Cadmium – it's complicated
Cadmium in agricultural soils is an emotive topic about which a lot of confusing statements are made, Greg Sneath, executive manager of the New Zealand Fertiliser Association told Onions NZ’s Conference.
He is also participates in the Cadmium Management Group where Horticulture New Zealand is represented through the Vegetable Research and Innovation Board.
Cadmium is an element which behaves like zinc and occurs naturally at trace levels in soils. It is also found at a range of different concentrations in phosphate rock, from which phosphate fertiliser is manufactured. So, over many years of repeat application, it can very gradually accumulate in the top layer of soil, he said.
“What’s in the parent rock is what you get in phosphate fertiliser made from it.”
There is no feasible mechanism for removing it from super-phosphate but in products like diammonium sulphate (DAP) with a manufacturing process using phosphoric acid, again depending on the cadmium concentration in parent material, there is generally a slightly lower level.
For many years before the 1990s, Nauru phosphate rock was New Zealand’s primary source of rock for manufacturing phosphate fertiliser. It was relatively high in cadmium. However, levels in fertiliser are now reduced through sourcing and blending phosphate rock from a range of other sources, which contain less cadmium.
Cadmium in soil can be taken up by plants and thereby enter the food chain at trace levels. Absorbed cadmium can accumulate in the kidneys and liver and over a lifetime, can create a risk of adverse health effects.
“But food isn’t the only source of cadmium,” he said.
“Exposure is doubled through smoking; industrial exposure is also possible.”
For example welding rods can emit cadmium.
The Ministry for Primary Industries (MPI) Food Safety Authority monitors cadmium levels in the typical New Zealand diet, using the Total Diet Study. The results are well below the World Health Organisation’s (WHO) recommended monthly tolerable intake levels. The European Union limits are lower than those of the WHO, even though they are based on the same information. They make much more conservative assumptions, but typical dietary intake of New Zealanders is also well below the European recommend intake value. These values represent a safe level of intake every day over a lifetime, so an
occasional mild excedance in a food standard is of no consequence for an individual.
The Australia and New Zealand Food Standards provide limits for cadmium in vegetables, as consumed. Root and tuber vegetables have a limit of 0.1 milligrams of cadmium per kilogram fresh weight, but this does not apply to onions, which are classed as a bulb vegetable. This means the principle of “as low as reasonably achievable” applies.
When it comes to exports, onions have to meet individual country standards. For example, the European Union standards for vegetables, excluding leafy vegetables, fruit, herbs, stem and root vegetables and potatoes, are 0.05 milligrams of cadmium per kilogram of fresh weight.
Sneath said plant variety and species had quite an influence on cadmium levels, with leafy green vegetables typically showing greater uptake of soil cadmium, compared with other vegetables. Fruit shows the lowest. But different varieties for each crop can show considerable differences in uptake.
Soil cadmium accumulation is influenced by the length of time phosphate fertilisers have been applied as well as the cadmium content of fertiliser applied and the rate of application. Soils in the Waikato and Taranaki region had been found to have typically higher levels, due to their long history of phosphate fertiliser application and soils with a high phosphorus requirement.
Sneath said the Fertiliser Association worked with MPI, the Ministry for the Environment, regional councils, and primary sector groups in the Cadmium Management Group, which meets twice a year. It has implemented a Cadmium Management Strategy and >
provides a governance role, as well as monitoring progress and supporting research and education.
The fertiliser industry maintains a voluntary limit of no more than 280 milligrams cadmium per kilogram of phosphorus. It had developed a Tiered Fertiliser Management System, endorsed by the Cadmium Management Group to control accumulation based on soil cadmium levels. In Tier 0 where soil cadmium is within the range, which occurs for background levels, no restrictions apply, but at Tier 1 where levels are slightly elevated, from 0.6 to 1.0 milligram per kilogram of soil, some restrictions on the rate and type of fertiliser apply. At Tier 2, from 1.0-1.4mg/kg soil, and Tier 3, from 1.4-1.8 mg/kg soil, choice of fertiliser becomes more restricted. Then at Tier 4 with levels of 1.8 mg /kg soil or more, limits are imposed to stop any further accumulation unless a site specific study is carried out to identify the risks and pathways for potential harm.
The programme is discussed with farmers at field days, seminars, small groups and mostly a one-to-one approach, due to the complexity and sensitivities of the topic.
Mitigation options to reduce plant uptake are important. Onions NZ has contributed to recent research on plant uptake of cadmium with sampling carried out in Pukekohe, Matamata, Canterbury and Hawke’s Bay. Soil samples were taken where one variety of onions was grown, selecting five samples from each plot, from 10 different growers on 19 different sites.
Soil cadmium does not necessarily reflect the plant uptake. For example, Canterbury soils had lower cadmium with little variation, but there was one site at which onions showed increased uptake. This demonstrates that the relationship between soil cadmium and plant uptake is a complex one.
The same plant variety might show different uptake results in different areas. For example, Sneath said the same variety of onion grown in Pukekohe and Pukekawa showed different uptake levels, showing there were different factors at play. This was also observed with spinach grown in different regions.
“In these trials we’ve looked at soil carbon, and soil pH but the relationship isn’t always clear,” he said.
“We can’t fully explain differences in uptake with these characteristics.”
Sneath said there were a number of practices which growers could use to limit plant uptake of cadmium. These are presented in the Tiered Fertiliser Management System, such as monitoring their soils, using fertiliser with lower cadmium levels, avoiding fertiliser with a high level of chloride and growing varieties which take up lower levels of cadmium, if these are known.
“Soil pH is a key issue because cadmium is more available in low pH soils,” he said.
The behavior of cadmium was also modified by the level of zinc in the soil, as chemically they are similar in nature. Cadmium is generally more available in sandy soils and there is evidence that increasing soil organic matter might mitigate plant uptake.
“On their own each of these practices may not be as effective in reducing cadmium uptake as they are in combination.”
Growers should also make sure their soils were well up in the recommended pH range, maintain organic matter in the soil as high as possible, keep zinc at the high end of requirements to overcome any deficiency and buy fertiliser from a reputable source, especially any zinc supplements.
▴ Greg Sneath – lots more to learn about cadmium.