QUESTION: Does compost turn into soil?
ANSWER: When you make compost, the final product definitely resembles soil, especially visually. However, under the surface, compost and soil are not quite the same—whether you’re comparing compost to topsoil, garden soil, or potting soil. Let’s take a look at the differences between the two and clarify when and why to use compost and when it’s best to use soil.
In short, compost is a soil additive that is more rich in nutrients than soil (similar to how fertilizer has more nutrients than soil). It is also looser in texture than soil and therefore does not support the root systems of plants like soil does, and it has much more of a tendency to settle and become compacted than soil. Compost also does not retain water like soil, which will hold moisture to keep it available for plants. Instead, water flows right through the loose grains of compost.
While you can situate plants in a container filled only with soil and the plants will thrive, compost alone cannot support healthy plant life. The rich nutrients of compost can be detrimental and even toxic to plants if they are growing solely in compost. They would not be able to stand up straight if they were rooted in compost, and the compost would become so compacted that it would seem to be trickling out of containers as the days went by.
Used properly, compost is mixed into soil, ideally at a level of about five percent. It can also be spread in a layer one to three inches thick over garden beds and worked down into the soil below to a depth of six to 12 inches.