As an urban gardener, I know that the quality of the soil I grow my edible plants in is important. Cities are notorious for having soil that is contaminated by heavy metals – be it from the residue of industry, construction, or traffic fumes. But it was only when I moved to a ground-floor Brooklyn apartment with a large backyard that I have faced the problem myself.
Until then, I had grown vegetables, fruits, and herbs in pots on terraces. In containers it is very easy to control the quality of the soil: You buy the good stuff, pre-mixed or bagged (or blend your own, if you are a serious DIY-er). And while replacing all of the soil in an in-ground garden is not technically impossible, it is prohibitively expensive for most gardeners. If you can, do. If you can’t, there is hope. Read on.
Let’s backtrack. It was fall and I was already growing and harvesting beautiful salad leaves when I decided to send a soil sample from our vegetable patch to Cornell University to be analysed. The results made me queasy: our soil had high lead levels - 560mg/kg. The “safe” level in New York state is 400mg/kg. I winced at the thought of the handfuls of salad my husband and I had been eating.
Intense hours of reading followed. Raised beds are the ideal answer in any heavy metal situation. For me, the cost of the cubic feet of good soil we'd have to bring in was prohibitive – we were renting, after all. Or I could use phytoremediation: growing specific plants to suck up and sequester lead. I could even grow mushrooms; mycoremediation is very effective at removing heavy metals from soil. But those methods take years. I wanted to grow food safely, and soon.
There was another important number in the soil test results: a low, acidic pH of 5.4 – (7 is neutral, below is acidic, above is alkaline). As I read about lead contamination, I began to appreciate that significance. It turns out that pH is key to lead remediation in soil.
Low pH makes lead available for absorption by plants. In acidic conditions root crops absorb the most, leaves next, and fruit (seed bearing parts of the plant) the least, if at all. But further research suggested it might not be something to freak out about after all. According to a 2002 University of Minnesota paper produced by its Department of Soil, Water and Climate, “In general, plants do not absorb or accumulate lead." This was (very good) news to me.
In fact, you're more at risk of lead contamination from residue on the skins of plants that haven't been rinsed properly (rather than anything the plant has absorbed). Small children crawling or playing in gardens with soil high in lead, and gardeners who do not wash their hands, are also at risk of ingesting lead.
So, what about all those salad leaves we ate? According to a 2013 Cornell University study by the Department of Crop and Soil Sciences, the lead in our garden thankfully wasn't enough to leach into our lettuce. "For lettuce, Pb [lead] levels remained well below the recommended limit even at a soil total Pb concentration of 915mg/kg." That is a lot of lead. Much more than ours.
Still, the lower the pH and the higher the lead levels, the more can be absorbed.
Although it took some time for me to come down from the very high branches of my panic tree, eventually I came back down to earth – which, after reading the science, wasn’t as frightening any more.
Because there is a solution: Raise the soil’s pH. Changing the soil from acidic to near neutral (known as sweetening) makes lead inaccessible to plants.
I assumed I needed garden lime, as it is alkaline. But it also contains magnesium, and our soil test revealed borderline high magnesium levels (too much stunts and even kills vegetables). Instead, I learned about the use of oyster shells and eggshells to sweeten soil from Steve Masley's website Grow it Organically – they are pure calcium. For more rapid results, I chose to buy crushed oyster shells for immediate application. Masley’s recommended application rate was 4-6lbs of crushed shells per 100 square feet, every year or so.
From late fall of that year through to early summer, I applied a total of 12lbs of crushed oyster shells (it looks like white dust and is sometimes called oyster shell flour), and 3 lbs of homemade crushed eggshells (we ate a lot of eggs!) to our 100 square foot vegetable plot, digging it in to about six inches. I more than doubled Masley’s recommendation. First, because I did not know how many points it would raise the soil's pH, and also because the quality of the soil affects the sweetening: Organic matter acts as a buffer, slowing the process down. According to our soil test, we had lots of organic material, so more seemed like a good idea.
I sent a second soil sample to Cornell one year after the first. The new test revealed that the soil’s pH had risen from 5.4 to 6.6. From acidic to optimum, and near-neutral. This was pretty dramatic. (And the overall soil quality score went from 54 and Low to a score of 75 and Excellent.) I planted vegetables with an easy mind, adding root crops like potatoes and garlic to the menu.
How to Make Crushed Eggshells
Making your own eggshell powder is inexpensive but it takes time to collect enough shells. Ask your friends to save their shells or ask a local eatery to save theirs for you.
Every time you use an egg, keep the shells. Allow them to dry if the egg was broken raw. Store them in a large airy container or jar and occasionally stomp them down to make room for more. When you have enough in weight to work with, turn them into powder.
To pulverize them, add them in batches to a food processor, ideally using a pastry blade (they may dull a sharp blade). Process until the shells are powder-fine. They will not be nearly as effective if you can see pieces of shell. While you are working, cover the food processor with a damp tea towel to prevent the very fine dust from escaping. And open it outside once the dust has settled. Weigh, and apply.
And remember to wash your vegetables. Because the lead isn’t in them, it’s on them.
Photos: © Marie Viljoen