the improvement was caused by a ‘change to’ low water-soluble phosphorus fertilisers”. What forms of ‘low-soluble P’? How many monitoring sites? How many years?
New Quinfert Field Advisor Brittany Stratton is based in the Manawatu/Whanganui area. Brittany completed a degree in Agricultural Science at Massey University and is keen to help farmers optimize their farm production with fertiliser products that minimise adverse effects on the environment.
RPR Revisited 5:
The Sources and Causes of Soil P Losses and the Role of RPR in Reducing Them
1Bert F. Quin and 2Gordon Rajendram
1Quin Environmentals (NZ) Ltd
2Eurofins NZ Ltd
Presented to the
New Zealand Soil Science Society Conference
Napier, New Zealand, 3-6 Dec 2018
- The key drivers of P leaching from soils are the degree of saturation of the soil’s P sorption capacity, and the volume of drainage.
- The higher the percentage of saturation of the P sorption capacity (Pscs%), the greater the concentration of P maintained in the soil solution, and therefore the greater the losses of P in any drainage (Sharpley 1995). This applies to both Pi and Po.
- Saturation occurs gradually through either the diffusion of P into soil mnerals via imperfections in their surfaces (Fig.1a), or its occlusion by being coated by Fe and Al oxides while physically bound to soil mineral surfaces (Fig.1b).
Figure 1: The mechanisms for the gradual saturation of soil solution P sorption capacity
RG McLaren, KC Cameron: Soil Science, 1990, pg 211-212
- The percentage saturation of the phosphate sorption capacity (Pscs%) is very important environmentally, but unlike the Netherlands for example, New Zealand soils and farms are not tested for this.
- Instead, only a relative assessment of the P sorption capacity of the soil is made (the phosphate retention or PR test, expressed on a 0-100 scale) for use in assessing maintenance P.
PR or ASC – what’s in a name?
- The PR test currently is used only to help assess required maintenance P inputs and soil Olsen P levels to maintain a given level of farm production. It was renamed the Anion Storage Capacity (ASC) about 15 years ago.
- The term ASC is a misnomer and should never have been adopted. A ‘store’ or ‘storage’ is by definition somewhere you deliberately put something until such time that you want to get it out to use it.
- When we apply soluble P fertiliser however, we have little or no control over the rate it which it will be adsorbed (into ‘storage’), or how quickly the soil P concentration will reach equilibrium again.
PR vs ASC – What’s in a name (cont’d)
- Just as importantly, the rate of desorption into plant available form is rarely capable of maintaining optimum production for more than 2 or 3 years after fertiliser is withheld.
- So this sorbed P is not a ‘store’ at all; it is better described as a bank ‘savings’ account which unfortunately pays no interest and can only be withdrawn at rates that are far too low to maintain a reasonable state of living.
The goal; a dual-purpose soil agronomic and environmental management test
- Where soluble P is used, the concentrations of P in the soil water are generally high enough to give rise to eutrophic levels of P in any drainage. Only 0.015 mg/L dissolved reactive P (DRP) is required for drainage water to be eutrophic. This represents only 0.1 kg/ha with an annual drainage of 400mm; not economically significant, but enormously important environmentally.
- Unfortunately, the Olsen P test, while reasonably useful agronomically, is by itself far to blunt an instrument to be used in environmental management.
- Measurements of CaCl2-P in drainage water have shown ‘change-points’ near the top of the pasture production response curve, above which losses of P in drainage water accelerate (Fig. 2, McDowell 2012). But the change-point varies markedly with different soils, so this approach has little practical benefit.
- However, McDowell and Condron (2004) had already produced a rather snazzy predictor of DRP in drainage from the Olsen P and PR over a range of soils, viz:
DRP = 0.069 (Olsen P/PR) + 0.007
- The Olsen P/PR ratio is essentially an expression of the degree of soil P sorption capacity. Very importantly, it bridges the gap between purely agronomic soil testing and environmental management.
- It is disappointing that more has not been done to promote the use of the ratio in environmental management.
- Setting a limit of say 0.35 for the Olsen P/PR ratio for pastoral soil development throughout New Zealand would bring major improvements to water quality, without significantly reducing farm production.
Relevance to RPR
- RPR has been proven to maintain any given level of pasture production with considerably lower Olsen P levels on most soils, by many researchers (Fig. 3a).
- This is a consequence of the fact that far more of the P uptake from RPR is coming more directly from gradually dissolving particles of RPR. The Resin P test (Fig. 3b) provides a far better assessment of this drip-feed.
- When soluble P is applied however, soil water P concentrations around dissolving granules can reach hundreds of mg P/L, overloading plant P uptake requirements and soil sorption rates for a period of weeks or months. During this pre-equilibrium period, large P leaching losses are possible from many soils, with phosphate retentions as high as 50%.
- This overloading and resulting susceptibility to loss does not occur with RPR. It is important to note however the fact that not all phosphate rocks can be used as direct application P fertilisers capable of maintaining high levels of pasture production.
- Only reactive phosphate rocks or RPRs, as a function of having at least 20% substitution of phosphate by carbonate in the crystal lattice (which gives them a much higher solubility product in mildly acid soils), can maintain sufficient concentrations of P in the soil solution for vigorous pasture production (Figure 4).
- Dissolved Reactive P (DRP). The concentration of DRP in run-off is driven very largely by specific fertiliser applications and by the form of P applied.
- Many studies around the world, including several in New Zealand (eg McDowell and Catto 2005), have demonstrated the very high levels of DRP that can occur in the first 2 or 3 run-off events after application of soluble P (Fig.5). This reflects the very high levels of soluble fertiliser P in the near-surface soil water.
Figure 5. P concentrations in surface runoff: Water soluble P versus non water soluble P
- 10 kg P/ha or more can be lost in single run-off events (eg Nash 1998). This can occur even when a run-off occurs months after application, if prior soil moisture levels have been too low for complete dissolution of fertiliser granules and the movement of P into the soil.
- These losses of DRP simply do not occur with RPR.
- This raft of evidence should have lead to RPR being strongly recommended for use in all sensitive catchments by soil fertility research scientists. Unfortunately, the potential role of RPR to mitigate P run-off has been deliberately reduced in recent publications from NZ.
- RPR’s overall ‘ranking’ as a P run-off mitigation option was reduced to a poorly justified ‘0-20%’ by McDowell (2012), lower than many of other mitigations listed, including flood irrigation management, sorbents in and near streams, dams and water recycling, applying alum and red mud to pasture etc, pure clover swards in sensitive areas, not applying fertiliser P to ‘hot-spots’ etc. And who pays for installing and maintaining all this?
- The fact that the unnecessary application of soluble P in the first place is the root cause of virtually all P loss is no longer mentioned in published papers by scientists funded under the Mitigator project; nor is it in the ‘public’ version of Ballance’s Mitigator® webpage.
- Instead of promoting the one-step practical solution of switching from soluble P to RPR, which alone of all mitigations saves the farmer money as well, a raft of expensive and site-unproven ‘remedies’ that are costly for the farmer to install and maintain are being presented.
- One excuse given for not promoting the benefits of using RPR as a P run-off loss mitigation is results from studies showing that while DRP losses are greatly reduced with RPR, particulate P losses are not. Despite DRP making up 30-70% of total P losses in most cases (Hart et al 2004), the attitude now appears to be ‘why bother using RPR if particulate P losses are not reduced also?’
- This logic, as well as being deeply flawed in itself, ignores the fact that all particulate P loss comparisons between soluble P and RPR have been done on areas with a background of soluble P use.
- Trials sites that have not had a history of RPR applications cannot be used to assess the effect of particulate P loss with RPR for a number of reasons:
- RPR particles are much denser (Bulk Density 1.65) than soluble P (BD 1.0-1.1), and are therefore far less prone to being carried off in run-off.
- This density difference also means that RPR particles ‘sink’ into the soil must faster, further reducing their susceptibility to run-off.
- As seen in Fig. 4, RPR is not capable of producing P concentrations in soil water much greater than 0.2 mg/L.
- The use of soluble P however produces very high concentrations of weakly adsorbed ‘Olsen’ P near the soil surface, often much higher than in the standard 0-75mm sampling depth.
- Much (>25%) of this loosely-bound P in near-surface soil is easily desorbed when the soil particles enter a body of water, for equilibrium reasons demonstrated decades ago by Australian researchers (eg Barrow 1983).
- There is overwhelming evidence that the use of RPR instead of soluble P would greatly reduce both P leaching and the loss of soluble P in run-off events.
- The advantage of RPR is likely to apply to particulate P losses as well, on farms where RPR has been used for a period of years.
- There needs to be greater focus on introducing P soil tests that are useful for environmental management rather than just productivity advice. The introduction of limits on the Olsen P to PR ratio, for example a ratio of 0.35, would have very considerable environmental benefits.
- Ways must be found to create a soil fertility research environment in NZ that is far more open to discussion and scrutiny than is currently the case. Approval of research topics, the publishing of trial data, and the availability of environmentally-protective products are under the control of profit-focused management staff of the two superphosphate-manufacturing cooperatives.
- Instead of hiding behind Mitigator® and the huge expenditure of farmers money on it, scientists involved should at the very least be openly stating why they are not supporting the use of RPR as a practical and effective P-loss mitigation, given despite the huge amount of evidence in favour of this benefit.
‘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