Gardening season is officially here in Calgary, and every day I see more people ready to purchase soil for their new raised or in-ground beds. They are often looking for the cheapest place to buy “top soil” and so I felt this post is timely.
First, a definition: What is top soil? According to Better Homes and Gardens “Topsoil is the top layer of the earth’s surface. Topsoil is dark in color and high in organic matter, which makes it very easy to till and fertilize ground for growing plants. It is scraped from the ground and sold in bags or bulk, often called “black dirt”.” I thought this was an interesting definition, and it touches on the points I want to make.
Top soil is not always dark in colour or high in organic matter. In the Calgary area, top soil is usually what is scraped off by developers as they create new neighbourhoods, and may not be very high in organic material at all. (It may also incorporate a lot of the sub soil, as our top soil is not always very thick.) As the soil is processed, the soil biology is destroyed, and many of the volatile nutrients, such as nitrogen, are lost to the air. What is left is generally very high in clay, and fairly low in organic material or humus.
This is in a local garden. You can see the clay that has baked in the sun, and the weeds that are trying to grow.
So what? Well, let’s talk about what plants grow well in degraded soil. Weeds! The initial colonizers of degraded soil are weeds such as pigweed or stinkweed. They don’t mind the lack of organic material too much. They can grow well in compacted soil. The high clay content of our local soil means that top soil is likely to become compacted very easily. Clay is the smallest particle, and it bonds chemically together. This is why potters use it to make dishes. When it is protected from the sun by a thick layer of decaying organic material, and the soil biology is intact, clay can be an excellent growing soil, as it holds nutrients and water well. However, once it loses that protective layer and the aeration caused by worms and other organisms, clay can bake in the sun and become hard-pan, which prevents water and plant roots from penetrating and leading to the inability of plants to access the nutrients in it.
Rich soil full of humus and organic material. (From my veggie garden)
Does tilling help? Well, yes, in the short term tilling can loosen the clay and allow roots to penetrate, but it does nothing to help the soil biology recover, and the clay will bake again on the first sunny day after a rain.
What are the alternatives? In my experience, most gardens do not need more top soil, because they already have enough clay. Instead: gardens need that rich layer of humus, also known as compost. Commercially available compost will add nutrients and organic matter to the soil below, and if it is fresh and alive, it will also contribute to the soil biology.
So I don’t need top soil? Probably not, unless you’re using it for grading or building up the level by more than 3-4 inches. That clay content works really well for creating the structure of your land, as compost just sloughs off in the rain. However, if you’re creating a raised bed to grow vegetables, or planning to add sod, or creating a bed to grow trees, shrubs or perennials, compost is a must.
What do I do if I’ve already added top soil? Don’t panic! If you’re finding the soil is really high in clay, and very heavy and hard, then add a product called zeolite. It is a volcanic mineral that breaks apart clay both physically and chemically. Then add 2-4 inches of straight compost on top. This will protect the soil from the sun, and will allow nutrients to penetrate below as the soil organisms create tunnels. Many people will tell you not to add that much around a tree, but I’ve found compost doesn’t create problems. (You can always slope it down to the base of the tree.)
Recipes? Sure! Here is my recipe for a raised bed: 80% compost, 10% coir (coconut fiber that holds water) and 10% zeolite.
Now you know why top soil isn’t tops!
This is what good growing soil looks like. (Chocolate cake texture!)