Soil Organic Matter: 5 Questions Answered

By Greg Selby | June 9, 2025

Soil health is the foundation of successful farming, and one of its most important ingredients is soil organic matter (or SOM). While humans can’t see SOM with the naked eye, it plays a critical role in improving soil productivity, conserving water, reducing inputs, making soils more resilient, and forming the physical, chemical, and biological trails of healthy soil.

For growers, understanding SOM isn’t just about science—it’s about protecting their livelihood, increasing ROI, and preparing their land for the next generation.

In this post, we answer five of the most essential questions about SOM:

What is soil organic matter?
Why does soil organic matter?
What impacts soil organic matter?
How to protect soil organic matter?
What makes soil organic matter so powerful?

Let’s dive in.

1. What is soil organic matter?

Soil organic matter is made up of everything in soil that comes from once-living organisms.

According to Cornell University¹, SOM consists of:

Fresh plant residues (like leaves, stems, and roots)

Active organic matter (detritus) that’s being broken down by microbes

Stable organic matter (humus), which can stay in soil for centuries

Organic matter is what makes soil alive. Dirt doesn’t have SOM—because dirt lacks biological activity. Simply put, dirt is dead.

Across the U.S., SOM levels vary widely–from under 1% in arid desert regions to 5% or more in parts of the Midwest and Pacific Northwest.² The ideal percentage of organic matter in soil depends on climate and soil type, but in most regions, maintaining levels between 3–6% is a strong indicator of healthy, productive land.³

2. Why is soil organic matter so important to growers?

SOM improves nearly every key function of soil. More SOM means better water retention, stronger structure, enhanced nutrient cycling, and higher microbial activity. All of these things lead to more resilient crops and better yields.

Water retention & drainage

SOM acts like a sponge. One mind-blowing fact is that soil can hold up to 90% of its weight in water, acting like a sponge.⁴ That stored water helps plants through dry/drought periods.

Soil health & structure

Connected to water retention is the concept of soil structure. The better the structure, the more effective soil will drain, which then will reduce runoff and waterlogging.

SOM binds soil particles into stable aggregates, improving porosity and reducing compaction. This means better air exchange, stronger root growth, and an improved microbial habitat.

Nutrient cycling

As SOM decomposes, microbes (i.e. the tiny organisms in the soil) break it down further and release nutrients like nitrogen, phosphorus, and potassium. These things act as food to soil, making it even more healthy.

SOM is especially excellent at nutrient cycling because it improves something called cation exchange capacity (CEC), which is soil’s ability to hold and deliver nutrients.

Microbial activity

SOM provides a rich food source for microbes, fungi, and earthworms, which break down organic matter and enhance soil fertility. This microbial activity also helps suppress plant pathogens and promotes symbiotic relationships, such as mycorrhizal fungi assisting with nutrient uptake.

Carbon sequestration

SOM helps to store carbon in the soil—a process that helps mitigate climate change because it removes carbon from the atmosphere. Stable organic matter like humus can lock in carbon for hundreds to thousands of years, making SOM a natural climate solution.⁵

By increasing SOM through sustainable practices, soils can act as long-term carbon sinks, helping mitigate climate change impacts.

3. What factors can impact SOM?

SOM is influenced by both natural and human-made factors:

Climate: Hot, dry regions decompose SOM faster and accumulate less of it.
Soil type: Clay soils tend to protect SOM better than sandy soils.
Land management: Practices like heavy tillage or monocropping deplete SOM.
Organic inputs: Compost, manure, cover crops, and microalgae all help build SOM.

For example, in the California desert, where low SOM is common, growers often rely on intensive irrigation. Conversely, in the Midwest, higher SOM means soil holds water longer, reducing irrigation needs and increasing drought resilience.

4. How can growers protect and build SOM?

Thankfully for growers, SOM can be regenerated through smart management practices. Here are four ways growers can protect and build SOM.

Minimizing soil disturbances

Tilling soil breaks apart soil structure and speeds up SOM loss. Reducing or eliminating tillage helps retain SOM, protect microbial communities, and keep carbon in the ground.

Managing organic inputs

Adding compost, returning crop residues, or applying live native microalgae provides the food and biomass microbes need to build SOM. The diversity and quality of inputs matter because balanced carbon inputs fuel life in soil.

Reducing erosion

SOM is vulnerable to wind and water erosion. This is especially the case with bare or exposed soil — two major causes of SOM loss. Keeping soil covered with cover crops or crop residue reduces erosion and protects SOM-rich topsoil.

Supporting microbial communities

Soil microbes are the drivers of soil organic matter decomposition and nutrient cycling. Moisture, temperature, pH, and organic inputs all impact microbial health, and supporting microbial life supports the entire SOM cycle.

5. What’s surprising about SOM?

SOM might only make up a small percentage of soil (2–8%), but its impact is disproportionately large.⁶

Here are a few surprising facts:

Soil organic matter only makes up a small fraction of soil.

Despite being crucial for soil health, SOM typically accounts for just 2-8% of total soil composition. Even this small percentage has a huge impact on water retention, structure, and fertility.⁷

Soil organic matter can store more carbon than the atmosphere.

Soil organic matter holds more carbon than the Earth’s atmosphere and vegetation combined. This makes it a key player in mitigating climate change.⁸

Not all organic matter is the same.

SOM exists in different forms, such as:

Fresh plant residues: The remains of dead plants and roots, including leaves, stems, and roots, which are in the process of decomposing.
Active organic matter (aka detritus): The portion of organic matter that is actively decomposing by microbes and fungi, contributing to nutrient cycling and soil fertility.
Stable organic matter (humus): The highly decomposed, stable form of organic matter that has a slow turnover rate and can remain in the soil for centuries, improving soil structure and long-term fertility.

Soil organic matter is constantly changing.

SOM isn’t static; it’s in a continuous cycle of decomposition, transformation, and renewal by microbes, fungi, and plant roots.

This dynamic process influences nutrient availability and soil health over time. In fact, the presence and diversity of fungi in soil can be indicators of soil health⁹, as a well-balanced fungal population suggests a thriving, functioning soil ecosystem.

It boosts both water retention and drainage

Because of its sponge-like and porous nature, SOM holds moisture while also improving infiltration. That means less runoff, better water filtration, and stronger drought resilience.

In fact, increasing SOM by just 1% can help soil hold thousands of extra gallons of water per acre.¹⁰

Final thoughts: The hidden power of SOM

When soil health is discussed, organic matter is at the heart of the conversation. It is the living link between productivity, profitability, and long-term sustainability. For growers, investing in healthy soil — and rebuilding soil organic matter — means: