Soil organic matter

Organic matter is the lifeblood of fertile, productive soil. Without it, agricultural production is not sustainable.

Organic matter is any living or dead animal and plant material. It includes living plant roots and animals, plant and animal remains at various stages of decomposition, and microorganisms and their excretions.

On farms the main sources of organic matter are plant litter (plant roots, stubble, leaves, mulch) and animal manures. Earthworms and microorganisms decompose these materials. The process of decomposition releases nutrients which can be taken up by plant roots. The end product of decomposition is humus, a black crumbly material resistant to further decomposition. A complex chemical substance, humus stores plant nutrients, holds moisture and improves soil structure.


The rate of decomposition of organic matter depends on the soil’s temperature, moisture, aeration, pH and nutrient levels.

The warmer and wetter the climate, the faster the rate of organic matter breakdown. Cooler areas have higher levels of soil organic matter because it does not break down as quickly in low temperatures.

Waterlogged organic matter breaks down very slowly because microorganisms necessary for decomposition cannot exist where there is no oxygen. Soils formed from waterlogged organic matter are known as peats, and contain a high percentage of organic matter.

Acid soils with low pH usually contain greater quantities of organic matter because microorganisms become less active as soil acidity increases.

Benefits of organic matter

  • Improve soil structure
    As organic matter decays to humus, the humus molecules ‘cement’ particles of sand, silt, clay and organic matter into aggregates which will not break down in water. This cementing effect, together with the weaving and binding effect of roots and fungal strands in the decomposing organic matter, makes the soil aggregates stable in water.
  • Improves drainage
    These larger, stable aggregates have larger spaces between them, allowing air and water to pass through the soil more easily.
  • Holds moisture
    The aggregates are also very effective in holding moisture for use by plants. Humus molecules can absorb and hold large quantities of water for use by plant roots.
  • Provides nutrients
    Organic matter is an important source of nitrogen, phosphorus and sulfur. These nutrients become available as the organic matter is decomposed by microorganisms. Because it takes time for this breakdown to occur, organic matter provides a slow release form of nutrients. If crops are continually removed from the soil, there is no organic matter for microbes to feed on and break down into nutrients, so fewer nutrients are available to plants.
  • Improves cation exchange capacity
    Humus molecules are colloids, which are negatively charged structures with an enormous surface area. This means they can attract and hold huge quantities of positively charged nutrients such as calcium, magnesium and potassium until the plant needs them. Clays also have this capacity, but humus colloids have a much greater CEC than clays.

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).


Anything you read about healthy garden soil and how to improve your soil mentions organic matter. While only a small fraction of agricultural or garden soils—between 3 and 6 percent1—consists of organic matter, it plays a crucial role in soil productivity. Being knowledgeable about organic matter is important even if you have “good” garden soil because all soil needs to be replenished with organic matter from time to time. Amending soil with organic matter is not a one-time project but an ongoing process. 

This article explains what organic matter is, what differentiates it from organic material and fertilizer, and why organic matter is beneficial for soil productivity in many ways.

What is Organic Matter?

Generally speaking, organic matter comes from living materials that fix and store carbon and deliver it as a source of energy to the soil. More specifically, organic matter is divided into three types, depending on the time it takes for the organic matter to fully decompose. Active organic matter consists of fresh plant and animal residues that take from a few months to a few years to decompose. This type of organic matter is soil that is very much alive because it is filled with lots of active microorganisms. At the other end of the spectrum is passive organic matter, also called humus. It is the stable form of organic matter where the decomposition has already been completed and there is no longer any microbiological activity. Slow organic matter is somewhere between active and passive—it is organic matter that takes decades to decompose2, such as bones.

Organic Matter vs. Organic Material

Organic material is the source of organic matter: leaves, compost, manure, plant residues etc. As organic material decomposes, it changes its form and mass; it is an unstable material. About 90 percent of the organic material disappears during the decomposition process into organic matter. That’s why soil formation, of which the integration of organic matter is an initial step, takes so long—up to 100 years to form one inch of topsoil.3