QUESTION: Do plants grow better in soil or compost?
ANSWER: Plants should not be grown in compost alone. Compost is a soil additive that makes soil richer by increasing the nutrients it contains and also improves the texture of the soil. However, without mixing compost with soil, plants end up getting too much of a good thing. When nutrient levels are excessive, they can actually be toxic to plants. In compost, this can especially happen with the nitrogen and salt compost contains as well as the micronutrient boron.
In addition to giving plants a surplus of nutrients that can be damaging, compost also makes a poor growing medium because of its texture. Compost is so much looser than garden soil that plants may be unable to stand up straight, as their root systems will be unable to find the support they need in compost. And because it’s so much looser than soil, compost has a tendency to become compacted or settle much more dramatically than soil.
This is especially obvious in containers, which will seem to be losing compost as time goes on because it will settle so significantly. The final problem with compost’s texture that makes it inappropriate as a growing medium is that it does not retain water and keep it available for the plants growing in it. Instead, any moisture from rainfall or that is provided by the gardener will simply drip through the compost without sticking around.
For these reasons, compost should only be used in conjunction with potting soil, topsoil, or garden soil. The ideal ratio of compost in soil is five percent. When you’re planting a garden bed at the beginning of the season, you can spread one or two inches of compost across the surface of the soil. Then mix the compost down into the soil to a depth of six to 12 inches. Alternatively, you can use this technique to amend your garden soil in the fall, ahead of time before planting in the spring.