Crop nutrition and heavy metals

Manufactured fertilizers, animal manures, biosolids and recycled industrial wastes may all contain heavy metal impurities. These heavy metals may accumulate in the soil with repeated applications of crop nutrients and thus increase the background levels of heavy metals in soils. Atmospheric deposition is also a significant source of heavy metals in soils. Because they are elements, heavy metals are not subject to chemical decomposition.

Cadmium is the metal of greatest concern because of potential adverse effects on the kidneys, bones. However, there is no conclusive evidence to date of adverse effects on human health from the use of phosphate fertilizers. This has been acknowledged in risk assessments carried out by the Organisation for Economic Co-operation and Development (OECD) and the Scientific Committee on Problems of the Environment (SCOPE).

Efforts to manage the presence of heavy metals in crop nutrients generally concern arsenic (As), chromium (Cr), lead (Pb), mercury (Hg), nickel (Ni), copper (Cu), zinc (Zn), although other metals are also addressed by certain regulations.

Arsenic-containing sources of crop nutrients are generally recycled wastes or chicken manure. Arsenic is known to be carcinogenic and toxic. However, little direct ingestion comes from plant materials.

Some forms of chromium are highly carcinogenic and are found in industrial wastes and sewage sludge. Exposure is generally due to dusts and soil sediments rather than through uptake by crops.

Lead can be found in wood products recycled as sources of crop nutrients. It can follow calcium and iron pathways in our bodies when either or those nutrients are lacking.

Although mercury can be found in recycled nutrients, it is not generally found in edible parts of plants.

There is little direct evidence of toxicity related to the ingestion of nickel. In fact, it is a vital micronutrient for some organisms.

Copper is also an essential trace element.

Zinc can be toxic to plants in excess, but it is needed in small amounts and is also critical for the functioning of the human immune system. Dietary deficiencies are related to a number of health problems.

Managing heavy metals in soils and agriculture

There are several options for minimizing the accumulation of heavy metals in soils over time. The first is to limit the addition of heavy metals. The presence of these metals is regulated either as a ratio of the target nutrient (phosphate or a defined micronutrient) or with regard to total application over a specified period of time by a number of countries (and states and provinces).

Another option is to minimize the transfer of heavy metals from the soil into the food chain through appropriate agricultural management practices. These include controlling soil salinity, acidity and preventing iron and calcium deficiencies in the cultivated crops. Another option is to grow crops that either do not show a propensity for taking up heavy metals or ones that store the metals in non-edible parts of the plant. Such characteristics can be enhanced through plant breeding.

Finally, plants that do absorb heavy metals can be used to remediate contaminated soils, but it is important that the plants then be disposed of in ways that ensure the heavy metals will not enter the food chain or otherwise pose a threat to human health.