Cadmium in Surface Soils
116 56 116
< 2mm fraction
0.6 0.5 0.4 0.3 0.2 0.1
56 94 49
Cadmium may be present in Manitoba’s soil, but plant-breeding research and other strategies are keeping it out of our food.
Soft, bluish white, the heavy metal cadmium is a known toxin and carcinogen that is typically found paired with zinc ores. It is rare, but it occurs naturally in trace amounts in all soils, and is commonly used in batteries and plastics.
Begun in 1994, a plant-breeding program aimed at producing low-cadmium durum wheat cultivars has been “extremely successful,” according to Cynthia Grant, a soil scientist at AAFC Brandon who has done extensive research on the issue of cadmium contamination in crops.
“It was a poster-child program for successful breeding in the world, actually,” she said, adding that Manitoba scientists played a major role in it.
The accumulation of cadmium and its distribution within the plant is affected by genetics, as well as the environment, as well as the soil characteristics and management practices that are put in place.
Canada’s grain-handling system also helps out. By streaming harvests from countless farms across the Prairies, grain from farms with high levels of cadmium are diluted to within acceptable levels by the time it is loaded onto ships for export.
In other good news for farmers on soils with elevated cadmium levels, Codex Alimentarius, the international agreement on food safety, recently raised the proposed acceptable level of cadmium contamination in food from 0.1 parts per million to 0.2 ppm.
“Combined with the increase in the limit and the genetic manipulation that was done, the majority of our soils are now producing durum wheat that is below or meets that 0.2 limit,” said Grant.
“There are still some areas that will produce high levels, but the blending process that we have now will see the durum wheat meeting the limits.”
After durum, the other two crops with the greatest tendency to accumulate cadmium from the soil are flax and sunflowers. Flax isn’t a staple crop in Manitoba, and most of it goes to industrial uses, and sunflowers aren’t a staple food like wheat.
“If we do have flax that goes into a health food market, we want to be careful that we are sourcing it from an area that doesn’t produce high cadmium,” said Grant. “Flax, just by its nature, tends to accumulate higher levels.”
The real concern with cadmium contamination is with rice, “a perfect storm” crop which both accumulates the heavy metal easily and is also low in zinc and iron, two minerals that restrict uptake of the contaminant in the gut.
The textbook case for cadmium poisoning involved a community of subsistence farmers in Japan who had been using heavily contaminated irrigation water downstream from a zinc smelter.
Since they were poor and consumed mainly their own rice and little else, the heavy metal accumulations over decades damaged their kidneys and other organs.
The impact was most severe in a rare form of extreme osteoporosis. Because cadmium in their diet over time replaced the calcium, their bones became so weak and brittle that they could not support their own body weight.
The condition mainly affecting the elderly due to extreme, long-term cadmium poisoning was named “Itai-Itai,” which in Japanese means “Ouch-Ouch” disease.
“So this 100-mile diet is a great idea in some respects, but in terms of reducing trace element excesses or deficiencies, it’s not a great idea,” Grant said.
That also means Canadian farmers who wish to market their produce based on their geographical location would be well advised to make sure their soil is low in cadmium or other heavy metals.
Grant added that it would be extremely difficult to consume enough flax to develop adverse health effects from cadmium, and the benefits from omega-3 fatty acids are optimized at low doses anyway.
SOIL LEVELS VARY
According to maps from the Geological Survey of Canada, in Western Canada, soil cadmium levels vary widely, and largely depend on what kind of parent material the retreating glaciers deposited on the landscape.
Other factors such as chloride-based salinity, water table level, and soil type have a big effect on bioavailability.
In Manitoba, cadmium levels follow a wide band stretching diagonally across the province from west to east, beginning in Swan River and down to the Red River Valley, in a range from 0.3 ppm to as high as 0.5 ppm.
This corresponds to areas with predominantly clay-based soils that originated in shale deposits carved out by the glaciers about 10,000 years ago from farther north that were ground up and left behind as the ice sheets melted.
In the sandier soils on either side of this belt, including a small pocket in the southwest corner and the eastern side of the province, cadmium levels are typically low.
Cadmium can also build up in the soil over the long term via the use of phosphate fertilizers, particularly if it is made of phosphate rock from so-called “dirty” mines. These tend to be in sedimentary rock deposits, such as Nauru in the South Pacific, and a mine in the state of Idaho.
“If we are using higher-cadmium phosphate fertilizer, there is potential for accumulation,” she said. “It’s not a case of, ‘Oh my God, we’ve put on phosphate fertilizer and we’ve destroyed our soil.’ But it’s something we’re watching long term.”
Igneous sources, which include Canada’s only phosphorus mine in Kapuskasing, are considered to be very “clean.” Existing regulations control allowable trace element levels in fertilizer.
There has in recent years been growing concern about dwindling global phosphorus reserves. In light of the fact that there are clean and dirty mines, there is concern in some quarters that supplies of “clean” P fertilizer may play out much faster than originally predicted. [email protected]