Gardening magazines say it, plant nurseries say it, landscapers say it, and seed catalogue companies say it: Add sterile topsoil when planting anything, even if the plant is going in a small container on the windowsill. For many new gardeners and those wanting some new landscaping in their yards, the idea of lugging in huge bags of soil when they've got perfectly good dirt just all over their yard already sounds pointless. Yet experienced gardeners and professional landscapers will tell you that you can't skimp on soil quality, and adding several centimetres of a sterile planting mix is necessary if you don't have and aren't willing to get your yard soil thoroughly evaluated.

Amending Soil Takes Time

Yard soil isn't always nutritious for the plants you want to have in your yard. Certainly, you're not going to excavate and remove a metre's worth of soil all over your yard, and the plants will end up partially in that soil anyway. But adding sufficient sterile topsoil and a good fertiliser on top of that yard soil is still necessary as amending yard soil takes time. When you plant something, you want the soil to both have the nutrition the plant needs, and to be at the pH level that the plant prefers. Nutrition can be changed rather quickly with fertiliser. But amending the soil to raise or lower pH can take at least one year and possibly even two. Adding a commercial potting mix formulated for acidic- or alkaline-loving plants allows you to add material that is already closer to what the plant needs.

Pollution May Be Present

Your current yard soil may also have remnants of pollution. Most urban soil is going to have some exposure to general pollution, but depending on where your property is located, yours could have been exposed to agricultural runoff or industrial pollution from long ago, before your neighbourhood was developed. You really should get your yard soil tested. However, if you're determined to just start planting, you'll need to add several centimetres of topsoil to give the plants a fighting chance. By the way, if you do have any suspicions that your property has been exposed to agricultural or industrial pollution, don't plant anything edible in the yard, even with topsoil. Save the edible plants for container gardens where the soil isn't touching the yard soil.

Weeds Could Grow Out-Of-Control

Soil contains weed seeds. Even topsoil that you get from a nursery can contain weed seeds if you're not careful about what you buy. (Some brands formulate their topsoil to be weed-free.) If you simply plant in your existing soil, without doing any sort of soil preparation, such as using solarisation to kill existing weed seeds, you could end up with many more unwelcome plants in your garden than you had thought possible. Adding several centimetres of seed-free topsoil can help prevent seeds in the old soil from germinating because that topsoil will cut off sunlight to the seeds.

Keep in mind that you can't just dump the topsoil on top of the yard soil. You'll need to create a transitional layer of mixed yard and topsoil and then add the main topsoil layer on top of that. The transitional layer gives roots a chance to grow a bit longer without running into a densely packed layer of old yard soil. Speak with garden centres and landscaping companies about recommended soil supplies.