Working with Urban and Suburban Soils
Maria Gulley is the horticulturist at Gardens of Growth, a full service landscape design, build and maintenance firm, where she also manages the company blog. She guests blogs about her experience volunteering as a Tree Tender for KIB and her passion for the value of urban trees and greenspaces, and enhancing our communities. When a soil develops naturally, it will end up with structure, pH, nutrient levels, and biology that are appropriate for supporting the plant life of its ecosystem. When humans build and work on soil, we alter it drastically. Some common problems for urban and suburban soils are compaction, loss of topsoil, and increased alkalinity (high pH).
Compaction ruins a soil's structure, the way in which the smaller soil particles clump together to form aggregates. Good soil structure has lots of small clumps that are rounded or somewhat blocky with plenty of pore spaces in between. Compaction compresses these clumps together into thin, flat pieces that we refer to as platy structure, and it gets rid of the pore space that should be holding water and air for roots to access. This structure reduces the soil's ability to absorb and drain water and makes it incredibly difficult for plants to take root and thrive. 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, skid-steers, excavators, or other machines drive over the soil. To reduce the impact, ask contractors to keep machinery to areas that will eventually be paved, like driveways and patios. Even if you haven't had large vehicles on your property in years, 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 stripes of compaction over time. If you know an area in your lawn or flower 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. Working with or on wet soil at all can cause compaction. Avoid planting after heavy rainfall, and keep heavy and light equipment off of muddy ground. 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.
Topsoil loss is a fairly standard problem for new houses. During the construction process, builders often remove much of the topsoil to get the right ground contours for the property. The top 3 to 6 inches of a soil profile contain nearly all the organic material, and large portion of the available 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.
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 the alkaline limestone can be brought closer to the surface. Concrete contains limestone, so wherever concrete is mixed or poured you can increase soil pH. If you have a professionally paved patio or path, it will be set on a layer of crushed and compacted material, and in Indiana that material is limestone. 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 soil returns to normal. Until then, you can stick with plants that tolerate alkaline soils (which is usually a good bet in our region anyway), 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 the middle or edge 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 acute pH problems. Limestone is an attractive material for walls and pavers, and these solid blocks won't do any harm to your plants. Understanding soil is absolutely essential to maintaining a happy lawn and landscape that will thrive for years to come. Usually we don’t think much about soil and our plants, other than thinking about whether soil is too wet or too dry. But in reality, a healthy soil has an ecosystem of its own that interacts with your plant roots and everything else around it to provide us and the environment with all sorts of benefits. It provides what plants need to produce food, shelter, and beauty, helps to purify water before it reaches aquifers underground, and provides a home for many animals. Take care of yourself and your environment by understanding your soil.