QUESTION: Can I use compost instead of topsoil?
ANSWER: Yes, you can use compost instead of topsoil, and in general, compost is much better than topsoil in every way. Compost may be somewhat difficult to create, but it is much better for the environment and your plants. Composting saves your green waste from going into a landfill, creates a disease-free, weed-free soil that can be used for a variety of purposes and saves you money too, keeping you from needing to purchase expensive bags of soil amendments every growing season.
Topsoil is the layer of partially decomposed organic matter, or humus that lies between the surface and the subsoil. Long ago, topsoil could be described as a deep, nutrient rich, organic layer. Now, in most developed regions around the world, topsoil is very thin, sometimes even nonexistent, due to erosion and other environmental factors. Oftentimes, what passes for topsoil these days, is actually just inert subsoil.
If the topsoil on your land is poor, make your own by adding two to three inches of a high-quality compost and mix it to a depth of six to eight inches. You want around five percent organic matter in your soil. You can reach that goal by applying compost to the top layer of the soil between each growing season, raking the compost into the top layer of the soil in between harvests.
Compost and topsoil are not interchangeable. Compost is not topsoil, but it can be used to make or improve topsoil. However, compost is the wrong product for many applications that call for topsoil to be used. For example, compost should never be used as fill dirt. Topsoil is not compost either, and it will not perform the same way that compost does. Adding topsoil to a growing medium does not ensure soil performance, and can actually lower the quality of a growing medium if the topsoil used is practically inert, containing little to no organic matter or active microbes.
Topsoil is literally the top layer of your soil, which should be high in nutrients and organic matter. Compost is decomposed food scraps, vegetable peelings, dried leaves, paper and cardboard, that is mixed together and left to decompose over time. The end result is a nutrient rich humus that is high in organic matter. Any high-quality topsoil will contain a fair amount of compost and topsoil can be improved by adding compost to it.