For the better part of two years now, Western Growers has been working with other interested parties to learn how naturally-occurring cadmium might be taken up by fresh produce items, understand the implications to public health and to develop a methodology to proactively address the issue of cadmium in the diet. We are particularly interested in cadmium in spinach, a commodity known to accumulate and concentrate this chemical element.
Through industry and government sampling programs, we now know that among dark leafy greens such as romaine, kale and spinach, only spinach seems to be of concern. The mean cadmium concentrations in spinach are above the levels seen in the U.S. Dietary Survey, indicating that California product exposes the public to higher levels than spinach produced in other areas. Since California is the major supplier of spinach, the industry here has banded together to ensure that growers and handlers possess the most current information to promote both consumer and regulatory confidence in spinach safety.
This effort is centered on “Healthy Soils” (HIS) which was formed in early 2014 to provide funding and leadership to address any potential adverse public health impacts of naturally-occurring cadmium in spinach. HIS is a coalition of trade organizations led by Western Growers and the Grower Shipper Association of Central California, along with numerous concerned spinach growers and handlers that came together to contribute time and money. The goals of the group are to: 1) conduct an industry representative sampling program to better understand the prevalence and range of concentration of naturally-occurring cadmium in spinach; 2) research agronomic and managerial practices that might reduce those concentrations where warranted; 3) educate growers and handlers; 4) equip the industry to respond to inquiries about this issue from regulators and buyers.
Here is some of what we know so far:
For cadmium in humans, the adverse effect occurs with chronic, life-long exposure to high levels of the element. Studies have shown that if a person is exposed to high enough levels of dietary cadmium for a very long period, a person’s kidneys can be impacted. Cadmium builds up in humans very slowly. It takes from 25 to 40 years of a high cadmium diet to get levels to plateau in humans, and 40 years of low cadmium diet for these levels to go back down. The kidney impacts that are of concern take decades of dietary exposure to humans before they would be observed. The safe levels of dietary intake for cadmium, set by authorities such as the World Heath Organization and the Environmental Protection Agency, are for long periods of exposure (at least a month of high level exposures for both authorities though the effects take much longer periods of exposure). Shorter-span exposures (such as the amount of cadmium in the food ingested during a single meal) are not even a subject of discussions in the documents produced by these authorities.
A single, short “excursion” from the normal levels of cadmium in leafy greens will not have any effect on human health, will not appreciably change the level of cadmium within the consumer, and will not in any way adversely impact those exposed. Kidneys will not be impacted by a short-term “spike” due to dietary exposure from leafy greens. If a single lot batch of a leafy green harvest has elevated levels of cadmium, a person may consume a few servings of the high-level greens over a week. Several elevated meals with four-fold higher “excursion” of cadmium from normal levels is not going to have any significant impact on the cadmium levels within the consumer and certainly will have no impact on the kidney resulting from chronic, life-time exposures to high levels of cadmium. If dietary cadmium caused short-term health effects, such as nausea, then short-term exposures would be a concern. But cadmium does not.
But that does not mean that as an industry we do not need to work diligently to reduce cadmium levels in the diet. So what can you do?
The first and perhaps most important thing to understand is whether your soils present a risk for cadmium uptake. Select soils in California, including soils in parts of the Salinas Valley, have naturally elevated levels of cadmium (Cd); most California soils have less than 0.5 PPM total Cd but select soils have been measured at up to 9.0 ppm. The second aspect to understand is the crop that is being contemplated for production in each block. A survey of vegetable fields showed that crop Cd uptake is roughly proportional to soil Cd content, but crop differences in Cd accumulation are substantial. Spinach accumulates two to three times the amount of Cd as romaine on a fresh weight basis, and broccoli accumulates even less than romaine. While soil Cd level is the primary determinate of crop Cd uptake, other factors such as zinc to cadmium ratio, soil amendments, pH and chloride in water and soil can affect Cd uptake as well.
A key factor is zinc fertilization. Zinc (Zn) and Cd are closely related ions. Zinc is an essential plant nutrient, but Cd is not. Plants are unable to distinguish well between these ions, and substantial uptake of Cd can occur if a high level of Cd is present in soil. Increasing the ratio of soil-available Zn to Cd suppresses Cd uptake. However, to significantly decrease Cd uptake, much higher levels of Zn application are needed than would typically be used to remedy a Zn deficiency. Across numerous field and pot trials, we have observed that applying 25-50 PPM elemental Zn on a soil dry weight basis (equivalent to 100-200 pound elemental Zn per acre foot of soil) suppressed Cd uptake by more than 40-60 percent. In addition, the effectiveness of any Zn application is affected by factors such as the form of zinc and how it is incorporated into the soil.
Other soil amendments such as compost, biochar, and humic acid have also been evaluated. It is believed that the high cation exchange capacity of these materials may sequester heavy metal ions such as Cd, making them less plant-available. To date, the results are mixed with a field trial combining zinc sulfate with compost application resulting in greater reduction in Cd uptake in spinach while a pot trial with added biochar suppressing Cd uptake in one soil but not in another. Humic acid materials have not reduced Cd uptake.
pH also plays a role with cadmium uptake as cadmium is more plant available at lower soil pH. Liming is therefore theorized to decrease crop Cd uptake, but experiments in the Salinas Valley have shown decreased uptake in some trials and no effect in others.
Chloride (Cl) in irrigation water, and from fertilizers, can increase crop Cd uptake. In a pot study, researchers observed that the uptake of Cd by spinach increased with increasing Cl in irrigation water, with Cl concentration above 100 PPM (approximately 3 meq/liter) causing the largest increase. This suggests that fertilizers containing Cl should be avoided in fields where Cd uptake may be a problem, and use of low Cl irrigation water, if available, would be ideal for spinach production in such fields.
Western Growers is working to advance an industry program that will engage spinach growers in the review of risk and implementation of best management practices. If we can solidify this program, it will demonstrate that industry is working together to limit cadmium uptake in foods. We encourage you to engage with us in this endeavor and to reach out to learn more and support the development of this program.