Nance Klehm is an ecological systems designer and bioinstigator based in Chicago. Like many old industrial cities, Chicago has manufacturing plants right within the city limits, and as a result its neighborhoods, soils and waterways have a buildup of contamination. It has been said that it is unsafe to grow food anywhere in Chicago unless you are willing to import clean soil. “Conventional remediation is all about removing the first two feet of soil, replacing it with crushed limestone and then capping it off with a frosting of topsoil,” explained Klehm. “There is a brownfield that was remediated by the USA EPA just around the corner from where I live. It’s about an acre and a half. There is an environmental justice group that finally got the go-ahead to use it as a growing space and small market area. But the amount of soil they would need to bring in to cover the site would cost almost US$30,000. Most operations that I know – if they are really small-scale or neighborhood-based – just ignore that expensive approach and grow directly in the soil. Or they do raised beds and they do it directly above cement. Or they will do a lasagna/sheet mulch approach with cardboard and wood chips up at least a foot and a half and then start loading up compost and growing on top, just waiting for the mulch to break down and build soil.”
To Klehm, you cannot remediate a site with plants unless you start first with the soil biology. “Soils can be so compromised and compacted. To even get them to the point where they are a good growing base for plants means I need to work with the microbes first,” Klehm said. “If I can get a soil in balance, so that it is really microbially active – so that it has the nematodes, the protozoa, the bacteria, micro-arthropods and the fungi – then that is the baseline. I usually spend a lot of time on that before I get into the plants.”
Describing her approach to remediation, Klehm had the following advice to offer: “I’d start with a baseline soil test, and then I would probably come in with the compost, the compost teas and plant extracts on that site first for a year and then test again. Then I would introduce a really diverse community of plants. And continue to feed the soil with microbes. Often in a site, we have to aerate the soil in some way. I spend a lot of time doing that before I even introduce plants.” On sites where there is significant soil compaction (which leads to anaerobic soil conditions) Klehm has used rebar to poke holes in the ground to help get more airflow into the soil. This creates a more aerobic environment, allowing for microbes from compost and compost tea to make their way down into the soil and remediate at a deeper level. “When I introduce plants, I usually introduce woody plants because they are tough, they have incredible root masses and they will anchor a site. I work with woody plants first and then other perennials. Then I go back to those annuals that you can pull out and dispose of,” continued Klehm.
When it comes to using plants for remediation, Klehm likes using plants that create an active rhizosphere in the soil and that stabilize the site and provide habitat for beneficial microorganisms. The rhizosphere is the area of interaction between the surface of the plant root and the area surrounding it, which is filled with bacteria, other microorganisms and soil debris. She also selects plants for the site that will produce lots of food for an active soil food web. Often when we see vacant lots or disturbed sites, they are covered with weeds and other pioneering species. What is interesting about weeds is that though they can replant a site quickly and carry out some key functions, when compared to other plants they actual pump out very little food (in the form of exudates) from their roots for the soil microbes. This is because weeds are trying to move through their life cycle quickly in order to further spread onto the site. “They are only sharing like 20% of their sugars with bacteria and fungi, whereas a tree is sharing 60%. Trees also grow an incredible root mass,” said Klehm. “I’ll work with perennial grasses, a lot of things that are considered cover crops, nitrogen fixers or green manures. But I also try to get a couple of trees in there because they will really send the roots far and wide and create a lot of sugar exchange, calling forth the fungi and bacteria.” When working with annuals and cover crops, Klehm views them in the following way: “I think of them as stabilizing the soil from erosion, creating an aerobic situation and making it ready for the plants that I want to use later, in two or three years. For me, remediation of a site takes at least five years or longer.”
On a neighborhood site that Klehm is transforming from a desiccated vacant lot to nourishing gardens, they have started with microbial remediation. “We began by building very large berms using the technique of layering carbon and nitrogen in them, and they are set to compost down and build clean soil. They are maybe three feet wide and two feet tall,” said Klehm. The berms will act as raised beds, so that plants can be safely grown in them on a quicker time line than it would take to remediate what is likely contaminated soil below. Following that, Klehm and her work party added cardboard and mulch to create pathways on the site.
Many urban gardeners and farmers stop at this point – they import or build soil up on a site, mulch the rest and move on to growing their veggies. However, where they stop is where much of Klehm’s work begins. “We built beds up but we also addressed down into the soil throughout the site. We cored into the ground underneath the paths and through the beds, and we put big cardboard tubes down that we had punched holes in. We filled them with compost to do a soil biology drench. The reason is that we are trying to use the biology of the site to unleash some of the minerals in the clay below. These cardboard tubes will stick up above the mulch. We can test the soil around them to see how we heal things over time. Then we planted a lot of trees and a lot of woody perennials. We are going to use the rhizospheres and the woody root masses to get the biology going. And once the fungal and bacterial communities are really strong, they will start helping to bind some of those contaminants, while also building nutrients through the site,” explained Klehm. “It is not just our legacy to grow food the way we want to with our low budget; we really need to heal what is below us. We do it because it’s our job.