Compost and soil organic matter

Organic materials, such as compost, plant residues or manure, do wonders for garden soil and plant health. If you’ve been adding organic materials to your garden, then congratulations! You’ve been doing great work to build soil organic matter and promote soil health.

Now, let’s take it one step further to ensure that your beautiful soil organic matter remains a benefit and not a liability to plants or to the environment. After all, it is possible to have too much of a good thing — even soil organic matter. But first, let’s look at what makes soil organic matter so important.

Soil organic matter: What is it and why is it important?

Soil organic matter (SOM) is the portion of soil that is composed of living and dead things in various states of decomposition, such as plant roots and microbes. Organic (carbon-based) materials that we add to the soil, like compost or organic fertilizers, will also contribute to SOM as they are incorporated and decomposed by soil organisms. And although SOM only accounts for a small fraction of soil by volume (2-8%), it’s very important for soil and plant health. SOM is where the magic happens!

Here are some of the things that soil organic matter does for your soil:

  • Provides essential nutrients for plants (such as nitrogen, phosphorus and sulfur) as it is decomposed by microbes.
  • Feeds and provides habitats for diverse soil organisms, including those that help fight plant pests and diseases.
  • Makes it easier for plant roots to thread through the soil to find water, air and nutrients.
  • Holds water in sandy (dry) soils and helps with drainage and oxygen availability in clayey (wet, heavy) soils.
  • Provides holding places for nutrients that plants need (SOM is a big part of the soil’s cation exchange capacity or CEC).