Earth Repair #1: Microremediation of Contaminated Soil & Composting
I'm grateful to Lynda Schneekloth, University at Buffalo Professor Emeritus and environmental activist, for her recommendation of Earth Repair by Leila Darwish. As we learn about contamination issues that affect our community, we must ask, “What can we do to heal this land?” Darwish's book offers practical information about remediating contaminated soil on a small-scale, grassroots level.
I will share a series of articles with practical insights I gleaned from Earth Repair. Each article focuses on one of the three types of remediating organisms: bacteria, plants, and fungi. I strongly suggest reading the full work for its inclusion of safety and legal considerations, suggestions for obtaining funding for materials, and much more detail. My intention here is to put this book on your radar and get you started with some widely applicable information.
When I picked up Earth Repair, I thought, “Yes! But—can natural organisms really make soil safe again once it's been exposed to industrial and ag chemicals, and heavy metals? What about radiation?” I was heartened to read some details that affirm this possibility. For example, when soil microorganisms compost dead plants and animals to make new soil, humic acid is created. This compound contains electrically-charged areas that attract and bind certain contaminants in a benign state (39). I also learned about research done following the Chernobyl nuclear disaster in Russia. In affected areas, it was found that the only foods not containing significant levels of radioactivity were those grown in soils rich in mycorrhizal fungi and other beneficial microorganisms (53).
Scott Kellogg includes this important caveat about bioremediation in his book, Toolbox for Sustainable City Living:
"Most bioremediation research has been done in controlled laboratory settings. Community -based, DIY applications of bioremediation have been few. The interactions between molecules, microorganisms, and human bodies are complex and not entirely understood. Much of the [bioremediation information in this book] is still of an experimental nature. At minimum, these treatments will not cause harm to the soil or to people applying them. There is no guarantee that soils will be safe following remediation. It will take many people participating in this experiment and recording their results to create a more complete picture of what processes work in what concentrations, against what contaminants, and under what conditions."
Microbial Remediation
Microbial remediation utilizes the ability of some bacteria to bind heavy metals in less bioavailable forms, or to break down organic contaminants into more benign substances. (In chemistry, organic means that the substance is carbon-based. This includes many industrial contaminants and agricultural chemicals). Because healthy soil rich in bacteria, fungi, and other organisms is imperative for healthy plant growth, we can't talk about using plants for remediation without starting with bacteria.
Conventional soil remediation, often completed by governments and contracted companies, might be the best route to take for highly toxic areas and large areas of contamination, such as oil spills. However, conventional remediation may have drawbacks. It often involves removing the top few feet of existing soil, laying down a rock layer as a buffer, and placing low-quality topsoil on top. This method is very expensive, and the contaminated soil is often placed in a landfill. When bacteria are used in conventional remediation, it is often fed a chemical fertilizer rather than plant-based nutrition (37). The small-scale remediation Darwish writes about strives to remediate existing soil sustainably, capitalizing on relationships and functions found in nature.
To support remediating bacteria, we can “farm” them and introduce them into the soil by making carefully monitored compost and compost tea. We can also alter soil conditions, such as pH, temperature, and humic acid level to make it more habitable for these species.
Making Thermophilic Compost
As an organic gardener, I have been making home compost for years. I was initially attentive about the ratio of carbon- to nitrogen-rich ingredients I was adding. I soon discovered that nature will inevitably break the pile down into decent-looking soil, no matter how negligent I am. However, when it comes to soil remediation, Darwish convinced me that my backyard pile may not have the bacterial diversity or quantity needed for remediation purposes. I will share some of the practices recommended to attain clean-up ready compost. When this method is used, the composting process should be complete in 16-18 days, followed by a few months of aging. I was surprised by the speed of the process, which seems to speak to its effectiveness.
1. Carbon to nitrogen ratio
Microorganisms that create compost require a carbon-to-nitrogen ratio of about 30:1 (41). Carbon-rich or “brown” compost ingredients include newspaper, dried leaves, cardboard, straw, and wood chips. They are typically drier materials. Nitrogen-rich or “green” ingredients include vegetable scraps, manure, grass clippings, coffee grounds, brewing waste, and urine. (Urine has a remarkably high level of nitrogen- perhaps the highest available to a small-scale gardener. And it's free!). Page 42 in Earth Repair offers an excellent list of the carbon-to-nitrogen ratio specific to each ingredient. Ingredients should be chopped into small pieces before being added to the pile.
Though we can't perfectly calculate this ratio, we can use our senses to determine that we are close enough. A pile with too much carbon won't heat up sufficiently; we will discuss temperature monitoring below. A pile with too much nitrogen will appear too wet and is more likely to have a foul odor. A healthy pile should have the moisture content of a wrung-out sponge (Darwish).
I was taught not to add wood chips directly to the garden as mulch, since the process of breaking down the chips pulls nitrogen out of the soil. However, Darwish praises wood chips in the compost pile. The large size of each chip helps to aerate and slow down compaction in the pile.
2. Nutrients
Darwish shares the importance of phosphorus in a compost pile due to its role in stimulating bacterial growth. Phosphorus-rich additions to the compost pile might include chicken manure, bone meal, rock phosphate, bat poop (guano), fish bones, and buckwheat (40, 41).
Other plants that are high in nutrients to enrich your pile include plantain, seaweed, garlic, yarrow, thistle, bracken fern, horsetail (rich in silica), nettle (rich in calcium, iron, and more) (41).
3. Forming the Pile
To achieve the proper temperature, a pile needs to be at least 1 cubic yard in size. (There's no way our home pile is getting hot enough!). It's best to have all the ingredients on hand at the beginning, to form the pile all at once. If materials are added slowly over time, this will also prevent adequate heat from forming.
When forming the pile, place a layer of browns with a larger surface area on the bottom, such as straw, wood chips, branches or twigs. This promotes aeration from the bottom of the pile (43).
From there, it is common to alternate nitrogen and carbon layers at 4-6” in thickness. This ensures that roughly the same amount of each type of material is added. Layers can be mixed together with a pitchfork as you go (43). Though a 30:1 ratio of carbon to nitrogen is needed, this does not translate to the volume of those types of material. All compost ingredients are a mix of both elements, and even the nitrogen-rich materials usually contain many times more carbon than nitrogen.
Since food scraps and brewers mash may attract pests, you may prefer to place those in the middle of the pile. Darwish suggests a top layer of browns to deter pests as well. You may also choose to “innoculate” your pile with finished compost or worm castings, thus adding beneficial bacteria (43).
If your pile is on the drier side, spray it with some water. Cover your pile with a tarp, first placing a stick in your pile with a bucket on top so the pile can “breathe” (43).
4. Aeration
A healthy compost pile should be aerobic, which means that the desired bacteria require oxygen. Aeration of the pile can be accomplished by occasional turning-- though not too often, since this can be detrimental to microbes. Some folks drill holes in plastic piping and build the pile around them, creating a “chimney” of airflow (44).
5. Temperature
A thermophilic pile should be at 131 – 145 degrees F (45). Use a compost thermometer to monitor your pile. Temperatures above 145 degrees mean that some beneficial bacteria may be killed off; flipping the pile at this point can restore safe temperatures.
Earth Repair briefly describes four phases of pile tending, including when to flip the pile, and what temperature fluctuations to expect. Check out pages 45-46 for this info.
6. Using Your Compost
When the compost is complete, it will look like dark brown soil, and smell earthy and pleasant. From there, the pile can rest before application. The less disturbance the pile receives, the more it will shift towards a fungally dominated pile (43). Piles that sit for 1-3 months tend to be bacterially dominant; 3-6-month-old piles may become fungally dominant. Ideally, a contaminated site receives both. I might add half of a pile to the soil in month #1, and the rest in month #6. Or, I might add all of it in month #3.
Some prefer to apply compost by tilling it into the soil, increasing its contact with contaminants. Others prefer not to disturb soil microorganisms with tilling, and “top dress” the area with compost by applying a thin layer to the surface. Darwish suggests a middle route, using a garden fork or broad fork to poke deep holes in the soil before applying compost.
Stay tuned for more about small-scale soil remediation!
Never miss an article: Join the Sweet Flag Herbs newsletter to receive an email when a new A Nourishing Harvest article is posted.
Sources:
Darwish, Leila. Earth Repair: A Grassroots Guide to Healing Toxic and Damaged Landscapes. 2013.
Kellogg, Scott. Toolbox for Sustainable City Living. 2008.
