Introduction: Urban gardeners face many soil quality challenges

Typical urban soil issues include:

• Compaction
• Too much or too little fertilizer
• Low organic matter
• Pollution from heavy metals, PCBs, fuel and PAHs (a by-product of fossil fuel combustion) pesticides, and drugs.

This training module focuses on Pb contamination and adding organic matter to your soil. It discusses Pb first.

We chose to focus on Pb, rather than other pollutants, because:

1) Elevated blood Pb is a bigger problem in Chicago than other places in the U.S. Overall in the U.S., about 1.6% percent of children have elevated blood Pb levels. But in Chicago? That number 11.2%.
Source: Dignam 2004, Levin 2008

2) There are simple, realistic steps you can take to limit your exposure to Pb through soil.

Part 1: Limiting Soil Pb Exposure

Below you will find a link that will take you to a video that talks about why soil Pb is a problem. The video is hosted by the "Vimeo" web-page. There is not a link on the Vimeo site to return to this web-page, so open the video link in a new window or click "back" on your browser after you have watched the video to return.

The video will also  give you some easy steps to take to limit your Pb exposure. To reinforce this information, or if you cannot access the video, similar  information is presented below the video in text and pictures.




Soil Pb Exposure

• Soil can be a significant source of exposure to Pb. Source: Mielke 1998
• A little Pb in soil is normal, but urban areas contain elevated amounts of Pb.
• Pb exists in urban soil because of deposits from industry, paint, and gas. Source: Clark 2006
• Even though we don't add Pb to gas and paint anymore, it sticks around as tiny dust particles.
• You can be exposed by accidentally eating soil, breathing in the dust, or consuming vegetables that have been grown on polluted soil.

Tips to limit and Monitor Pb exposure


1. Have your soil tested for Pb every 3 to 4 years (less if you're constantly replacing material in raised beds). Clean materials in raised beds can become contaminated through Pb dust.

2. Choose garden locations away from frames of houses and busy streets if possible. Those are places where deposits of Pb accumulated over time.

3. Cover bare batches of soil with grass or mulch to limit the amount of Pb dust blowing in gardening areas.

4. Wash your hands and produce with soap to limit accidentally eaten soil.

5. Use raised beds. Put landscape fabric down before filling the beds to stop plant root access to original soil.





6. If you're buying materials for a raised bed, try to validate it is from a clean source.

7. Keep soil pH above 7. At 7 and above, Pb tends to stay in insoluble forms, which are not accessible to plants and your body. When pH gets too high, though, your plants won't be happy!


8. If you think you're at risk for an elevated blood Pb level, ask your doctor for a blood test.

Part 2: Soil Pb Testing

Below you will find links to two videos; one that talks about how to test your soil for Pb and one about interpreting soil Pb test results. You will find similar information as text below the video links.

The videos are hosted by the "Vimeo" site. There is not a link on the Vimeo web-page to return to this web-page, so open the video links in a new window or click "back" on your browser after you have watched a video to return.




How to Sample Soil for Pb

1. Call testing labs to find out if they will analyze a small number of samples for "total Pb." Ask about price, the amount of soil they need, and other instructions.  You may get a better price per sample by sending more samples, so consider asking fellow gardeners if they want to sample their soil and send it with yours.

2. The EPA suggests sampling the top 1.5 cm, because that's where Pb dust may accumulate. In areas where soil is mixed, like garden beds, research has shown the same amount of Pb at the top of the soil and in rooting zones. So, it is possible to take a deeper sample to use both for Pb and nutrient analysis.

3. Clear the area your going to sample of debris and scoop surface soil into a paper bag. Take multiple samples from areas where you garden, and other areas of exposed soil (like pathways).

4. Keep track of where you sampled from.

5. Leave soil to dry in the paper bags

6. To save money, you can combine soil samples for an average. We suggest not combining soil from gardening areas and non-gardening areas, however. They may have very different Pb levels.

7. Put soil into labeled plastic bags.

8. Wash your hands

Interpret Soil Pb Numbers

Your soil test results will be returned with a number for Pb measured in ppm, or parts per million. That's the same as milligrams (mg) per kilogram (kg). A kg is about 2 pounds, and a thumbtack is about the weight of 300 mg.

Studies trying to quantify soil Pb in Chicago show a huge range. One study reported an average of over 2000 ppm in the soil they sampled. Other studies that sampled just within raised beds found an average under 200 ppm.

Here are the Environmental Protection Agencies soil Pb standards:

• At 400 ppm, soil is not safe for children to play in
• At 1200, bare soil limit in other areas
• At 5000 ppm, consider abatement - covering or removing the soil

The EPA has no specific standard for what is safe to garden vegetables in. It would be hard to get a number for that, because you'd have to measure how much Pb is in the soil, how much of that Pb is soluble (accessible to plants and your body), how much soil you accidentally eat, how much Pb your vegetables absorb, and how much of those plants you eat.

That being said, some researchers and organizations have tried to estimate a number. Their suggestions range from not gardening in soil over 300 to 600 ppm Pb to avoid elevated blood Pb levels.

Part 3: Organic Matter

What is organic matter?

Organic matter describes materials that are or were once part of living organisms. If you add peat or compost to your soil, you're adding organic matter.

The amount of organic matter in soils is variable. It is measured in percents. Three to eight percent is generally good for plants.

Benefits of adding organic matter can include:

• Improved water infiltration
• Improved water-holding capacity
• More nutrient availability
• Formation stable aggregates
• Reduced soil compaction   

Below you will find a link to a video about adding organic amendments to your soil. Below this you will find additional information about organic matter. The video is hosted by the "Vimeo" site. There is not a link on the Vimeo web-page to return to this web-page, so open the links in a new window or click "back" on your browser after you have watched the video to return.




You can increase organic matter in your soil by adding organic matter amendments, planting cover crops, or using organic mulches.

Organic matter amendments can't give you all the benefits at once. Different amendments are good for different things.

Adding sphagnum peat moss or coir is good for organic improving aeration and water retention in soil.



Other kinds of organic matter, like composted food scraps, composted manure, mushroom compost, and worm castings, benefit you by adding long-term nutrition to your soil.


Using organic matter

When you add any kind of compost to your soil, it's important it is finished. You can tell because there won't be big chunks of scraps or wood-chips in it. Unfinished compost can contain lots of carbon relative to the amount of nitrogen in the material. As microbes consume the carbon, they use up the nitrogen, meaning your plants won't have access to it. This is called immobilization.

Using finished compost, however, means there is a more ideal carbon to nitrogen ratio, so microbes won't rob your plants of the nitrogen.




It may not be necessary to add organic matter every year. Adding excessive amounts of materials that contain nitrogen contributes to pollution. Clues to use to know if you need more or less nitrogen are by getting your soil tested, and by looking at your plants. When you have your soil tested, you can request a C:N ratio. An good ratio would be 10 to 1 or 15 to 1 parts carbon to nitrogen. If your plants are very leafy and will not fruit, you might have too much nitrogen. If your plants have yellow leaves, you may need more nitrogen.



If you have an established garden and want to add organic matter to improve plant nutrition, add about a one half to one inch layer of compost to the soil. For every inch you add, you can till it in about twice that deep.

If you are gardening in containers or using a raised bed, you will want to use other amendments besides straight compost. An easy ratio to remember when making a mix from scratch is to use one part compost, one part sphagnum peat moss, and one part vermiculite or perlite by volume.

And when buying compost, make sure it's from a reliable source because factors like salt content and pH vary widely and affect its quality. If you want to buy large quantities of a soil amendment, your Extension office may be able to give you some direction.