Category Archives: Sciencelike

Soil Lead Tests

We got our soil lead tests back this morning.  We tested the soil we grow in and the soil that surrounds our growing areas for lead.  We imported soil that was certified as clean, so we’ve known that it would be appropriate for growing, but we wanted to test it so that we could monitor the way the lead levels change over the seasons.  We hope to test it twice a year.  Here are our results:

Chicago Ave Garden:

Growing Areas – 23 ppm

Grass Areas – 1100 ppm

St Louis Garden:

Growing Areas – 27 ppm

Grass Areas – 450 ppm

According to Carl J Rosen from the University of Minnesota Extension, “Background concentrations of lead that occur naturally in surface agricultural soils in the United States average 10 parts per million (ppm) with a range of 7 to 20 ppm” and “Generally, it has been considered safe to use garden produce grown in soils with total lead levels less than 300 ppm”.  So our growing areas are very well within the range of safe levels of soil lead.  The grass areas around our gardens, however are too high (much too high at the Chicago Ave garden).

Rosen also noted that soil lead isn’t readily accumulated in the plants we eat, but that we risk ingesting lead by eating the soil itself:

“The most serious source of exposure to soil lead is through direct ingestion (eating) of contaminated soil or dust.In general, plants do not absorb or accumulate lead. However, in soils testing high in lead, it is possible for some lead to be taken up. Studies have shown that lead does not readily accumulate in the fruiting parts of vegetable and fruit crops (e.g., corn, beans, squash, tomatoes, strawberries, apples). Higher concentrations are more likely to be found in leafy vegetables (e.g., lettuce) and on the surface of root crops (e.g., carrots)… There is more concern about lead contamination from external lead on unwashed produce than from actual uptake by the plant itself. If your garden is close to busy streets or highways, remove outer leaves of leafy crops, peal all root crops, and thoroughly wash the remaining produce in water containing vinegar (1 percent) or soap (0.5 percent).”

At our farm, we do wash all of our produce, but after reading this, we are going to test out washing them with a vinegar solution to see if it doesn’t negatively impact the freshness and taste of our veggies.

We’ve also decided we are going to take some stronger measures to keep dust from blowing into our Chicago Ave growing beds.  We’re going to increase the woodchips border/buffer around the garden area in size and depth, and keep the vegetation around that border high to keep the dust down.

Today I also called about testing the grass clippings we have been collecting from mowing around the St. Louis garden.  We want to see if they will be safe to use as mulch on our beds.  We are also going to be testing our growing soil for pH and nutrients (Rosen also noted that: “Lead is relatively unavailable to plants when the soil pH is above [6.5]. If needed…. Lead is also less available when soil phosphorus [levels] are high”).  We’ll let you know how those go, once we get them back!