Urban Gardening – Thoughts from a Soil Scientist

Along with the trends of buying local food, buying organic, etc., there seems to be an increase in (or perhaps more accurately, a return to) gardening – especially in urban areas. Urban gardening is a great way to save money on food, a great source for fresh vegetables, and an easy way to introduce kids to where the food on their plate comes from. In fact, working with the soil has been proven to make you happier! However, there are a couple potential obstacles you should consider first before starting your urban garden.

"Graze the Roof" by Sergio Ruiz
“Graze the Roof” by Sergio Ruiz

First, in urban environments the possibility that soil could have been contaminated with heavy metals, petrochemicals, etc. is pretty high. Lead (which was once a common additive to gasoline and paint) is common in urban soils and can be adsorbed by the roots of the vegetables you grow. Because of this, that lead can eventually end up in the food on your plate. Most lead poisoning comes from ingesting lead (like eating lead paint chips…), so it’s important to know that the soil you’re using for your garden is safe. You should take some soil samples and send them to a lab in your state that can test for heavy metals like lead. Usually the “land grant” university in your state (in the US) will have a soil testing lab where these tests can be performed for a nominal cost. Other forms of contamination are possible as well, such as chemicals from cars, asphalt , laundry-mats, etc. These chemicals are more difficult to test for, so your best bet is to find out the history of your garden plot. These records should be available from your local city government, perhaps even online.

Second, urban soils are often compacted from foot, car, or perhaps machinery traffic. Compacted soils make it difficult for plants to grow, mainly because the plant roots are not strong enough to penetrate the compacted soil, and thus cannot gather enough water or nutrients for the plant to survive, let alone grow and produce vegetables. Compacted soils are especially common in newer housing developments where entire blocks of houses were built around the same time. The construction companies often remove all of the topsoil prior to building the houses. The soils are then driven over by construction machinery and compacted. Then sod is laid directly on top of the subsoil. This makes for soils with very poor growing conditions for both lawns and gardens.

A good alternative for areas with either contaminated or compacted soils is to use a raised garden plot with soil that was brought in from a reliable source. You can buy bags of potting soil from a local home and garden supply store, but a more economic alternative is to have a trailer full of topsoil trucked in to your backyard. When you build your raised garden, be sure not to use wood that has been treated. Some of the chemicals used to treat lumber are designed to kill fungi that break down wood. These chemicals (such as arsenic) can leach out of the wood and into the soil used for your veggies! However, untreated wood, though it might not last as long, will still last for decades and is probably cheaper anyway. There are lots of great designs and how-to sites that show you how to build a raised garden bed. Here’s one example from “This Old House”.

Raised-bed Gardening. Photo by Joe Mabel via Wikipedia Commons

Space is also another consideration. If you don’t have the space for a garden or a raised garden, then perhaps you need to think outside the box (raised garden pun intended). Container gardening is popular in urban areas, and it’s what I’ve done for the last decade or so that I’ve been in college. Container gardening is exactly what its called – growing ornamental or vegetable plants in containers. Containers can be traditional plant pots, buckets, plastic totes, or any other container with an open top.

The advantages of container gardening include:

  • Containers can be arranged to optimally use the space available
  • Potting soil can be used, and can be trusted to be lead/chemical-free
  • Work can be performed on a bench, thus avoiding working on your knees
  • Containers can be arranged to provide decoration for your outdoor space
  • Many objects found around the house can be cheaply converted into decent containers
Vertical Pallet Garden. Photo by Heather Foust

Vertical gardening is a version of container gardening that uses your space available most efficiently. Much like using shelves to save space inside your home, vertical gardens use shelves, stairs, racks, etc. to make use of vertical space outside of your home for gardening. The options for vertical gardens are only limited by your imagination. In this How-To, Heather Foust gives a good example (and directions) of making a vertical garden using a pallet and some other supplies (shown left). Also, here’s an interesting article from the Christian Science Monitor about vertical gardening in urban areas. 

The main disadvantage of container gardening is that you’ll likely have to water more frequently, but there are strategies to overcome that problem – see my prior blog post about saving water with container gardening. Another good resource is the University of Illinois Container Successful Container Gardening website.

In summary, the biggest obstacles to urban gardening are soil contamination, soil compaction, and space limitations. I’ve given you a few good alternatives to overcome those issues. Also, be sure to fertilize appropriately, lime as needed, and make sure the plants that you pick are appropriate for the sunlight that’s available. Your local garden supply store can help you with suggestions on those issues.

If you know of an urban gardening obstacle that I didn’t address, please leave a comment and I’ll see if I can help out. Also, if you can’t find a place to have your soil tested, let me know what city/state you live in and I’ll see if I can help find a soil testing lab close to you.

Happy digging!


7 thoughts on “Urban Gardening – Thoughts from a Soil Scientist

  1. “… urban soils are often compacted …” True, but you can use worms and/or certain plants that sprout roots that soften and aerate the soil as an initial measure if you are not in a hurry. If you apply soft tactics as this and can afford to wait, say, two years, not only will you have ideal top soil but likely will have an idea if the soil is so contaminated that it repels such organic “helpers” which then should give you reason to think again.

    1. Thanks for the comment. This is true. Worms can help alleviate compaction issues and create “macropores” through which plant roots can grow. Incorporating organic matter along with adding earthworms can also help improve compaction by both providing food for the worms, and by helping to build up soil structure over time. I’d be wary of relying on earthworms as indicators of contaminants though. They can concentrate some contaminants in their bodies without adverse effects, and then move those contaminants up the food chain (like DDT for example).

      There are also other plants that can be used for till the soil, such as “tillage radishes”. Be careful using tillage radishes in urban areas though, because they tend to collect sulfur in a small area, which eventually stinks like rotten eggs and can be mistaken for a natural gas leak.

      Thanks for the comment,

  2. It seems really important to make sure that the soil on your property is healthy enough to plant a garden. You made a very good point by stating the importance of checking the amount of contaminants in your soil. Any toxins can have an impact on how well your garden grows. What are your thoughts about split spoon sampling versus down the hole sampling? I would like to choose the best soil sampling method to grow a garden in my yard.

    1. Deanna,
      Thanks for the comment. Regarding toxins, in Urban vegetable garden the bigger concern is toxicity to those consuming the veggies – especially in regard to lead. I just want to make that point clear. Toxicity might cause some yield issues, but in urban gardens I’d be more worried about heavy metals.

      In regard to soil sampling methods – the exact method doesn’t matter so much as does the technique. You want to make sure that you take a representative sample of the soil. To do that, take multiple samples from throughout the garden area. Make sure each sample is equal in size to the other sample locations. For the method, a clean shovel or trowel will work just as well as fancy soil sampling equipment as long as the sample size is consistent and that multiple sample locations are used. I would recommend using a small spade (aka sharpshooter) or shovel and dig down to 6 or 8 inches. Be sure to use a shovel that has been cleaned well first. Place each sample in a clean bucket (that has never been used to hold fertilizer). Once the bucket is about half full, mix it thoroughly. Then take a subsample of this well-mixed soil and submit it to a soil testing lab for further analysis. There are lots of good videos on YouTube that show how to take garden soil samples. Watch a few that came from respected sources, such as university extension offices, if you want some more help on sampling strategy. Also, the soil testing lab to which you will be sending your samples should also have a pamphlet or video describing how to take a representative soil sample, and how to prepare it for shipping and analysis.

      Thanks for the comment.

  3. Good comments on treated wood use in raised beds. Old rail road ties have been used in raised beds and low retaining walls. Ties usually have been treated with heavy duty preservatives. What contaminants could we expect to find that has leached into the adjacent soil? I have a raised bed like this and I use it for ornamental annual flowers and not for food production. Am I being too cautious?
    Robert from Oregon

    1. Robert,
      Good question. Rail road ties have typically used creosote as a preservative, which contains many different compounds. I haven’t an extensive literature search on creosote in the past, so I can’t give you a great answer to your question. However, this link looks like a reputable source and a good place to start: https://www.atsdr.cdc.gov/phs/phs.asp?id=64&tid=18

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