1 - Heavy metal contamination of composted sewage sludge should not be a problem in most of New Mexico, but the Waste Water Department can probably give you information to confirm the safety of their product.

Q.

I live on the east Mesa on Jade Ave. I got some composted sludge from the water department last fall and worked it into some raised beds with local soil. Some iris and herbs are doing well, but before I plant anything else edible I wondered if there is any danger from heavy metals that would not be composted out.

- Richard G.

Las Cruces

A.

I suspect there is not be enough industrial waste containing heavy metals to create a problem in your area, but there is always a chance. However, there should be little problem with heavy metals in compost in most of New Mexico. Gardeners with questions about potential problems with heavy metals or other substances in composted sludge can contact their waste water department and ask for information from their tests to see if there is anything of concern. The EPA requires testing to be sure that the treatment plant is not releasing harmful material after treatment. If there is a potential pollutant in the waste stream coming to the treatment plant, the waste water department probably tests the input as well as output to identify sources of contaminants. They will work with such sources to eliminate or reduce contaminants of concern.

Heavy metals will persist in the waste if they are there. Since they are non-organic materials they will not decompose during the waste treatment and composting process, they would actually concentrate somewhat. That is why the treatment plant would be testing their product. If there were materials of concern, they would probably not be allowed to release it to the public.

In general, heavy metals are not soluble in alkaline soils such as are found in most of New Mexico. Plants can only absorb minerals that are dissolved. As insoluble minerals, they cannot be absorbed into the plants, even if they are there. As gardeners acidify soils, they may increase the solubility of some heavy metals, if they are there. Finally, if heavy metals are present, even if not soluble, they could be on the exterior of root crops grown in the soil. Washing and peeling these crops should remove any potential heavy metal contaminants. Root crops, carrots, radishes, turnips, sweet potatoes, and tuber crops such as Irish potatoes can be grown in soil not amended with the composted sludge if the reports indicate the potential presence of heavy metals.

The report from the treatment plant will help you determine your best course of action. You will probably learn from the report that there is no cause for concern.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.

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