Dirt Rocks! Urban Soils and Final Notes

Now we begin our final lesson on dirt. I've had a lot of fun teaching you about the essentials of soil science, and I hope you've learned some lessons you can apply in your yard. This information is vital to the proper planning and maintenance of any landscape. If you're new to the blog and would like to catch up on earlier lessons to understand this one better, click these links to see the Intro, Physical Qualities of Soil, and Soil Chemistry and Biology. I'm willing to bet that most of you reading this blog live in an urban or suburban area, and if so you have some special challenges to deal with. We'll spend the bulk of this post discussing these issues, and then I'll wrap up with a few final notes on potting mixes and soil testing.  Enjoy!

Photo by Maria Gulley

Photo by Maria Gulley

Working with Urban and Suburban Soils

When a soil develops naturally, it will end up with structure, pH, nutrient levels, and biology that are appropriate for its ecosystem. When humans build and work on soil, we alter it drastically. The picture on the right is from our property, formerly a dairy. It provides an example of the visible effects of human activity on soil. The top three layers (at least) represent years of changing land use, each stage leaving its own mark. Some common problems for urban and suburban soils are compaction, loss of topsoil, and increased alkalinity.


Some of this will be review from the lesson on physical qualities of soil, but there's plenty more to discuss. Compaction ruins a soil's healthy granular or blocky structure. The resulting platy structure reduces the soil's ability to absorb and drain water and makes it incredibly difficult for plants to take root. Soil can be compacted by heavy machinery, routine wearing by lighter loads, or handling while wet.

The best way to deal with compaction is to prevent it from happening in the first place. Compaction by heavy machinery usually occurs during construction when trucks and skid-steers and excavators and whatnot drive over the soil. To reduce the impact, ask contractors to keep machinery to areas that will eventually be paved, like driveways and patios. Laying down plywood or a thick layer of mulch in high traffic areas during a project can reduce compaction. Even if you don't have large vehicles on your property, you can still cause compaction with frequent traffic from lighter equipment like mowers, or even walking the same route over and over again. Vary your mowing pattern to avoid the kind of compaction you see in the image on the left. If you know an area in your lawn or planting beds will receive high foot traffic (four-legged foot traffic counts too!), consider putting in stepping stones or having a more permanent path put in so traffic stays confined to the designated path and reduces risk of compaction elsewhere. Working with or on wet soil at all can cause compaction. Avoid planting after heavy rainfall, and keep heavy and light equipment off of a saturated yard.

An air spade uses compressed air to blast away compressed soil without hurting tree roots. Photo by Maria Gulley

An air spade uses compressed air to blast away compressed soil without hurting tree roots.
Photo by Maria Gulley

Sometimes compaction is unavoidable, or you inherit a problem someone else created. Good structure isn't naturally restored to a compacted site, so you need to take action. On agricultural land, farmers can use specialized plows to shatter through compacted soil, but this method isn't usually well suited for the home lawn and garden. For lawns, the best option is aeration. Aerating machines remove plugs of turf a few inches deep to allow air and water into the root zone. In a flower bed, mixing organic material like compost or manure will help, and using a rototiller can relieve compaction in the top six inches or so. Do not use a rototiller under a tree, because most of a tree's roots are in the top foot of soil and tearing up the top few inches can do a lot of damage. When a tree is stressed by root zone compaction, an arborist can use a tool called an air spade that uses highly pressured air to remove the soil around the roots without damaging the tree. The loosened soil can be put back, and the tree will be much happier.

The most fertile soil is in the top layer. Photo courtesy of USDA.

The most fertile soil is in the top layer.
Photo courtesy of USDA.

Topsoil Loss

Topsoil loss is a pretty standard problem for new houses. During the construction process, builders remove much of the topsoil to get the right ground contours for the property. The top 3-6" of a soil profile contain nearly all the organic material, and a large portion of the nutrients. Even when a contractor claims to replace topsoil, it's usually not the higher fertility, granular structure original soil we want, but instead a heavy clay soil full of rocks that may already be compacted from improper handling.  Before any landscaping is done, make sure that the top layer of soil is suitable for plant growth. You can determine this through soil testing for nutrients and organic matter (discussed below) and by trying to insert a stake or wire flag into the soil when it isn't bone dry or soaking wet. If you can't get more than a few inches without serious effort, then your soil is probably compacted.

Increased pH

Construction work of any scale from a new house to a new paved path can result in a localized, temporary increase in soil pH. In Indiana our soils have limestone bedrock, so when digging deep enough to lay a building foundation or a solid paving surface the alkaline limestone can be brought closer to the surface. Concrete is also derived from limestone, so wherever concrete is mixed or poured you can increase soil pH. If you have a paved patio or path, it will be set on a layer of crushed and compacted material. Can you guess what that material is? That's right, it's our friend, limestone once again. So as you can see, there are plenty of ways limestone can ruin your garden's day after construction. Fortunately, this increase in pH will fade with time as the compounds that cause it are washed out of the soil. Until then, you can stick with plants that tolerate alkaline soils (Indiana natives are usually great choices for this), or you can acidify the soil with aluminum sulfate (available at your local garden center) if you start to see signs of nutrient deficiency. The farther you get from the new building or paving, the less you will need to worry about, and planting areas out in your yard may have no problems at all. It is important to keep in mind that it is the dust and tiny particles of disturbed limestone that cause these pH problems. Limestone is an attractive material for walls and pavers, and these solid blocks won't do any harm to your plants.

Potting Mix vs. Soil

If you've ever gone to the store to buy something to fill your flower pots, chances are you've come across a whole range of possible choices, from potting mix to potting soil, mushroom compost to garden soil. Potting mixes and soils are a whole separate beast from soils in the ground. In fact, most potting mixes don't contain any soil at all. Soilless potting mixes are engineered to be lightweight for easy handling, quickly draining to leach out salts that can harm plants, and high in organic matter to hold nutrients after water flows out. Soil and compost will easily compact in containers without the help of the organisms that aerate soil in the ground. Many potting mixes come with slow-release fertilizer mixed in to compensate for the lack of minerals in the first year, or you can fertilize on your own. If you're like me and forget to water your containers, you can also buy potting mixes with polymers that help moderate moisture, or you can buy the polymer separately and add it in (Soil Moist is one common product). While potting mix is the best option for containers, it’s also usually the most expensive. Potting soil is a cheaper alternative that is better for containers than garden or topsoil, but I prefer to combine it with potting mix rather than using 100% potting soil in a container.

Soil Testing

If you are making major changes to your landscape or having persistent problems you don't understand, it is recommended that you get a sample of your soil tested. A soil test will tell you pH, nutrient levels (usually for potassium, phosphorus, magnesium, and calcium), texture, cation exchange capacity, and organic matter content for a sample of soil. You can test just a problem area or take samples from many spots in your yard to get the big picture. There is more to collecting a soil sample than just putting some dirt in a bag. Check out this Purdue Extension publication to learn how to take good soil samples. Soil sampling is inexpensive (usually around $10 or less per sample for a basic test), and it can provide vital information. To find a soil testing site for your area, check this list.

That wraps up our series on the essential points of soil science! At the bottom you'll find some links to learn more about urban soils and the other topics we covered today. I am always happy to answer any questions about soil (or other aspects of landscaping and horticulture), so ask away in the comments or email me at maria@gardensofgrowth.com.

Additional Resources
Growing Gardens in Urban Soils (US Environmental Protection Agency)
Urban Soil Primer (USDA Natural Resources Conservation Service)
Soil Compaction in the Urban Landscape (University of Florida)
The Dirt on Dirt - Potting Soil (Proven Winners)
Collecting Soil Samples for Testing (Purdue University)
Certified Soil Testing Laboratories (Purdue University)

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