I recently spoke with a San Francisco resident that wants a veggie garden. Their plot is against the front of their house. They tested their soil for lead, found medium levels and removed nearly twelve inches of soil and retested. They found in the lower layer even higher levels of lead. They wanted to know what their options are for planting (if impatient, scroll to bottom for suggestions).

I did some looking around and found one approach is to use a phosphate spread in the affected soil. The suggestion comes from Microbiologist, Dr. Sam Fogel of Bioremediation Consulting Inc. He was initially quoted in a Boston Globe article on the subject where were he explained that Phosphorus binds with Lead and reduces Lead mobility and its biological availability. I went in search of academic findings to support this idea and did not find an overwhelming amount of research. However, there were a couple interesting articles. One is in Soil Science, “Lead Phosphorus Interactions in Equilibrated Soil Suspensions” by M. T. Aide and D. Huff. However, I found it hard to compare their experimental study with the true conditions of soil in a yard but they did report that “Phosphorus amendments limited the appearance of the exchangeable Lead.”

Another Article from the Journal of Environmental Quality, is “Chemical Immobilization of Lead, Zinc, and Cadmium in Smelter-Contaminated Soils Using Biosolids and Rock Phosphate” (catching title) by N. T. Basta, R. Gradwohl, K. L. Snethen, and J. L. Schroder, reported:

“Rock phosphate was the only treatment that reduced human gastrointestinal available Lead in both gastric and intestinal solutions, 23 and 92%, respectively. Alkaline organic treatments (lime stabilized biosolid, n-viro soil) decreases Cadmium transmission through the food chain pathway, whereas rock phosphate decreases risk from exposure to Lead via the soil ingestion pathway.”

I then contacted Dr. Fogel to follow up on what his own understanding of Phosphorus binding with Lead and the suggested quantity to use per square foot. Dr. Fogel informed me that he did not yet know the exact quantity to use per square foot and that I would have to determine the appropriate amount to use through experimentation (in partnership with my state agricultural school no less). After providing a very simple but decent test methodology he suggested using the highest level phosphorus spread (called Triple Superphosphate) and using a quantity according to agricultural recommendations. He used this method himself and found Lead levels 100 times less in his own garden plot.

I did find a critique of Triple Superphosphate in that phosphate binds to other minerals besides Lead, rendering them unavailable to plants. These other minerals included zinc, iron and manganese, which are needed for plant health. However, this is not an issue in this case as the garden will not be planted directly in the affected soil. If using regular rock phosphate (rather than triple super), you would use a greater quantity.

Quickly, to understand the difference
To make 0-20-0, rock phosphate is treated with sulfuric acid to make calcium phosphate (0-20-0) and calcium sulphate (gypsum).

To make 0-46-0, rock phosphate is treated with phosphoric acid. With this, much higher phosphate content, much higher N-P-K fertilizer formulas can be made. Less needs to be used.

For applying rock phosphate (0-20-0):
Pacific Calcium Inc., which carries organic soil amendments, suggests 500 to 2000 pounds of Montana Natural Rock Phosphate per acre or 10-25 pounds per 1000 square feet, or .01 lb-.025 lb per square foot.

Triple Superphosphate (0-45-0) one manufacturer, Simplot, suggests an application of 4 pounds per 1000 square feet, or .004 lb per square foot.

Returning the point of this blog, which what could help make the lead contaminated plot usable, here is what I suggested.

• - Mix a phosphate spread into a 6 to 8 inch layer of the exposed dirt (I’d use the triple super).
• - Replace the 12 inches of excavated soil with fresh clean soil.
• - Build 3 foot raised beds for planting (which should be filled with an appropriate organic soil)
• - Place either landscape fabric, or cardboard, and thickly mulch the exposed dirt path between the raised beds.
• - If this does not abate their concerns enough, they can also plant fruiting crops such as tomatoes, squash, peas, and corn because (they take up relatively little Lead) and avoid planting root vegetables, potatoes, and leafy vegetables, which take up more. I personally do not feel this last measure is necessary.

There remain some issues with this approach. One is that to determine the decreased level of Lead present, you’d have to continue to test. Another is that if the source of Lead is coming from old layers of paint or old pipes from the abutting house, Lead will continue to seep into the soil. Since little is yet known about the exact quantity and frequency of phosphate treatment required to keep Lead levels under control for planting, it is hard to guarantee effectiveness over extended years. However, I would imagine that for quite some time to come the raised beds would be safe to use (annual testing would help determine this).