Soil organic matter
- Abbreviation
- SOM
- Also known as
- SOM, organic matter
Soil organic matter (SOM) is the fraction of soil made of living organisms, fresh and decomposing plant and animal residues, and stable, well-decomposed material called humus. Although it usually makes up only a few percent of soil by weight, it strongly affects fertility, structure and water holding.
What is soil organic matter?
Soil is mostly mineral: sand, silt and clay. The rest includes organic matter: living roots and organisms, plant and animal residues at every stage of decay, and humus, the dark, stable material that remains after decomposition. In most cropland, organic matter accounts for only a few percent of the soil’s weight, yet it has an outsized influence on how the soil functions.
Soil organic matter is roughly half carbon, which is why it is closely linked to soil organic carbon and to discussions of soil carbon sequestration.
Why soil organic matter matters
- Nutrient supply. Organic matter stores nitrogen, phosphorus and sulfur that microbes release as it decomposes.
- Nutrient holding. It adds to the soil’s cation exchange capacity, helping it hold nutrients against leaching.
- Structure. It binds soil into stable aggregates, improving water infiltration, aeration and resistance to crusting and erosion.
- Water holding. It increases the water available to plants, especially in sandy soils.
- Biology. It is the food supply for the soil microbiome.
It is one of the most widely used indicators of soil health.
How soil organic matter changes
Organic matter is constantly being added, from roots, residues and manure, and lost, through decomposition and erosion. Its level reflects the balance. Tillage speeds decomposition and erosion, reducing organic matter over time. Practices that add more carbon and disturb the soil less tend to build it:
- Reduced tillage and no-till farming.
- Cover crops and diverse rotations.
- Retaining crop residues.
- Adding manure or compost.
Changes are slow. Measurable increases often take years, and results depend on soil type and climate.
Frequently asked questions
It depends on soil type and climate. Clay soils and cool, moist climates tend to hold more than sandy soils and hot climates. Tracking trends on the same field over time is more useful than comparing with a single target.
Soil organic carbon is the carbon contained in soil organic matter. Organic matter includes carbon along with other elements such as hydrogen, oxygen and nitrogen. Laboratories often estimate one from the other with a conversion factor.
Sources
- Soil organic matter; soil health indicators, USDA Natural Resources Conservation Service
- Soil organic matter and soil carbon, Land-grant university extension