Soil Organic Matter

Soil organic matter (SOM) is a category found in soil test results and it is critical for good soil health.

Healthy soil contains a variety of organic matter (Kate Russell)

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Soil organic matter levels can range from practically nothing to as much as 90%. Deserts are at the low end of the scale, while low lying, wet areas (think peat bogs) are at the high end. Most topsoils range from 1% to 6% soil organic matter. Soils containing 12% to 18% organic matter are called histosols. Histosols tend to be acidic, low in nutrients, and have poor drainage.

California desert (Pixabay)

Rain forms pools in bogs (Ivar Leidus) CC BY-SA 4.0

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Components of soil organic matter

Soil is made up of minerals (45-49%), water (25%), air (25%), and things that were or are alive. These lifeforms can be insects, plants or animals, in various stages of living or decomposing, microbes, and any substances created by those living things. These lifeforms, both alive and dead, and their secretions and exudates, are what make up soil organic matter.


Soil organic matter is approximately 5% living things, 10% fresh residue, 33-55% stabilized organic matter, and 33-50% decomposing organic material.


Organic matter and soil health

Maintaining healthy soil is a big part of the Integrated Pest Management (IPM) practices that allow us to grow plants with a minimum of chemical interventions. Healthy levels of soil organic matter provide biological, physical, and chemical benefits to your soil. Sufficient soil organic matter improves soil structure and water retention and infiltration. It also increases soil aggregation, or clumping, which increases the number of macropores and micropores through which water, air and roots can move. Organic matter improves soil biodiversity, and the absorption and retention of pollutants, while reducing soil compaction, crusting, and urban drool. Organic matter also creates a  buffer against changes in soil pH.


Organic matter and plant health

As plants, animals, and insects decompose, a variety of compounds become available to plants, increasing soil fertility and nutrient cycling and storage. These compounds include carbohydrates (sugars and starches), fats, lignin, proteins, and charcoal. As these compounds are broken down further, or mineralized, they increase your soil’s cation exchange capacity. This means plants are better able to absorb atoms and molecules of plant food through root hairs. Insufficient soil organic matter can cause mottling and other signs of nutrient deficiency.


Soil organic matter also acts as a carbon sink, reducing the amount of carbon in our atmosphere. As a major player in the carbon cycle, soil organic matter is believed to hold 58% of the Earth’s carbon. We can help keep it there (and out of our air) with no-dig gardening and cover crops.


How to increase soil organic matter
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Before increasing anything in your soil, send a sample to a lab for testing. There is no other way of knowing what, exactly, is present without a soil test. It would be rare for most soils to have a problem with increasing organic matter levels, but it’s better to be safe than sorry. Plus, then you’ll have all that other great information!


You can increase organic matter levels in your soil with these tips:

• stop rototilling - it disrupts important soil microbe populations and soil aggregates
• reduce soil compaction and erosion with cover crops and mulch
• fertilize only as needed [as per your soil test results]
• incorporate perennial legumes and cereal grasses, such as millet, oats, and alfalfa, into your landscape
• top dress with aged compost and manure
• rotate annual vegetable crops with green manure crops

Remember, soil organics matter!