What Else Can Come With Your Phosphate?

Cadmium, phosphate fertiliser and why the source - and form - of phosphorus matters

Phosphorus has played an enormous role in the development of New Zealand agriculture.

Many New Zealand soils require additional phosphorus to maintain productive pasture and cropping systems, and phosphate fertilisers have been used successfully here for generations.

But phosphate fertiliser doesn't begin in a fertiliser factory.

It begins with rock.

And that rock contains more than phosphorus.

Depending on where phosphate rock was formed, its mineral composition, calcium content, natural reactivity and trace-element profile can vary significantly.

One of those trace elements is cadmium.

Cadmium is particularly relevant in New Zealand because repeated phosphate fertiliser use has contributed to its gradual accumulation in some agricultural soils. The Ministry for Primary Industries identifies phosphate fertiliser as the primary ongoing source of cadmium accumulation in New Zealand agricultural soils, while also noting that cadmium concentrations in our soils remain relatively low overall.

Cadmium is a naturally occurring element.

It can occur naturally in phosphate-rock deposits and can therefore travel with the phosphorus from the original rock into fertiliser products.

The cadmium originates in the raw materials.

The amount present can vary substantially depending on the geological source of the phosphate rock.

That makes the question "Where did my phosphate come from?" more important than it might first appear.

For much of the twentieth century, New Zealand used significant quantities of phosphate rock sourced from Pacific deposits, particularly Nauru.

Nauru phosphate was an exceptional phosphate resource.

It was a high-grade calcium-phosphate material and was particularly well suited to manufacturing superphosphate.

Unfortunately, its geology also brought something else with it.

MPI records historical Nauru phosphate rock at an average of approximately 450 mg cadmium per kg of phosphorus and notes that Nauru contained some of the highest cadmium concentrations among phosphate rocks historically used by New Zealand.

In 1995, New Zealand's superphosphate manufacturers began a cadmium-reduction programme which resulted in the phasing out of Nauru rock and increasing use of phosphate rock from other sources.

This isn't about blaming farmers or today's fertiliser industry.

Nauru was considered a very valuable phosphate resource.

The issue was that we came to understand more about one of the naturally occurring trace elements that came with it.

There is an important distinction here.

Natural phosphate rock and Reactive Phosphate Rock, or RPR, aren't necessarily the same thing.

RPR refers to phosphate rock with sufficient natural reactivity to progressively dissolve under suitable soil conditions.

Interestingly, although Nauru phosphate was well suited to reacting with sulphuric acid during superphosphate manufacture, it wasn't especially reactive when judged as a directly applied phosphate rock.

Massey University fertiliser teaching material lists Nauru phosphate at approximately 22% citric-soluble phosphorus as a proportion of total phosphorus, while recognised reactive phosphate rocks generally exceed approximately 30% using that measure.

RPR stands for Reactive Phosphate Rock.

Unlike single superphosphate, RPR isn't first completely acidulated to produce highly water-soluble phosphorus.

Instead, a sufficiently reactive phosphate rock is applied directly and relies much more heavily on natural soil chemistry to progressively release its phosphorus.

Phosphate rock + sulphuric acid -> more readily soluble phosphorus before application

Reactive phosphate rock + suitable soil conditions -> progressive phosphorus release in the soil

Factors including the natural reactivity of the rock, particle size, soil pH, rainfall, soil calcium and other soil conditions can influence how quickly RPR dissolves.

Calling a phosphate material "natural", "direct applied" or "RPR" does not automatically mean it is low in cadmium.

Again, it comes back to the geological deposit.

Single superphosphate is commonly manufactured by reacting phosphate rock with sulphuric acid.

The reason is straightforward.

Phosphorus contained within the original phosphate mineral can be relatively insoluble.

Acidulation breaks down that mineral structure and converts more of the phosphorus into forms that dissolve readily and become available to plants.

That technology has made an enormous contribution to agricultural productivity.

But it raises another question:

Sulphuric acid doesn't create cadmium.

But breaking down the phosphate mineral can change the chemical environment and availability of cadmium already present.

Research indicates that, under some conditions, this can affect plant uptake.

International Fertilizer Development Center research compared untreated phosphate rock with partially and fully acidulated forms of phosphate rock.

In greenhouse research using upland rice on acidic soils, researchers found that cadmium uptake increased as the degree of acidulation increased.

The broad progression was:

with increasing cadmium uptake under those particular experimental conditions.

A more recent review covering decades of IFDC phosphate-rock research also discusses this finding.

This does not mean sulphuric acid creates cadmium.

It also does not mean every superphosphate will result in greater cadmium uptake than every RPR or directly applied phosphate.

Soil pH, phosphate source, plant species, organic matter, mineralogy and numerous other factors influence cadmium behaviour.

There are really two different questions.

How much cadmium is present?

And:

How available is it to plants?

Those are not necessarily the same thing.

A lower total cadmium concentration is clearly desirable if the objective is to minimise how much cadmium is being introduced to a soil.

But actual plant uptake can be influenced by soil pH, organic matter, mineral composition, crop or pasture species, chloride concentrations, existing soil cadmium and the chemical form in which cadmium occurs.

This is why we don't think the discussion should become:

That would be far too simplistic.

A low-cadmium superphosphate could contain substantially less cadmium than a high-cadmium RPR.

New Zealand's fertiliser industry currently operates a voluntary maximum of 280 mg cadmium per kg of elemental phosphorus in phosphate fertilisers.

That is a maximum industry benchmark, rather than a statement that every phosphate fertiliser contains anywhere near that amount.

Modern New Zealand fertiliser cadmium concentrations have also been reduced substantially compared with the historical Nauru era.

That is important context.

Today's phosphate fertilisers should not be represented as though they are chemically identical to phosphate products manufactured from historical high-cadmium Nauru rock.

But another useful comparison comes from Europe.

Under the European Union's Fertilising Products Regulation, relevant phosphate fertilisers are subject to a cadmium maximum of:

The EU also allows qualifying fertiliser containing no more than:

to carry a "low cadmium" statement.

There is an important difference in the units used.

New Zealand expresses its voluntary industry limit as mg cadmium per kg of elemental phosphorus, or P.

Europe expresses its limit as mg cadmium per kg of phosphorus pentoxide, or P2O5.

Those numbers therefore shouldn't be compared as simply '280 versus 60'.

When New Zealand's 280 mg Cd/kg P voluntary ceiling is converted onto approximately the same P2O5 basis, it equates to around 122 mg Cd/kg P2O5, compared with the European Union limit of 60 mg Cd/kg P2O5.

Cadmium benchmark comparison

Benchmark	Cadmium limit
EU limit	60 mg Cd/kg P2O5
EU "low cadmium" statement	<= 20 mg Cd/kg P2O5
NZ voluntary industry ceiling	280 mg Cd/kg P
NZ ceiling converted to P2O5 basis	approx. 122 mg Cd/kg P2O5

So, when compared on approximately the same phosphorus basis, New Zealand's current voluntary industry ceiling is roughly twice the EU limit.

The two regulatory systems aren't identical, and this comparison should be understood in that context.

But the difference in the benchmark is significant.

At Bio Charge, our position is straightforward.

That is Bio Charge's position.

It is not a claim that a phosphate fertiliser beneath the New Zealand limit is unsafe, unsuitable or non-compliant.

There is an important difference between a maximum and a target.

We don't believe the objective should be:

Our question is:

For cadmium and other unwanted trace metals, our long-term goal is as close to non-detect as appropriate quantitative analytical testing can reliably demonstrate, wherever practically achievable.

A maximum is a ceiling.

Bio Charge DAPR has undergone X-ray fluorescence (XRF) analysis.

XRF provides information about the elemental composition of the material tested.

We regard that as useful analytical information.

But we are equally clear about what that result does - and does not - establish.

XRF can measure major and trace elements, but its sensitivity depends on the instrument, calibration, sample preparation and material being tested.

A cadmium result must be interpreted alongside the laboratory's method, units and reporting limit.

We do not treat an element omitted from a report, or a result below a reporting limit, as proof that DAPR contains zero cadmium.

That would go further than the test supports.

Cadmium is naturally occurring.

Phosphate rock is a natural geological material.

So it would not be responsible for Bio Charge to promise that cadmium will never be measurable in every sample of naturally occurring phosphate.

And that isn't what we mean when we talk about our benchmark.

Our objective is to undertake appropriate quantitative trace-element analysis and work towards results where unwanted trace metals are below the reporting limit of an appropriate analytical method wherever practically achievable.

That means using quantitative analysis with a reporting limit suitable for cadmium in phosphate materials and commissioning more sensitive testing where needed.

Our approach is simple:

• Test properly.
• Report the actual result.
• Keep unwanted elements as low as practically achievable.

If future quantitative laboratory testing detects naturally occurring cadmium at a very low concentration, we believe the right approach is to report that result for what it is.

Transparency doesn't mean pretending natural geology contains nothing.

Cadmium isn't the only trace element that can occur in geological materials.

Depending on the phosphate source, other elements may also be present.

European fertiliser rules establish limits for a number of contaminants, including cadmium, lead, mercury, arsenic and hexavalent chromium, or Cr VI.

It is important to distinguish hexavalent chromium from total chromium. They are not the same measurement.

It is also important not to suggest chromium has the same established history in New Zealand agricultural soils as cadmium.

The evidence associating long-term phosphate fertiliser use with cadmium accumulation in New Zealand soils is considerably stronger.

Our approach isn't to turn every detectable element into a scare story.

For a visual introduction to the wider international discussion around phosphate fertilisers and cadmium, we have included the Safer Phosphates - Heavy Metals and Cadmium video.

Watch: Safer Phosphates - Heavy Metals and Cadmium

Safer Phosphates is an advocacy organisation rather than a New Zealand regulator or independent research institution. We therefore regard the video as useful background material rather than the primary evidence supporting the New Zealand-specific claims in this article.

Those claims are instead based principally on information and research from organisations including the Ministry for Primary Industries, Massey University, the Fertiliser Quality Council, the Fertiliser Association of New Zealand and published international research.

Bio Charge DAPR is a New Zealand-sourced direct-application phosphate material.

It provides approximately 6% phosphorus, 13% calcium and around 1% potassium.

It is applied directly rather than first being completely acidulated with sulphuric acid to manufacture conventional single superphosphate.

That is a factual difference in the way the materials are processed.

It does not, on its own, prove that Bio Charge DAPR contains less cadmium than every superphosphate or RPR product.

Nor does it establish that one fertiliser will always result in lower plant cadmium uptake than another.

Those claims require data.

We want to know what is in the phosphate we're supplying.

We want our testing to become more detailed rather than simply relying on our current XRF analysis.

And we don't view the current New Zealand industry maximum as the number to aim underneath.

Phosphorus remains essential to New Zealand agriculture.

Superphosphate has made an enormous contribution to this country's agricultural productivity.

RPR also has an established role where the phosphate source, soil and climatic conditions are appropriate.

This isn't about declaring one form of phosphate universally good and another bad.

It's about recognising something much simpler:

New Zealand's experience with Nauru demonstrates that particularly well.

Nauru was a valuable, high-grade phosphate resource.

Unfortunately, it was also naturally high in cadmium.

Today we have better science, better analytical tools and considerably more understanding of long-term soil accumulation.

So we believe it is reasonable to ask better questions:

• Where did it come from?
• What else came with it?
• How has it been processed?
• What does the laboratory analysis actually show?
• What benchmark should we be aiming for?

At Bio Charge, our position is simple:

For readers who want to look further into the science and New Zealand context, these are the main resources behind this article.

Ministry for Primary Industries - Monitoring cadmium in New Zealand soils - Covers phosphate fertiliser, cadmium accumulation and New Zealand soil monitoring. Open source

Ministry for Primary Industries - Cadmium in New Zealand Agriculture - Detailed historical information covering Nauru phosphate, cadmium concentrations and New Zealand's cadmium-management response. Open source

Ministry for Primary Industries - Managing cadmium in grazing farm systems - Practical information on phosphate fertilisers and managing long-term cadmium inputs. Open source

Fertiliser Association of New Zealand - Managing contaminants - Industry information covering phosphate rock, cadmium, historical sourcing and New Zealand's voluntary fertiliser limits. Open source

Fertiliser Quality Council - Monitoring cadmium levels in New Zealand phosphatic fertilisers - Ongoing New Zealand phosphate fertiliser monitoring information. Open source

Massey University / Fertiliser and Lime Research Centre - Phosphate Rocks and Phosphorus Fertilisers - Information on RPR reactivity, citric solubility and phosphate-rock characteristics. Open source

European Union - Regulation (EU) 2019/1009 on fertilising products - Current EU regulatory source for cadmium and other contaminant limits in fertilising products. Open source

International Fertilizer Development Center / Plant and Soil research - Review covering phosphate-rock reactivity, acidulation and cadmium uptake research. Open source

US Geological Survey - X-ray fluorescence information - Background on elemental analysis using X-ray fluorescence (XRF). Open source

Safer Phosphates - Heavy Metals and Cadmium - International background video used for general education in this article. Open source

Educational note: Phosphate-rock composition, fertiliser performance, cadmium concentration and nutrient availability vary with geological source, manufacturing process, application rate, soil characteristics and farming system. References to fertiliser categories describe general scientific principles and research findings and should not be interpreted as claims regarding any particular manufacturer or individual commercial fertiliser product.