how to manage high soil arsenic?
Our residential soil here in Sacramento, California, tested at 70 mg/l for arsenic; test lab says regulatory limit is 41 mg/l. I understand the biggest concerns with growing food in this soil would be arsenic uptake in root crops and arsenic contamination of above-ground plant parts from bits of soil that might land there. So far I understand our options may include:
--avoid root crops and carefully wash above-ground row crops
--don't use compost made from waste of plants grown in this soil
--add iron to the soil to bind the arsenic
--keep soil pH low (and avoid adding phosphorus) to make arsenic less available for plant uptake
--raise soil pH (and add phosphorus), then grow crops and discard them to remove arsenic from the site
--plant only in raised beds with imported clean soil
--replace all the soil (not an option we really want to consider).
What do you think of these options?
Do you have others to suggest?
We would like to plant fruit trees and grapes - would that produce be safe to eat? - any special precautions necessary?
Thank you - Paul
Accumulation of arsenic in the plants is generally low so plants seldom accumulate arsenic in levels hazardous to humans. The arsenic is also toxic to plants and you may see some plants perform poorly in this soil. The main human health risk is from arsenic in the water or eating soil particles adhering to the plant parts. Washing and/or peeling removed soil particles that adhere to the plant surface. Fruit from trees or vines should not pose much risk if the plants do not have a problem growing in the soil. Some fruit trees are sensitive to arsenic and you may see arsenic toxicity. Peaches are very sensitive, http://ucanr.edu/sites/fruitreport/Nutrition_-_Fertilization/Individual_Nutrients/Arsenic/
Cherries and other stone fruit may also be sensitive. I could not find anything on grapes. My feeling is that if your trees and vines grow well the fruit or juice would be safe to consume. If that fruit was the only thing you ate I would worry about long term risk but not from eating the fruit sometimes during the year.
Now about Arsenic in the soil. While arsenic was widely used as a pesticide to kill insects and weeds before the invention of synthetic pesticides after WWII, most of the arsenic in California soils is naturally occurring. Arsenic behaves similarly to phosphorus in the soil because they form similar ions. Arsenic is generally bound to clay particles in the soil and not very soluble in the soil solution. Soils with high clay contents can hold more arsenic and can absorb more arsenic from the water and are held fairly strongly by the clay particles. Lowering the soil pH will make the arsenate ion more soluble. Additions of phosophate will replace some of the arsenate on the clay complex, but adding enough phosphate to replace most of the arsenate and flush it from the soil would probably cause problems from too much phosphorus in the soil. It would also depend on the ion content of the water you were using to flush the soil and many other nutrients would be lost. I would only add phosphorus if a soil test indicated that it was needed in your soils, but do not be afraid to use complete fertilizers. I would not be afraid to try growing fruit trees or grapes in these soils. If I wanted to grow root vegetables such as radishes I would use raised beds and bring in some soil.You will learn a lot as you try to grow plants in this soil and the addition of some soil amendments, such as composts would probably help.
Mark L., thank you so much! You are a godsend. We are greatly indebted to you.
Sorry that took so long
not a problem! Paul
Mark, We planted an apricot tree in the soil that tested at 70 mg/l arsenic. It has grown well and fruited already. In an abundance of caution, we recently had the leaves tested for arsenic; the results came back 0.076 mg/kg (76 ppB). I'm getting the impression that is fairly low, which is good news. Some questions though: --Do you agree that is low? --Is it reasonable to assume that concentration in leaves roughly corresponds to concentration in fruit? Our test lab would only test leaves. I understand FDA allows 0.5 ppm to 2.0 ppm (500 to 2000 ppB) arsenic in certain foods, well above 76 ppB. For infant rice cereal, FDA proposed an "action level" of 100 ppB of inorganic arsenic, still above 76 ppB. Thanks so much for any insights you may be able to offer.
Well, I think that is pretty good. Yes, I think that is low. It is a thousand fold reduction. I see that apricot is sensitive to arsenic so the fact it does not show toxicity symptoms is encouraging. I can't find much about arsenic transport in plants to differentiate between fruit and leaves. I did find one source which indicated the general order of accumulation was roots>shoots>leaves>fruit. This means the fruit levels should be lower than the leaves.
Thank you SO much, Mark!