FAQ: How does compost help soil?
Most of compost’s benefit comes from its organic matter content, i.e., decaying plants and animals. This organic matter contains the carbon, nutrients, and beneficial microbes that make compost the perfect amendment for so many soil types. How does compost help soil?
Compost is about 50-60% organic matter on a dry weight basis. A bit more than half of that is organic carbon.
This organic matter acts like a sponge, helping soil hold water and reducing runoff. It also increases pore space to facilitate movement of air and water laterally through the soil, which creates transportation routes for microbes.
Compost is a soil conditioner. Texture, moisture retention, and a host of other factors contribute to a well-conditioned soil. A deep layer of light, friable soil encourages root development. These root systems provide additional pathways for water and beneficial organisms.
Healthy soil also provides a welcoming environment for those beneficial microbes, as well as other “critters” like earthworms.
The presence of active microbial populations improves nutrient uptake and contributes to the degradation of pollutants, too. The combination of improved uptake and fewer nutrients lost through runoff results in a reduced requirement for synthetic fertilizers.
For many growers, compost is the only amendment needed for a beautiful, abundant garden.
But don’t get carried away
Compost is a soil amendment – not soil. Too much organic matter can be as bad as not enough.
Your target is a soil with a 5% organic matter content. The best way to determine soil organic matter (SOM) percentage is with a soil test.
Soil testing is easy, inexpensive, and always a good move. If you never tested your soil or can’t remember the last time you pulled a sample, this is the year. Every three years is recommended.
You can buy a gizmo for a few dollars online or at the local garden center. Make sure it tests for SOM along with moisture, salts, etc.
For about the same money, you can also use your local Cooperative Extension Service. If you are not familiar with this excellent resource, we found this site which allows you to search for your county office by ZIP Code.