Organic matter

Organic matter—also referred to as natural organic matter (NOM)—is a broad category of carbon-based compounds present in both natural and human-made settings, on land and in water. It originates mainly from the waste and remains of living things like plants and animals, but organic molecules can also form through non-living chemical processes. This material plays a key role in moving nutrients through the environment and helps the planet’s surface hold onto water.

**Formation** Living organisms release organic compounds into their surroundings while alive. After death, their bodies—along with shed parts like leaves and roots—are broken down by microbes and fungi. Through oxidative polymerization, the smaller fragments from this decay can recombine into larger, more complex organic molecules. The exact makeup of natural organic matter depends on where it came from, how it was transformed, its age, and the environment it sits in, so its properties and functions vary widely.

**Natural ecosystem functions** Organic matter is widespread in ecosystems and moves through them as soil microbes decompose it, making nutrients available. After breaking down and reacting, it can enter soil and surface water through flow. It supplies food for living organisms and acts as a buffer in water, helping to keep pH neutral—a function thought to help counteract acid rain.

**Source cycle** Some organic matter not already in soil arrives via groundwater. When groundwater saturates the surrounding soil or sediment, organic matter can move freely between them. Groundwater itself contains natural organic matter from sources like kerogen and coal deposits, soil and sediment organic matter, and material that seeps in from rivers, lakes, and marine systems. Organisms decay into organic matter, which is then transported and recycled. Not all biomass moves, though; some stays in place, turning over only over millions of years.

**Soil organic matter** Soil organic matter comes from plants, animals, and microorganisms. In a forest, for instance, leaf litter and woody debris fall to the ground—this is called organic material. Once it decays enough that its original form is unrecognizable, it becomes soil organic matter. When it further breaks down into a stable substance that resists further decay, it is called humus. So soil organic matter includes all organic material in the soil except what hasn’t yet decomposed. A key property of soil organic matter is that it improves the soil’s ability to hold water and nutrients, releasing them slowly to support plant growth. Humus also helps soil particles stick together, which lets nematodes and microscopic bacteria easily break down nutrients. To quickly increase humus, you can add compost (decomposed organic material), plant or animal waste (like leaves, trimmings, or manure), or green manure (plants grown specifically to be mixed into soil). These materials feed nematodes and bacteria, which produce more humus, giving plants the nutrients they need. Soil organic matter is vital for all ecology and agriculture, and it is especially important in organic farming, where it is heavily relied upon.

**Priming effect** The priming effect involves sharp changes in how soil organic matter (SOM) breaks down, triggered by relatively small interventions in the soil. It is usually caused by sudden or ongoing additions of fresh organic matter (FOM). Priming effects often speed up mineralization because the FOM provides extra energy and nutrients, boosting microbial activity. After FOM is added, specialized microorganisms are thought to grow quickly and decompose only this new material. In these areas, SOM turnover can be at least ten times faster than in the surrounding soil. Other triggers besides organic matter inputs—such as adding mineral fertilizer, root secretions, mechanical soil treatment, or drying and rewetting—can also cause short-term changes in turnover rates. Priming effects can be positive (speeding up mineralization) or negative (slowing it down, leading to nutrient immobilization). Most documented changes involve carbon and nitrogen, but the effect also occurs with phosphorus, sulfur, and other nutrients. The priming effect was first identified in 1926 by Löhnis, who studied how adding fresh green manure to soil intensified the breakdown of organic matter and affected legume plants.