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1. Economic and environmental benefits of reactive phosphate rock (RPR)

By Filed Under: Enquire, Quinfacts

Background

Both the economic and environmental benefits of using RPR as the source of phosphate on New Zealand’s soils have been deliberately understated by the fertiliser manufacturing industry in the past decade. With no significant independent importation and scientific promotion of RPR, farmers who want to keep using it have found it increasingly difficult to access true RPR at a reasonable cost.

Three years ago, the industry was caught out selling substandard material as RPR, and now describe those products as DAPR (Direct Application Phosphate Rock) instead. The only true RPR that is being imported by the superphosphate manufacturers  – Sechura RPR from Peru – has an undesirably high cadmium (Cd) content. And there are increasing reports that the unacceptable practice of blending this with cheap, lower-Cd manufacturing rock is continuing.

Why is this happening? Why are not greater efforts being made to supply true RPR to NZ farmers? The industry itself accepts that superphosphate manufacturing in New Zealand is on a slow but inevitable decline into oblivion, as they import more and more high analysis DAP and now MAP and even TSP to meet demand from their farmer shareholders. More and more of the sulphuric acid used to convert manufacturing phosphate rock into superphosphate is being imported directly rather than being made from imported elemental sulphur as traditionally done.

Even the argument that superphosphate has a ‘smaller carbon footprint’ than high analysis manufactured fertilisers does not stand up to scrutiny of the environmental emissions from superphosphate manufacturing, and the much higher  costs, per unit P, of internal storage, freight and spreading costs with superphosphate. High analysis fertilisers containing 20-22% P require less than 40% of the tonnes to be stored, transported and spread and New Zealand.

So, given the increasing importation of high analysis fertiliser into NZ, why has the importation of true RPR with low Cd content virtually stopped? The reasons given – mainly poor supply – simply do not stack up. As an example, the industry says they will not import  Algerian RPR – similar to Tunisian RPR but with lower cadmium – because it ‘does not meet NZ’s citric acid test’. This is incorrect; it demonstrates either a surprising lack of understanding of the deposit, or a deliberate attempt to confuse the farmer. More about this in a future Quinfacts. Other RPRs that could be imported are the Israeli Arad and the lower-cadmium end of the Tunisian deposit.

RPR has by far the lowest carbon footprint of any form of phosphate fertiliser there will ever be; it is simply dug out of the ground, crushed or washed and sieved to get rid of most of the accompanying clay and other minerals, and shipped here, to be mixed with elemental S and other nutrients and applied. Superphosphate served its purpose for 100 years building up available P levels in New Zealand soils, but they simply do not need to be increased anymore, just maintained where they are. RPR does that perfectly, as many, many trials have shown.

The science of the benefits of RPR

RPR has been researched extremely thoroughly in New Zealand. It will maintain pasture production and soil phosphate reserves at least as well as superphosphate in any situation except a combination of very low rainfall and higher than optimum pH. If annual fertiliser application has to be withheld, for example for farm income reasons, RPR maintains production far better than superphosphate. Sulphur requirements can be easily maintained by adding typically 7 kg elemental sulphur (either as fine S or sulphur-bentonite dispersable prills) for each 10 kg P required. Where soil sulphur levels have been allowed to run down to deficient levels (typically <5 ppm), the possibility of deficiency in early spring is easily avoided by the incorporation with the RPR/elemental S of any number of sources of sulphate-S, eg gypsum, sodium sulphate, sodium thiosulphate, calcium thiosulphate, ammonium sulphate and potassium sulphate, at about 10 kg S/ha.

RPR is a sandy mineral formed completely naturally by the decomposition of fish skeletons and shellfish on the sea floor, and their adsorption of phosphate from sea water, over hundreds of thousands of years. For cost reasons, almost all of the deposits being commercially mined are ones that have been raised above sea level by earthquakes or drops in sea level at some point in time. No chemicals are added to the product; beneficiation simply involves crushing and/or washing to remove impurities such as clay and organic matter. This increases the P content; this is further increased by drying to remove moisture. RPR is therefore a truly natural or ‘organic’ product, and is approved for use by organic farmers across the globe, as are gypsum and elemental S.

All true RPRs have an inherent liming effect of about 500kg lime per tonne of RPR applied. This is due to the fact that when RPR is applied to acid soil (as are all soils in NZ), the soil acid reacts with the RPR to release plant-available water-soluble phosphate. The soil acid consumed in the process reduces how much lime needs to be applied in the future to maintain the soil pH. This is particularly important in hill country where application costs of lime are very high. A typical soil in NZ needs about 250kg/ha lime annually to maintain the soil pH (this is typically done by applying 1 tonne every few years; this practice is simply to reduce the higher spreading costs of more frequent application of smaller amounts. The addition of elemental S reduces but does not eliminate the liming benefit of RPR. Long-term lime requirements are typically reduced by 30% where RPR/S is used to replace straight super, or 50% where it is used to replace sulphur-super.

On medium-high rainfall situations on low to medium P-retention soils, sulphur-super needs to be used instead of straight super. This is simply because the sulphate content in it is being leached too quickly to be of much use. As well as being economically wasteful (the farmer has paid for this sulphate), as it is leached from the soil it automatically takes with it ionically-equivalent quantities if calcium, magnesium, potassium and sodium. All of these unnecessarily leached nutrients ultimately need to be replaced, if production is to be maintained.

And finally, there is P run-off. A host of field studies in New Zealand and many other countries around the world have invariably shown that soluble P fertilisers such as superphosphate are far more prone to P run-off after rainfall events or irrigation than is RPR. This effect can remain for many weeks after application. The P lost in run-off enters streams, rivers and lakes where it is, alongside nitrate-nitrogen leached from cow urine patches, the biggest cause of eutrophication of waterbodies in New Zealand. Some recent industry-funded studies have argued that reducing ‘hotspots’ such as gateways and excessive soil P levels are more important than the form of P fertiliser used. However in my view, these studies are largely industry-backed smokescreens to try and hide the fact that if RPR was being used, these other loss mechanisms would simply not occur in the first place, or at least be greatly reduced. These hotspots are essentially caused by excess uptake of soluble P by pasture in the months after application. This excess uptake of P does not happen with RPR. More about this in a later Quinfacts.

2. How to define a true RPR

By Filed Under: Enquire, Quinfacts

Background

Any phosphate rock will release some P into plant-available form when applied to acid soils. The question is, in the pastoral context, will it release P at a sufficiently fast rate to maintain the needs of a high production pasture? Most phosphate rocks do not. Even if very large quantities are applied they still do not, as the dissolution in the soil reaches equilibrium.

This is why, to be effective as a fertiliser, most phosphate rocks have to be chemically converted into a more soluble form of phosphate using a concentrated acid. The simplest process, to make single superphosphate, known as SSP or ‘super’, is to react the finely-ground phosphate rock with concentrated sulphuric acid. The acid reacts with the apatite tricalcium phosphate mineral which makes up the phosphate rock to form a mixture of water-soluble monocalcium phosphate (about 45% of the weight) and gypsum (about 55% by weight). Various greenhouse gases and others such as poisonous sulphur dioxide are emitted during this process. The product is then dried and granulated in most countries, or simply semi-granulated in the case of New Zealand, giving a product containing about 9% P. Typically a total of 8% P is plant available, with the rest being made up of unreacted phosphate rock and complex calcium-iron-aluminium phosphates. The gypsum provides about 11% S, all present as water-soluble sulphate-S.

If a wider ratio of sulphuric acid to phosphate rock is used, phosphoric acid is formed which can be separated from the gypsum. This phosphoric acid can then be reacted with more phosphate rock to form triple superphosphate (TSP, 20% P) or with ammonia to form diammonium phosphate (DAP, 18% N and 20% P) or monoammonium phosphate (MAP, 10% N and 22% P). These 3 ‘high-analysis’ fertilisers (and phosphoric acid itself),  make up the vast majority of phosphate fertiliser traded internationally, because the high analysis greatly reduces transport, storage and spreading costs compared to SSP. SSP usage in NZ is slowly but surely giving way to imported TSP, DAP and MAP, amended with added elemental S to suit individual farm needs, rather than applying a blanket amount as with SSP.

It has however been known for nearly a century that some phosphate rocks can be used directly as a phosphate fertiliser on acid soils. All of New Zealand soils, and most of Australia’s agricultural soils, are acidic. The phosphate rocks that can be used with some level of effectiveness on acid soils are loosely defined as direct application phosphate rock or DAPR. With this number, there is a smaller group defined in New Zealand and Australia and an increasing number of other countries as reactive phosphate rock, or RPR. The fundamental definition of an RPR in the pastoral context is a phosphate rock that is reactive enough to release sufficient plant available P to maintain the growth of high-producing pastures. There are several laboratory methods used to help determine whether a DAPR is sufficiently reactive to be defined as an RPR. These methods are designed to avoid the need to do a whole series of new field trials whenever a new potential RPR is located or commercialised.

In the case of both New Zealand and Australia, nation-wide series of field trials comparing various DAPRs with superphosphate (SSP and TSP) were conducted in the 1980s and 1990s which ran for 4-6 years, followed in some cases by no-application for 1 or 2 years to measure the residual effects of the different fertilisers. Many other single-location trials and laboratory and glasshouse studies focusing on specific aspects were also conducted, and much research was conducted on predictive solubility tests.

The science of determining which phosphate rocks qualify as RPRs

In both New Zealand and Australia, it was determined that the simplest, reasonably accurate laboratory method to ensure that a particular DAPR would maintain pasture production as well as the same amount of P as soluble phosphate on acid soil, and therefore be described as an RPR,  was whether or not at least 30% of the total P content of the DAPR would dissolve in a dilute organic acid (2% citric acid) in a short 30-minute extraction. RPRs that contained more than 30% ‘citric P’ or ‘citsol’ (as the test has come to be called) performed no better in this role as a maintenance P fertiliser, although they will give a slightly faster P response in very P-deficient situations. Although this citric P definition is not controlled legally, it is sufficiently strongly industry-recognised in both countries that using the term RPR to describe DAPRs that do not meet the 30% citric P test is unacceptable in the market, as both Ballance and Ravensdown have found to their cost in recent years.

When MAF analytical research chemists Mike Brown and myself first introduced the test back in the 1980s, it became rapidly accepted. A particular strength of the test was that, by doing the test on RPRs as sold, it provided an automatic control regarding whether the product as sold to farmers was fine enough. If the product was too coarse, the reduction in surface area meant that it did not dissolve fast enough – either to be fully effective as a fertiliser in the field, or to meet the 30% citric solubility requirement.

Over time however, two significant shortcomings in the citric acid test have become apparent. One of these produces an artificially low citric P for some true RPRs; the other allows one particular RPR (Sechura) to be blended with poor-quality phosphate rock and still produce a test of 30% or more. Let’s explain both problems.

  1. The effect of the presence of free lime or dolomite in RPRs. A small proportion of true RPRs contain a percentage of 10% or so free lime or dolomite mixed with the carbonoapatite mineral. Two examples are the small Hamrawein deposit on the Red Sea coast in Egypt and the massively larger Algerian RPR deposit. Much of the latter contains 10% free dolomite. This is even better from an agronomic point of view, as even less lime needs to be applied to maintain soil pH than with other RPRs. However, the presence of this dolomite selectively consumes about 20% of the citric acid in the laboratory test, leaving less of it to dissolve the P. So Algerian RPR, if mined where dolomite is present, can come up with a slightly lower citric P than 30%, even though it is just as good as a fertiliser. When applied to the soil, the individual grains of RPR and dolomite land in microscopically different locations, each consuming different micro-pools of soil acid. Fortunately, it is relatively easy to determine whether sufficient lime or dolomite is present to affect the citsol test.
  2. Sechura RPR has a different chemical composition in the crystal lattice to other RPRs, caused by the presence of hydroxide ion replacing some of the carbonate. This has no effect good or bad on its agronomic effectiveness – field trials have shown that it performs identically to other RPRs on pasture; but it does have a much higher citric solubility, ranging from 38-46%. Unfortunately, some unscrupulous companies have used this as a means to cheat farmers, by blending low-cost, low-solubility non-RPRs with Sechura, knowing that the average citsol can still meet 30%. Unfortunately for the farmer, the percentage of P present that is not an RPR will still be ineffective. One solution to this real problem is to insist that all companies selling RPR identify the source of their product, and sign a declaration that samples taken for Fertmark or other analysis contain only that RPR and no other source of P. It still requires honesty on the importer’s part however.

The a-axis measurement – unbeatable test for RPRs. I have been campaigning for 20 years for New Zealand and Australia to adopt an alternative RPR test, called the a-axis dimension test. It is a relatively simple test but requires a quite expensive piece of laboratory equipment – an x-ray diffractor or XRD.

The XRD measures the dimension in Angstroms of the a-axis of the carbonoapatite crystal lattice. RPRs have a smaller a-axis measurement due to at least 20% of the phosphate in the lattice being substituted by carbonate the in the crystal lattice (note this is totally different carbonate to the carbonate in the form of free lime or dolomite present in some RPR deposits). Having a smaller a-axis makes the crystal less chemically stable and therefore more susceptible to being attacked by soil acid. The a-axis measurement that is used internationally to define the most reactive phosphate rocks should be a maximum 3.329 A (angstroms). Phosphate rocks with a higher a-axis than this, such as these from Morocco, cannot maintain a sufficient concentration of P in the soil solution to ensure maximum growth of the vigorous ryegrass/clover pastures grown in New Zealand and Australia. The only known exception to this a-axis rule is Sechura RPR, which because it has more hydroxide (OH–) in the crystal lattice than other RPRs, has an a-axis of 3.334 A. Partly for this reason, it is vital that the location of origin, and proof of reactivity, of any phosphate rock being mixed with Sechura RPR be declared. Amazingly, this is not a requirement under Fertmark in New Zealand.

 

3. Using RPR in low soil P, low rainfall, high pH
or extremely high P retention (>97% PR) situations

By Filed Under: Enquire, Quinfacts

Background

RPR is a natural, slow-release mineral formed on the sea floor over hundreds of thousands of years. Deposits that have been raised above sea level by changes in sea level or earthquakes are cheaper to mine.

When RPR is applied to acid soils, the soil acid attacks the phosphate mineral, releasing plant-available P in a sustained fashion. True RPRs release P fast enough to easily maintain the growth of high-producing pastures, with only a few limitations.

The science of using RPR in limiting situations

The first is that the soil has to have some level of acidity; otherwise the RPR will remain undissolved (as it has for eons in the North African desert). Because all pastoral soils in NZ and virtually all in Australia are acidic anyway, this isn’t an issue. It was originally advised that RPR should not be used at soil pH levels above 6.0, which is still slightly acidic (neutral pH is 7.0). The only reason for this recommendation was that RPR did not perform too well on a site that had a soil pH of 6.4. But this site also happened to have a very low rainfall (550mm). On farms with higher rainfall or irrigation, especially where existing soil P levels are already at or close to recommended maintenance levels, RPR has been shown to work perfectly well at soil pH levels up to 6.4. Few if any pastoral soils have been limed to a pH higher than this; it is simply not necessary for optimum production and induces trace element deficiencies.

The slow-release nature of RPR makes it less suitable where a rapid increase in soil P levels is required, eg in dairy conversions. This is particularly the case on high P retention soils.

Here are my recommendations for using RPR in less than perfect situations –

  1. Capital applications in very low soil P levels (less than two-thirds of the bottom of the recommended range):

Use a mix of RPR and a low-gypsum (non-superphosphate) soluble P, eg TSP, DAP or TSP. The water-soluble component should make up at least one half of the total P applied, until soil P levels reach the recommended range.

  1. Very low rainfall (<700 mm, no irrigation):

Following the end of a drought, apply the first P application as TSP, DAP or MAP.

  1. Soil pH in the range 6.4-6.7:

Use a mix of RPR and TSP, DAP or MAP. The water-soluble component should make up at least one-half of the total P applied.

  1. Very high P retention soils (PR>96%).

Use a mix of RPR with TSP, DAP or MAP for the first 5 years. The water-soluble component should make up at least 33% of the total applied P.

4. What every farmer should know about cadmium (Cd)

By Filed Under: Enquire, Quinfacts

Background

The cadmium (Cd) issue arose in New Zealand and Australia purely and simply because the manufacturing phosphate rocks both countries originally used to make superphosphate (Nauru and Christmas Islands) contained elevated levels (50-100 ppm) of Cd. All of this Cd ended up in the superphosphate, and therefore in the soil. Cd exists at low levels in all soils. But if too much is present in the fertiliser, the levels in the soil increase over decades to the point where elevated levels show up in pasture and then in animals, which concentrate it in their liver and kidneys, as happens to many poisons that cannot be excreted in the urine. The highest levels are found in soils which have had the highest application rates of superphosphate for the longest period of time; these are typically the dairy farms in the Waikato.

The highest levels of Cd are found in the liver and kidneys of young stock. To avoid the entry of excessive Cd into the human food chain, offal from young animals is not permitted to be sold for human consumption.

The reasons why some phosphate rocks are higher in Cd than others is not completely understood. Different hard ‘manufacturing’ phosphate rock deposits and different RPR deposits can contain very different levels of Cd. There can even be wide variation in levels within a particular deposit (more about this later). The concentration of Cd is thought to be largely due to the level of Cd in whatever organic matter is present in the phosphate rock as mined; that is, little is present in the phosphate rock crystal lattice itself. For this reason, the Cd can be removed by roasting the phosphate rock at high temperature. The organic matter in the phosphate rock is burnt off, along with the Cd in it. Ideally this is captured from the emissions. This roasting is expensive, and it also reduces the solubility of the phosphate rock, so it is not so suited for deposits that are used for direct application.

Internationally, most high Cd phosphate rocks (even those that otherwise are suitable for direct application) are used to make phosphoric acid. The reason for this is that most of the Cd precipitates out of the phosphoric acid and ends up in the gypsum. The phosphate fertiliser made from this phosphoric acid generally contains less than 20% of the amount of Cd originally present, but the enriched level in the gypsum by-product reduces its suitability for land application purposes, such as improving soil drainage and reducing salinity.

The science – and pseudo-science – of how Cd is managed by the industry in NZ

I first brought the issue of Cd accumulation to NZ farmers in the 1990s. Quinphos Fertilisers initiated a self-imposed limit of 2 ppm Cd for each % P in a product. This meant that an RPR containing 13% P could not exceed 2×13 = 26 ppm Cd. I based this limit on some excellent published independent scientific research conducted by Massey University, which showed that, at this level of Cd, there was no accumulation of Cd in the soil, and therefore in the food chain. At the time, we were importing Egyptian ‘Kossier’ RPR which contained only 12 ppm Cd, and Tunisian RPR, which ranges from 20-25ppm at the western end of the deposit to about 80ppm at the eastern end. We insisted that we receive RPR only from the low-Cd end; none of our shipments exceeded 25ppm. Averaged over the two RPRs, we had 19 ppm.

After years of ‘deny, deny’ that there was even an issue, the industry very slowly moved to adopting a self-imposed Cd limit of 280 micrograms of Cd per kg of P in any phosphate fertiliser. This equates to a 25 ppm Cd limit for a 9% P super, or 36 ppm for an RPR containing 13% P, nearly 40% higher than Quinphos’ maximum limit. This just isn’t good enough.

A decade ago the industry in effect admitted the seriousness of the Cd situation it had got NZ farmers into by introducing a ‘4-class’ categorisation of Cd in NZ soils. Farmers whose soils were in the highest Cd category could now only use fertiliser containing almost zero Cd.
What are the implications for RPR users? There are no RPRs with absolutely zero Cd (a cynic would say that the only motivation for having this pointless category at all was to prevent RPR being used). The main RPR now being imported by the industry (Sechura) is right at, or above, the industry’s own limit of 36 ppm. I have visited the Sechura deposit in Peru (as well as virtually all the other RPR deposits), as part of consultancy work for an international mining company. Unlike say the Algerian RPR deposit, which is essentially one thick homogeneous layer all containing 16-20 ppm Cd, Sechura has 8 different layers separated by clay; these layers range from 20-60 ppm Cd, with the lowest Cd layers being at the bottom. It is simply not possible to selectively mine the lowest Cd layers without enormous cost increase. The minimum weighted average achievable with current mining methods is about 38 ppm in my assessment. An importer may fluke one shipment at say 32 ppm, but the next is just as likely to be 45 ppm.

The very good news is that there are plentiful supplies of low Cd RPR. The Arad deposit in Israel is one. The Red Sea coast RPR deposits in Egypt are another. NZ’s underwater Chatham Rise phosphorite nodules – not mined as yet – are another. By far the biggest known to date, and already being mined, is the huge Algerian RPR deposit. All these RPRs are sufficiently low in Cd to lead to a slow but sure decline in the Cd levels in NZ and Australian soils if used as the source of phosphate.

5. The effect of the sulphur form used on the ratio of S to P required

By Filed Under: Enquire, Quinfacts

Background

Pasture plants need to take up 7 kg of sulphur (S) for every 10 kg of phosphorus (P). So, if both nutrients are being applied in reasonably efficient forms, a fertiliser containing 9% P should only need to contain 9×0.7 = 6.3% S. Near the coast, where there is significant input of sulphate in rainwater, the amount of S required is less or even zero.
But most people know that single superphosphate or ‘super’ contains 9% P and a fixed 11% S, almost twice the calculated amount of S required. Furthermore, in higher rainfall areas and low-medium P retention soils, even this amount is insufficient, and super has to be amended with elemental S to make sulphur-super. Why is this? It is simply because much of the sulphate-S gets leached from the soil before it can be used by the pasture (taking valuable cations such as calcium, magnesium and potassium with it). Even though 10-35% of applied soluble P becomes fixed in progressively unavailable forms in our soils, this is still more efficient than is sulphate-S, except in very dry conditions.

The science of S fertilisation

It has been known for decades that, except in very dry conditions and on very high P retention soils, finely ground elemental S is far more efficient than sulphate -S. This is why elemental S rather than say gypsum (calcium sulphate) is added to straight super if more S is required, and elemental S is the ‘go-to’ product for adding to the likes of TSP, DAP and MAP.
It is no surprise therefore that by far the most common blend of RPR and elemental S sold by Quinphos was 92% RPR and 8% fine elemental S. With 12.7% P in the RPR, which also contained about 0.8% S as sulphate, this gave a product containing 8.7% S and 11.7% P ; an S:P ratio of – you guessed it – 0.7 to one.

A big proviso is that the elemental S used has to be truly fine – mainly less the 250 microns and all less than 500 mm. This can be irritating to the eyes if not dampened. These days, the cost of prilling molten sulphur with bentonite clay is far less than it used to be. These small water-dispersable hemi-spherical prills contain hundreds if tiny sulphur particles in each prill, which disperse easily in the soil.
The only situations where sulphate is likely to be needed are on extremely low rainfall areas or in cold late winter/early spring if soil S levels are low. Sulphate of ammonia provides a good N boost and takes care of the S requirement in the latter case.

 

Urine Patch Detection & Treatment with Spikey ® – Trial Results

By Filed Under: Enquire, News & Opinion, Quinview

LOW-COST DETECTION AND TREATMENT OF FRESH COW URINE PATCHES

Geoff Bates1, Bert Quin1,2 and Peter Bishop2,3
1Pastoral Robotics Ltd, Auckland New Zealand
Email: gbates@ateliertech.co.nz, bert.quin@gmail.com
2Advanced Agricultural Additives Ltd, Tokomaru, Manawatu
3Bishop Research Ltd, Tokomaru, Manawatu

Abstract
Leaching of nitrate-nitrogen (NO3-N) from cow urine patches is the most serious threat facing the future viability – environmental and possibly economic – of grazed dairy farms in New Zealand. Proposed solutions to this problem have largely focussed on changing the fundamental basis of the NZ year-round grazing model to one extent or another, all requiring increased capital expenditure on and maintenance of housing or stand-off pads, and capital and running costs on collecting and distribution of manure.

Solutions that permit full-time grazing are largely focussed on means of better matching the energy and protein ratio of the feed to the cow’s intake requirements, such as by different plant species, or mixes of crops and pasture. All these will require much research, and new skills to be learnt by the farmer.

Previous published attempts to reduce losses from the actual urine patch include (i) the Taurine® tail-activated N-inhibitor dispensing device (a work in progress), and (ii) sensing where urine patches are likely to be with chlorophyll level assessment in pasture. However, by the time significantly greater growth can be assessed, most of the urine-N is already present as nitrate.

This paper describes the to the farm-tested working prototype stage – of relatively inexpensive and easily manufactured vehicle-towed equipment which uses the measurement of surface-soil electrical conductivity to detect fresh urine patches with very high accuracy (Spikey®, patent application 617342). The equipment simultaneously sprays the urine patch with ORUN®, a mix of the commonly used urease inhibitor NBPT and gibberellic acid (GA3), and where more dissolved organic carbon (DOC) is required, the dicarboxylate polymer AlpHa® as well (patent applied for). This mix is assessed as being is at least as effective in reducing nitrate leaching as DCD (now removed from the New Zealand market).

In the future, it is intended that a robotised vehicle (Mini-ME™) will tow the equipment, releasing farm labour. In addition to substantial extra grass growth and N recovery of pasture, achievable at relatively low cost of implementation, initial results indicate potential reductions in both nitrate leaching and nitrous oxide volatilisation of at least 30%.

Introduction
To substantially reduce nitrate leaching and emissions of ammonia and nitrous oxide from urine patches in a practical fashion, while maintaining a full grazing system required the authors to address three challenges:

  1. How to detect and localise the urine patches, and before significant transformation of the urine-urea had occurred;
  2. Selection of methods of moving the detection equipment over the area of interest.
  3. What to treat the fresh urine patches with, and how.

This paper presents the approach taken to address these challenges, and the outcomes.

Detection and localisation of the urine patches
Early experiments into the detection of the urine patches investigated fluorescence, temperature variation and grass response but none of these techniques yielded a practical system that could be applied soon after deposition of the urine but did not require immediate application. The most easily identifiable traces of urine deposition were found to be a changes in soil moisture and conductivity. It was realised that significant gains were to be made if the development lead to a system that could be implemented in conjunction with follow on application of urea, particularly the wetted prilled urea-based technology ONEsystem® (Quin et al. 2015).

The urine detection and treatment system specification developed was for a system capable of searching a typical paddock immediately after grazing, in the typical rotational grazing system used in pastoral dairy farming. Paddock searching could be achieved in conjunction with fertiliser application (where part or all of the paddock is covered). The ideal system was envisaged as being used daily on the area grazed that day would be once a day (that is, a requirement for detection up to 24 hours after urine deposition). However it was considered that even a maximum detection period of only 10 hours after deposition would allow the system to be deployed after every milking on the most common twice a day milking dairy system.

Early investigation showed surface conductivity measurement worked best in moist soil conditions and was severely limited in drier soils. In dry conditions, poor electrical contact restricted the current flow measurements to levels close the minimum resolution of the system, leading to erroneous measurements. Further experimentation lead to a method where both the contact resistance and the soil conductivity influence the measurement. The results presented here are based on this technique.

The prototype used for the data collected (Fig. 1) had a total of 7 measurement cells. This number is not limiting, and future longer iterations (modules) and compound iterations (multiple modules) are planned. Development is currently underway of a 6-metre arrangement consisting of three 2-metre modules that are designed to be towed behind a farm vehicle. The outside modules would fold upwards for passing through gateways. A single 2-metre module would be suitable for towing behind a small robotic vehicle, as discussed later.

bates-fig1
Figure 1. Spikey®, named after the appearance of the contact disks.

The experimental technique developed to evaluate the performance of Spikey® under varying soil and soil-moisture conditions involves topping and raking a section of pasture that has not been grazed for at least 20 days. The topping/raking is undertaken to remove excess pasture, as is the case after grazing. Urine collected from cows or occasionally synthetic urine (formula urea 273 g, glycine 67.5 g, K2CO3 326.2 g, K2SO4 32.2 g, KCl 127.2 g, made up to 20 L in water) was then deposited at a fixed spacing at various times during the test period and marked with stakes and paint. At the test time Spikey® is towed over the urine patches, typically at 8 to 12 km/h. Some testing was undertaken at 4 and 16 km/h. When Spikey® is being tested, a mark is inserted by the driver of the tow vehicle in an extra channel of the data to indicate the location of the urine patch, thereby assisting data analysis.

The following figures include the raw data from the 7 sensing cells and the urine patch indicators which are the blue marks with sub-zero values.

Fig. 2 shows the data for very dry soil with a damp surface; 2 mm of rainfall occurred during the testing, following more than a month without rain. Three urine samples were placed at 3 different times during the day. The three samples placed consisted of fresh cow urine (collected that morning), old urine (collected 2 weeks prior) and synthetic urine. It is noted that every urine patch except one can be clearly detected by visual analysis of the data; the one just after the 15 second mark was less obvious. Some other data spikes are present. These are due to localised high conductivity effects, most commonly dung pats.

Fig. 3 shows the same urine patches detected 24 hours later (32 hours after the first deposition). It can be seen that all the patches are still clearly defined. The non-urine patch sourced spikes (most as mentioned could be observed to be due to dung pats) tended to be physically much smaller than the urine patches, and could be isolated by their relative size. This is evident in Fig. 4, where the spike caused by a non-urine patch was only evident in one channel of data, whereas the urine patches caused 2 or more multiple channels to be activated. However, even with the differentiation possible through channel comparison, there is likely be a small percentage of false positive responses. This not considered to be important. If required, the choice could be provided to deliberately include detection and treatment of cow pats, which themselves are a source of N losses, albeit a less serious one than urine patches.

fig2
Figure 2. Detection of urine patches placed at various times during the day and detected at 7pm that day, dry soil, damp surface. Blue marks below zero indicate the placement of a urine patch.
fig3b
Figure 3. Detection of urine patches placed at various times during the preceding day (same patches as in Figure 2) and detected at 6:20pm, dry soil, damp surface.
fig4bb
Figure 4. Comparison of response to urine and dung patches, dry soil, damp surface. The signal amplitude is increased with moist soil, as can be seen in Fig. 5, where the data was collected in late spring before this summer’s drought had started.
figg5b
Figure 5. Urine patch detection in moist soil (South Auckland late November) with one dung patch placed at the end of testing. Data taken on the same day as deposition at 5 pm.

The converse of this is that the signal is much smaller in dry soil (Fig. 6). However, the background noise is also reduced, avoiding any reduction in urine-patch detectability.

fig6bb
Figure 6. Urine patch detection in dry soil, data collected 2 hours after placement.

Methods of moving the equipment to detect and treat the urine patches over the area of interest

Two practical methods of moving the sensing system over the freshly grazed paddock have been implemented The first involves the use of a farm vehicle such as the quad bike, as shown in Fig. 7, similar to that used in the FLRC demonstration on Massey University No. 4 Dairy Research Farm. Tow behind configurations of Spikey® are now able to be supplied for research applications.
The second method, also demonstrated at the research farm, involves the use of a small autonomous farm vehicle or robot, as shown in Fig. 8. This robot, referred to as Mini-ME®, uses differential GPS to follow a pre-defined path in an accurate, repeatable and auditable manner.

fig5spikey
Figure 7. Spikey® being towed behind a farm vehicle.
fig6spi
Figure 8. The Mini-ME® autonomous robot towing Spikey®.

Treatment of urine patches with ORUN® – Optimised Recovery of Urine N
The objective of reducing nitrate leaching and ammonia nitrous oxide emissions, if successfully attained, leads directly to increased plant N uptake. This in turn increases pasture growth and farm productivity.

However, in the aftermath of the DCD/Eco-N® fiasco, it was considered vital to achieve this outcome using only accepted, widely-used chemicals; ones which were considered to be of very low toxicity, and furthermore that decomposed sufficiently rapidly in the soil to avoid any potential milk residue problems.

Granular fertiliser urea treated with nbpt (typically 0.2-0.5%) has been widely used on grazed dairy pastures in NZ (SustaiN®) and other countries since 2002, primarily to reduce ammonia volatilisation. However, it has also been shown to reduce nitrate leaching and increase pasture response to fertiliser urea (Zaman et al. 2005, 2008, Blennerhassett et al. 2007). The global mega-analysis conducted by Abalos et al. (2014) demonstrated that nbpt was more effective than DCD in improving plant N uptake from urea on acid soils.

The growth promotant gibberellic acid (GAs) has been used widely on dairy pastures since the 1990s, typically at rates of 20 gm/ha GA3 per application. It is proven to produce substantial increase in growth through cell elongation, particularly in cooler weather (Matthew et al. 2009, Ghani et al. 2014)

Finally, it is possible that the growth of pasture affected by urine application can become limited by the availability if dissolved organic carbon (DOC) in the root zone. The potential growth is very high, given the non-limiting supply of available N and the further stimulus provided by GA3. Where DOC is a possible limitation to N uptake therefore, it was considered useful to include a source of organic carbon that would be decomposed into plant-available form over the typical ‘lifespan’ of a urine patch (1-3 months). The fulvic acid polymer Alpha® was selected for this purpose.

In due course therefore, the treatment mix of choice, described as ORUN®, was made up of the following 3 products and their rates-

  • Urease inhibitor N-(n-butyl) thiophosphoric triamide (nbpt), applied at a rate of 2 kg/ha of urine patch area (approx. 45 mg/urine patch)
  • Gibberellic acid (GA3) at 30 mg/ha (0.75 microgram/patch)
  • Where needed, Alpha®, a source of dissolved organic carbon (DOC) at 15 L/ha (0.4 ml/patch).

The 3 products were dissolved in water at a dilution sufficient to apply 300 L/ha.

ORUN® trial results and discussion
Field trials with cow urine were conducted on a Manawatu mottled fine sandy loam loam at Massey University between June 2014 and February 2015. In the first trial (winter to early summer 2014), dry matter production and N uptake from urine-affected areas, and the movement of mineral N through the soil profile were investigated. In the second trial (summer 2014/15), nitrous oxide emissions were measured over a 5-week period, using static cylinder technology.

  1. The nbpt component keeps the urine-urea in the urea form urea for longer, thereby delaying its conversion to ammonium and then nitrate. This gives the pasture more opportunity to recover these forms of N.
  2. This increased period before the urine-urea is transformed allows more time for the urea to move laterally in the soil solution, thereby increasing the effective area of the urine patch, typically by 30%.
  3. The GA3 in ORUN greatly stimulates the already high pasture growth on the urine-affected patch area (by increasing cell elongation), and also on the increased area resulting from lateral diffusion of urea. It can be used year-round for this purpose as there is little overlap of patches from one grazing to the next, GA3 can be applied in this manner after every grazing.
  4. These factors combined to increase pasture dry matter and N uptake in the total urine affected area by percentages that are at least comparable in magnitude to the increases obtainable with DCD (Figs 9 and 10).
  5. As a consequence of additional pasture N uptake with urine+ORUN®, there was a reduction in soil nitrate below the root zone in the 40 to 70cm zone on Sep 30, from 25 kg N/ha to 15 kg N/ha, indicating a minimum 40% reduction in nitrate-N leaching.
  6. Measurement of nitrous oxide greenhouse gas emission from urine-patches (Trial 2, conducted Dec 2014- Jan 2015) showed a reduction of approximately 30% with ORUN® treatment, as shown in Figs 11 and 12.
fig9sp
Figure 9. Cumulative dry matter comparison between non urine-treated pasture (control), urine-treated pasture, and urine+ORUN® – treated pasture.
fig10sp
Figure 10. Cumulative herbage N uptake (kg N/ha) comparison between non urine-treated pasture (control), urine treated pasture, and urine+ORUN® treated pasture. Field trial conducted July – Dec 2014.
fig11sp
Figure 11. N2O flux comparison; control, urine and urine+ORUN®.
fig12sp
Figure 12. Cumulative N2O flux comparison between non urine treated pasture (control), urine treated pasture, and urine + ORUN treated pasture. Field trial conducted Dec 2014-Jan 2015.

 

Conclusions
Treatment of urine-affected areas with ORUN® nbpt/GA3/AlpHa® spray significantly increased urine-affected pasture response and N uptake; by 1.8 and 2.2 times the increases over the control attained by urine alone. The increased recovery of soil minera reducing both estimated nitrate leaching and measured nitrous oxide emission by at least 30% compared to non-treated urine patches. The authors are confident that refinements to the ratios and concentrations of the ingredients in ORUN® will bring about further reductions in N losses.

The methods of detection and treatment of urine patches developed in this research are practical to implement within existing farm systems and involve a relatively low investment of approximately $40,000 for a 3-module, 6 meter-wide Spikey® unit . It is estimated that this would require 45 minutes per day to tow the unit over the pasture grazed that day on a 150 ha dairy farm. The ORUN® cost of chemicals is estimated at less than $50/ha per year, allowing for 8 applications.

It is calculated that the increased utilisation of urine-N in extra DM produced would produce an additional 1.5% pasture growth per annum averaged over the entire farm. This would have an estimated positive contribution to farm earnings of approximately $8-10,000k per annum at a milksolids payout of $5/kgMS.

The one-off investment of $40,000 for Spikey ®, $7500 pa for ORUN®, and 200 hours labour per year ($4000 pa), less an $8-$10,000 pa increase in milksolids production, can be compared to an actual loss of annual earnings of $84k in 2013/2014 suffered by the 160 ha SIDDIC Dairy Research Farm at Lincoln as a result of reducing stocking rate sufficiently to achieve an Overseer® – calculated 10% reduction in nitrate leaching (Pellow 2015).

References
Abalos, D., Jeffery, S., Sanz-Cobena, A., Guardia, G. and Vallejo, A. (2014). Meta-analysis of the effect of urease and nitrification inhibitors on crop productivity and nitrogen use efficiency. Agriculture, Ecosystems and Environment 189: 136-144.
Blennerhassett, J.D., Zaman, M., Ramakrishnan, C., Quin, B. F. and Livermore, N. (2007). Summary of New Zealand SustaiN® trials to date. Proceedings of the Workshop ‘Designing Sustainable Farms – Critical Aspects of Soil and Water Management’. Massey University Occasional Report No. 20, ISSN 0112-9902, pp 111-116.
Ghani, A., Ledgard, S., Wyatt, J. and Catto, W. (2014). Agronomic assessment of gibberellic acid and cytokinin plant growth regulators with nitrogen fertiliser application for increasing dry matter production and reducing the environmental footprint. Proceedings of the New Zealand Grassland Association 76: 177-182.
Matthew, C., Hofmann, W.A., and Osborne, M.A. (2009). Pasture response to gibberellins: A review and recommendations. New Zealand Journal of Agricultural Research 52: 213-225.
Pellow, R. (2015). The real (rather than modelled) cost of meeting N limits – Part 1: An ongoing case study of a Canterbury dairy farm. In: Moving Farm Systems to Improved Nutrient Attenuation. (Eds. L.D. Currie and L.L Burkitt). http://flrc.massey.ac.nz/publications.html. Occasional Report No. 28, Fertilizer and Lime Research Centre, Massey University, Palmerston North, New Zealand. 13 pages.
Quin, B.F., Gillingham, A.G., Spilsbury, S., Baird, D. and Gray, M. (2015). Improving the efficiency of fertiliser urea on pasture with ONEsystem®. In: Moving Farm Systems to Improved Nutrient Attenuation. (Eds. L.D. Currie and L.L Burkitt). http://flrc.massey.ac.nz/publications.html. Occasional Report No. 28, Fertiliser and Lime Research Centre, Massey University, Palmerston North, New Zealand. 11 pages.
Zaman, M., Blennerhassett, J.D. and Quin, B.F. (2005). Increasing the utilisation of urea fertiliser by pasture. Proceedings of the Workshop ‘Developments in Fertiliser Application Technologies and Nutrient Management’. Massey University Occasional Report no. 18, ISSN 012-9902, pp 276-284.
Zaman, M., Nguyen, M-L., Blennerhassett, J. D. and Quin, B.F. (2008). Reducing NH3, N2O and NO3-N losses from a pasture with urease inhibitors and elemental S-amended nitrogenous fertilizers. Biol. Fertil. Soils 44: 693-705.

Acknowledgements

The authors are grateful to Prof. Mike Hedley and Dr Dave Horne (Fertilizer and Lime Research Centre, Massey University, Palmerston North), and Dr Carolyn Hedley (Landcare Research Institute, Palmerston North) for their help with the ORUN® trials.

 

‘Knowing is not enough; we must apply. Willing is not enough; we must do.’  Goethe