Understanding Soil Organic Matter: Why It Matters and How to Manage It

I get a lot of questions about soil organic matter(SOM) . What is it? Why do I need to monitor my organic matter levels? What is a good result and what is bad result? What do I do if my results are bad? Let's explore what SOM is, the ideal levels to aim for, and what you can do if your soil is lacking—or has too much—organic matter. 

What is Organic Matter in Soil?

According to the USDA, “Soil organic matter (SOM) is the organic component of soil consisting of three primary components: small (fresh) plant residues with small living soil organisms, decomposing (active) organic matter, and stable organic matter (humus).”

Here, “organic” refers to naturally occurring substances and organisms in the soil, not the more commonly understood retail definition (products created without synthetic additives). Examples of organic matter in this context are manure, crop residues, cover crops, and compost. These elements all contribute to the vitality and resilience of soil.

Why Do You Need to Monitor Organic Matter? 

Without sufficient organic matter, your soil’s health will suffer. Organic matter helps create an ecosystem that enhances plants’ ability to absorb essential nutrients, including those in your fertilizer applications. It also improves water retention and drainage, helping roots access moisture while preventing over-saturation.

Organic matter’s effect on soil structure supports beneficial microbial activity. Healthy microbial activity, in turn, strengthens the soil ecosystem, helping it resist diseases and pests that could otherwise destroy your crop.

What Are Healthy Levels of Organic Matter?

Organic matter is reported as a percentage, and healthy soils typically have between 2% and 6% organic matter. If your organic matter levels are within this range, your soil is likely well-balanced and ready to support healthy plant growth.

What to Do If Your Levels Are Too Low or Too High?

If your SOM level falls below 2%, it’s time to take steps to improve it. Start by preserving the organic material you already have through soil erosion measures like a no-till farming regimen. Adding organic matter sources, like compost, grass clippings, or mulch, can also help bring SOM levels up over time.

Can SOM be too high? Yes! Organic matter levels above 6% may indicate excessive phosphorus. For example, if you’ve been using manure to increase SOM, your annual soil test might reveal dangerously high phosphorus levels. Phosphorus pollution, often caused by excess application of fertilizers and amendments, is one of the main threats to waterways.

Phosphorus is the leading cause of pollution in our waterways and gets into the system when farmers and homeowners apply more of it than the soil and its plants can absorb. And while the right level of phosphorus is a key to plant health, too much can actually kill plants. A surprising number of our soil test customers show toxic levels of phosphorus coming from organic lawns and gardens. Always remember – too much of a good thing can turn into a bad thing.

If you want to dive deeper into this topic, visit the USDA site for a good read.