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Organic Matter

or-GAN-ik MAT-er
🌱 Soil Health By the Fed 'n Happy Team · Updated October 20, 2026

Definition

Organic matter (OM) refers to any living or once-living material in soil, plant residues, animal waste, microorganisms, and their decomposed remains. It is the foundation of a healthy, productive soil ecosystem.

Soil cutaway showing fresh crop residues breaking down into active organic matter worked by microbes and worms, then stable humus
Organic matter, plant residues, animal waste, and their decomposed remains, forms the biological foundation of productive soil. — Source: Fed 'n Happy (original illustration)
Quick Facts — A Genuinely Debated Claim
Organic matter's water-holding effect is widely cited, but the size of that effect is actually contested in the research.

Certain types of organic matter can hold up to 20 times their weight in water under lab conditions (Reicosky 2005). But a large-scale meta-analysis of over 50,000 measurements (Minasny & McBratney 2018) found organic matter's real-world effect on field water capacity is often smaller than commonly assumed. The honest takeaway: organic matter reliably improves water retention, but treating "20x" as a guaranteed field result oversimplifies the science.

Sources: Food and Agriculture Organization (FAO), European Journal of Soil Science (Wiley)

Why It Matters

Even a 1% increase in soil organic matter can improve nutrient holding capacity and biological activity, and usually water retention too, though how much varies by soil. Most degraded agricultural soils have OM levels below 2%; thriving soils aim for 5% or higher.

How It Works

Organic matter cycles continuously: plants capture carbon from the air, organisms consume plant material, and decomposition returns nutrients to the soil. Each cycle builds soil tilth, structure, and fertility.

Benefits

  • Improves water retention (how much varies by soil)
  • Feeds soil microbiota
  • Buffers soil pH
  • Increases cation exchange capacity
  • Reduces erosion

Sources: Journal of Soil and Water Conservation, University of Georgia Extension (C1299), USDA NRCS, via University of Hawaii CTAHR

How to Build Soil Organic Matter

Building organic matter is a multi-year process, but consistent inputs and reduced disturbance compound over time.

Add Compost and Worm Castings

These directly add organic matter, and castings arrive already partly processed into more biologically active, plant-available forms.

Plant Cover Crops

Keeping living roots and plant residue in the ground longer through the year adds a steady stream of organic inputs, even outside the main growing season.

Reduce Tillage

Tillage exposes organic matter to oxygen, accelerating its breakdown back into the atmosphere. Minimizing disturbance helps organic matter accumulate rather than deplete.

Expect a Multi-Year Timeline

Meaningful increases in organic matter typically take several growing seasons of consistent practice, even under favorable conditions.

Sources: Food and Agriculture Organization (FAO), University of Georgia Extension (C1299), USDA NRCS, via University of Hawaii CTAHR

Organic Matter vs. Humus vs. Fresh Residue

All are part of the same continuum — the difference is how far along decomposition each one has come.

Fresh Residue Active Organic Matter Humus
Stage of decomposition Recently added, largely unprocessed Actively breaking down Fully stabilized, resists further breakdown
Turnover speed Fast, quickly consumed by soil organisms Moderate, changes form as it decomposes Very slow, can persist for centuries
Nutrient release Rapid initial release as breakdown starts Ongoing release as decomposition continues Slow, steady, long-term release
Contribution to CEC Minimal until it starts breaking down Growing as decomposition progresses High, humus has among the highest CEC of any soil component
Best for Feeding soil biology and short-term nutrient supply The active engine of ongoing soil fertility Long-term soil structure and fertility

Sources: Food and Agriculture Organization (FAO), University of Vermont Extension, USDA NRCS Illinois (Soil Tech Note 13A), University of Georgia Extension (C1040)

Common Mistakes to Avoid

Organic matter is often reduced to a single percentage, but a few real habits shape how much of it actually builds up.

Citing "20x its weight in water" as a guaranteed field result
That figure comes from lab conditions for certain organic matter types. Real-world field impact is smaller and more variable, according to large-scale meta-analysis.
Tilling more than necessary
Tillage exposes organic matter to oxygen, accelerating its breakdown. Frequent or deep tilling works against the goal of building it up.
Expecting fast results
Meaningful increases in organic matter take multiple growing seasons of consistent practice, not a single application or year.
Treating all organic matter as equally stable
Fresh residue cycles quickly; humus can persist for centuries. A high organic matter percentage doesn't tell you how much of it is actually long-term stable carbon.

Sources: European Journal of Soil Science (Wiley), University of Georgia Extension (C1299), University of Vermont Extension

Frequently Asked Questions

What is soil organic matter? +
Organic matter (OM) refers to any living or once-living material in soil, plant residues, animal waste, microorganisms, and their decomposed remains. It is the foundation of a healthy, productive soil ecosystem.
What percentage of organic matter should healthy soil have? +
Most degraded agricultural soils measure below 2% organic matter, while thriving, biologically active soils often reach 5% or higher. Even a 1% increase can improve nutrient holding and biological activity, and usually water retention too, though how much varies by soil.
Is it true that organic matter holds 20 times its weight in water? +
Certain types of organic matter can hold up to 20 times their weight in water under laboratory conditions (Reicosky 2005). That said, a more recent large-scale meta-analysis (Minasny & McBratney 2018) found organic matter's real-world effect on field water capacity is often smaller than commonly assumed, so the practical impact varies by soil type and shouldn't be treated as a guaranteed multiplier.
What's the difference between organic matter and humus? +
Organic matter is the broader category, any living or once-living material in soil at any stage of decomposition. Humus is the specific, stable end product of that decomposition process, the most processed and longest-lasting fraction of organic matter.
How can I increase soil organic matter? +
Adding compost, worm castings, cover crops, and reducing tillage all build organic matter over time. Worm castings in particular arrive already processed into more biologically active, plant-available forms compared to raw, undecomposed material.
How quickly does organic matter break down? +
It depends on the fraction. The active fraction cycles relatively quickly, readily changing form as it decomposes and releasing nutrients along the way, while humus, the most stable fraction, can resist breakdown for centuries.
Does tilling reduce soil organic matter? +
Yes, generally. Tillage exposes organic matter to oxygen, accelerating its breakdown and release back into the atmosphere as CO2, which is part of why reduced-till and no-till practices are associated with higher organic matter levels over time.
Fed 'n Happy Insight

Adding worm castings is one of the fastest ways to increase biologically active organic matter — unlike raw compost, castings are already processed into plant-available forms.