This factsheet explains the nature and properties of soil organic matter (SOM) and describes its benefits for crop production. It provides guidance on management practices that can be used to increase SOM in agricultural soils, with specific references to Ontario research. Read on to learn more.
Why organic matter matters
Soil organic matter makes up only a small portion of soil, but it has a large impact on soil functions. Building and maintaining SOM helps to:
- Support higher and more stable crop yields.
- Improve tolerance to drought and other stresses.
- Provide greater quantities of soil-supplied nutrients.
- Improve soil structure, aggregate stability and trafficability.
Organic matter is part of a dynamic, ongoing biological cycle involving plants, soil life, minerals, air, water, and amendments such as manure.
How organic matter exists in soil
Organic matter exists as a continuum of materials that cycle through the soil at different rates, rather than as distinct, fixed categories. There are three primary types of SOM, as defined by speed of cycling:
1. Fresh inputs and living biology (fast‑cycling SOM)
This part of organic matter turns over quickly, from days to a few years. It includes living organisms, such as bacteria and fungi, as well as fresh plant residues and recently applied manures. This pool:
- Drives decomposition and nutrient release.
- Produces sticky substances that help form and stabilize soil aggregates.
- Feeds the soil food web and fuels biological activity.
2. Microbial residues and processed organic matter (intermediate SOM)
As microorganisms break down residues, much of the original plant material is transformed into microbial by‑products and dead microbial cells. This pool:
- Supplies a steady release of nutrients.
- Plays a key role in building soil structure.
- Is the connection between fresh residues to longer‑lasting organic matter.
The efficiency of the microbial processing of fresh organic materials controls how much is converted into stable organic matter versus released to the atmosphere as carbon dioxide. Higher microbial efficiency is believed to help build SOM faster. While the science is evolving, efficiency is generally understood to be enhanced by adequate availability of soil nutrients (especially nitrogen), presence of high-quality materials with a low carbon-to-nitrogen ratio, optimal microbial growth conditions (e.g., not too dry), and good soil structure.
3. Mineral‑associated and protected organic matter (slow‑cycling SOM)
Rather than being “untouchable” or unable to be processed by the soil system, long‑lived organic matter is believed to persist because it is physically protected inside aggregates or chemically attached to clay and silt particles. This pool:
- Provides much of the soil’s long‑term carbon storage.
- Supplies slow, steady nutrient release.
- Improves water‑holding capacity, especially in sandy soils.
- Supports stable soil structure over time.
This material can persist for decades to centuries, depending on soil texture, management, and disturbance, though emerging evidence suggests that some types of mineral-associated organic matter can be fast-cycling (Jilling et al., 2025).
Soil organic matter effects
Organic matter influences nearly every soil function. Below are a few key effects of SOM on soil:
- Cycles and stores nutrients, both by directly holding nutrients like nitrogen and sulphur, and by increasing retention of cations like magnesium and potassium.
- Improves soil structure, with enhanced pore space for air and water movement. This results in greater water infiltration, reduced runoff, better resistance to compaction and improved root growth.
- Helps buffer against changes in pH, including after nitrogen fertilizer applications.
Impact of management practices on soil organic matter
Ontario researchers have identified several key practices that contribute to increased soil organic matter under the province’s unique climatic and soil conditions. They include:
- Perennials and small grain crops grown in rotation. Perennials are associated with higher soil organic matter, according to both small plot research (Jarecki et al., 2018) and a province-wide soil sampling campaign (Saurette et al., 2024). Small grain cereals, such as winter wheat, also increase organic matter when included in rotation (Van Eerd et al., 2014).
- Application of organic amendments. Sources that have a higher percentage dry matter and carbon content (e.g., solid cattle manure) have a greater positive effect than low-carbon amendments such as liquid hog manure (Liang et al., 2021).
- Cover crops, which have been found toincrease organic carbon – a component of SOM – in soil by 8% relative to no cover crop controls, based on 19 long-term trials across North America (Peng et al., 2023). Locally, a long-term trial on a sandy loam soil in Ridgetown, found that cover crops in a field crop-processing vegetable rotation resulted in greater organic carbon compared to no cover crop controls (Chahal et al., 2020).
No-till practices are generally not associated with higher overall levels of SOM relative to conventional tillage under Ontario’s climate and soil conditions. No-till has been shown to redistribute organic matter closer to the soil surface (Deen and Kataki, 2003). One exception is the long-term rotation, tillage system trial at Ridgetown, which has shown an increase in SOM attributable to no-till in the top one metre of soil (Van Eerd et al., 2014).
Bottom line
Soil organic matter is a dynamic soil property that increases or decreases over time depending on:
- The regularity and quantity of carbon inputs.
- How efficiently microbes process organic materials.
- The protection of carbon within soil structure and minerals.
Practices such as diversified crop rotations, application of organic amendments, and cover cropping – which contribute carbon, protect aggregates, and maintain living roots – are critical to building SOM over time under Ontario conditions.
Initial draft generated by Microsoft Copilot. Revised, fact-checked and re-written by Jake Munroe, OMAFA.
References
Deen, W. and Kataki, P.K. 2003. Carbon sequestration in a long-term conventional versus conservation tillage experiment. Soil & Tillage Research. 74 (2): 143-150. https://doi.org/10.1016/S0167-1987(03)00162-4.
Jarecki, M., Grant, B., Smith, W., Deen, B., Drury, C., VanderZaag, A., … & Wagner‐Riddle, C. 2018. Long‐term trends in corn yields and soil carbon under diversified crop rotations. Journal of environmental quality. 47(4), 635-643. https://doi.org/10.2134/jeq2017.08.0317.
Jilling, A., Grandy, A.S., Daly, A.B., … & Whalen, E.D. 2025. Evidence for the existence and ecological relevance of fast-cycling mineral-associated organic matter. Communications Earth & Environment. 6:690. https://doi.org/10.1038/s43247-025-02681-8.
Liang, C., Hao, X., Schoenau, J., Ma, B. L., Zhang, T., MacDonald, J. D., … & Angers, D. 2021. Manure-induced carbon retention measured from long-term field studies in Canada. Agriculture, Ecosystems & Environment. 321: 107619. https://doi.org/10.1016/j.agee.2021.107619.
Peng, Y., Rieke, E.L., Chahal, I., Norris, C. E., Janovicek, K., Mitchell, J.P., … & Van Eerd, L.L. 2023. Maximizing soil organic carbon stocks under cover cropping: Insights from long-term agricultural experiments in North America. Agriculture, Ecosystems & Environment. 356: 108599. https://doi.org/10.1016/j.agee.2023.108599.
Saurette, D., Burns, T., Blackford, C., Warren, J., Arntz-Gray, J., Kelly, R., Jefferies, D., Munroe, J., Nemeth, D. Ontario Topsoil Sampling Project: Soil Health Baseline Study. 2024. Ontario Ministry of Agriculture, Food and Agribusiness. https://www.ontario.ca/page/ontario-topsoil-sampling-project-2024.
Van Eerd, L.L., Congreves, K.A., Hayes, A., Verhallen, A., Hooker, D.C. 2014. Long-term tillage and crop rotation effects on soil quality, organic carbon, and total nitrogen. Canadian Journal of Soil Science. 94: 303-315. https://doi.org/10.4141/CJSS2013-093.