Managing contaminants

Fertilisers are essential for viable, economic production in agricultural and horticultural farms. Phosphate fertilisers are derived from phosphate rock, which contains trace levels of a range of elements. Some trace elements like zinc are essential for the health of animals and plants. Others are not. If they occur at excessive levels in the soils, they can have an adverse impact on the environment or human health.

Global and New Zealand research indicates that cadmium and fluorine are the elements in phosphate rock most likely to accumulate in agricultural soils over many years of phosphate fertiliser use. There are no indications for concern from the levels of contaminants currently in New Zealand soil. Nonetheless, we must actively manage phosphate fertiliser use and monitor soils to ensure that the risks from soil contaminants remain low over the long term.

Cadmium

Cadmium accumulation in agricultural soils is a consideration world-wide. The New Zealand fertiliser industry has been proactive in setting industry-initiated controls for fertiliser and soil cadmium. It also undertakes monitoring and research to ensure that the risk is both understood and properly managed. As farmer owned co-operatives, it is in the industries’ best interest to manage soil cadmium well, to ensure levels don’t compromise future land-use options.

Sources of cadmium

Cadmium occurs naturally at low levels in the earth’s crust, atmosphere and water. It is found in the rock used to make phosphate fertilisers. There is no commercially viable way to remove it from superphosphate, so what occurs in the rock is retained in the fertiliser. However, higher analysis products like DAP or Triple superphosphate tend to have lower cadmium concentrations per kilogram of phosphorus, than superphosphate.

• Plants take up cadmium from the soil. This means it can enter the human food chain.
• Cadmium does not tend to accumulate in milk or meat to any appreciable amount. It accumulates primarily in the kidneys and to a lesser extent the liver of animals. To avoid the risk of cadmium entering the food chain, the sale of offal from ruminant animals older than 30 months is prohibited in New Zealand.
• Naturally high levels of cadmium are also found in some shellfish and crustaceans.
• Smoking cigarettes is a major route for exposure to cadmium.

Effects of cadmium

In the case of ongoing, low-level exposure to cadmium through food products for animals and humans, most of the cadmium is not absorbed and therefore passes through. What is absorbed by the body tends to accumulate in the kidneys. If excessive amounts are ingested over a lifetime, there is a risk of kidney damage.

Food Standards

For staple food products, food standards provide for safe levels of cadmium based on regular ingestion of the food products over a lifetime. The World Health Organisation also recommends levels of exposure which are safe over a lifetime’s intake at those levels.

The amount of cadmium in the typical diet of New Zealanders is monitored by the Ministry for Primary Industries (MPI) using the Total Diet Study. Results show that the levels of cadmium remain well within World Health Organization guidelines – and well below levels that affect human health.

Fertiliser monitoring

To limit the accumulation of cadmium in soil, the New Zealand fertiliser industry has set maximum limits for the cadmium content in phosphate fertilisers (280 mg Cd /kgP). Levels in phosphate fertilisers are routinely audited by the New Zealand Fertiliser Quality Council. The weighted average cadmium level in phosphate fertilisers between 2003 and 2015 was reported as 184 mg/kg P – well below the agreed industry limit.

Soil and water monitoring

Members of New Zealand’s Cadmium Management Group monitor cadmium levels in agricultural soils in New Zealand, as well as the concentrations in surface water and estuaries. Across all soil samples, the median soil cadmium value was found to be highest on dairy land.

Farms with higher soil cadmium levels were mainly based in Waikato and Taranaki. This can be attributed to their long history of phosphate fertiliser use, which the pasture needs to thrive in this region. Before 1996, the industry predominantly used Nauru rock phosphate for the manufacture of superphosphate, which had much higher cadmium levels than rock phosphate used subsequently. It is believed soil cadmium levels in these regions are largely a legacy of this early period, in combination with these soils generally having a high demand for phosphate fertiliser.

Regional council data shows cadmium levels in surface water, groundwater and sediments are below accepted guideline values and unlikely to have adverse environment effects. Continued monitoring is essential to ensure it stays that way.

Cadmium Management Strategy

The New Zealand fertiliser industry is an active member of the Cadmium Management Group, facilitated by the Ministry for Primary Industries. The group comprises representatives from Ministry for the Environment and Ministry for Primary Industries (including Food Safety Authority), representatives from regional councils, and the primary industry sectors. The Group is collectively responsible for New Zealand’s Cadmium Management Strategy, which aims to ensure that cadmium in rural production areas poses minimal risks to health, trade, land use flexibility and the environment over the next 100 years, at least. The Group has adopted a Tiered Fertiliser Management System for managing the rate of soil cadmium accumulation and ensuring that soil cadmium levels remain within acceptable limits over the long term. This system provides an important role in the soil monitoring, fertiliser management and education aspects of the overall Cadmium Management Strategy.

More information on cadmium management is on the MPI website, or see our Research page for New Zealand research.

A review of the state of knowledge of cadmium in New Zealand agricultural systems can be found here.

Fluorine

Fluorine is a common element in the earth’s crust and is found in phosphate rock. It accumulates in soils with fertiliser application. There is no indication of risk to livestock, human health or the environment from current levels of soil fluorine.

Excessive fluoride intake by livestock can lead to a condition called ‘fluorosis’ which can in rare situations lead to animal mortality. It is recommended to prevent animals from grazing paddocks immediately after phosphate fertilisers have been applied, and to prevent them from accessing areas where phosphate fertiliser is stored.

Fluorine has not received much consideration internationally. Research is being undertaken by the New Zealand fertiliser industry to ascertain current levels in New Zealand soils and to understand any potential impact on soil health and function.

Read about the latest PhD research on fluorine, supported by the Fertiliser Association of New Zealand.