We recently had our soil tested by Hills Laboratories (www.hill-laboratories.com). We requested two tests: one called ‘basic soil’ and another called ‘heavy metals + mercury’ (with lead being the prime concern).
The ‘basic soil’ test was intended to serve as a guide for any further soil amendments and management. It offers ph, levels of calcium, magnesium, phosphorus, potassium, sodium, cation exchange capacity, and base saturation.
The ‘heavy metals + mercury’ test was requested because we are gardening between two high traffic roads in which lead may have accumulated from the days before unleaded fuel. We also sanded the window frames and eaves on our old brick house about ten years ago and suspected this may have been leaded paint. Other houses adjacent or near that may have been demolished or stripped could also have been a source of contamination.
Our soil lead level is 700 parts per million. Natural levels vary from about 10-20. Most sources I found recommend abandoning edible gardening above 200-400ppm. There is no known ‘safe’ level of lead exposure. So while I was not surprised, having it confirmed was disheartening and scary. I went to my doctor to explain the situation and requested a blood lead test.
My blood lead test came back normal. I was surprised that it was not even a little elevated. I think I may have part of the explanation though….
Many sources state a strong correlation between certain soil conditions and the inhibition of plant uptake of lead. The three I found most cited were: as the ph rises above 6.5, when phosphorus levels are high (binds lead as lead phosphate) and when organic matter is high. Based on the results of our ‘basic soil’ test I know that our ph is 6.9 and our phosphorus is very high. I take for granted that our organic matter is high too. It looks like the lead may be unavailable to our plants. I have been eating leafy greens almost daily from this garden for 6 years now so I strongly suspect this is what is happening. I will now get a plant tissue lead test to check (will keep you posted).
From what I have read, the greatest risk is not actually via the plants you eat but through direct exposure to the soil (dust inhalation, toddlers eating soil etc) so I am surprised my blood lead is not elevated at least somewhat by the direct exposure i.e. gardening.
Alternatively, my soil sampling method (zig zag 10 different spots all over) may have happened upon one localised point of high contamination which then skewed the result. The majority of the garden may actually be clean. I just may not have eaten anything or enough of it from the contaminated area to show in my blood. However, although the 700ppm is surely not an even spread through the garden, I do suppose elevated levels would be consistent throughout (based on the rationale for testing in the first place).
Accidentally and fortunately, some of my soil amending guesswork over the years has probably contributed to the favourable soil conditions for inhibiting lead uptake. Plenty of compost (especially in the first few years), straw and woodchip mulch and chop and drop mulch has increased organic matter and probably phosphorus. I have added about 25kg lime flour (calcium carbonate) which would have raised the ph. I have added about 25kg gypsum (calcium sulphate), which does not alter ph but it functions as a clay breaker/flocculater. It may have helped by unlocking mineralogy and biology in a way which affected lead. I have also added 20kg Rok Solid rockdust, seaweed periodically and lots of coffee grounds. These have contributed generally to organic matter content and biological activity.
I recommend anyone concerned organise a soil test. If the result is high consider whether or not the plants are actually taking it in. Get the plant material tested. Get your body tested. Take remedial action with the soil (with ‘basic soil’ and possibly other tests in hand) and test at a later date. Fruits do not take up lead anywhere near as much as leaves/bulbs/stems/tubers etc, so if nothing else, plant a fruit garden with self-sowing and perennial ground covers that nurture bees, birds, insects, the soil and your crop plants and get your salad somewhere else.
Happy spring, Rory
george street orchard 