Understanding Soil Organic Matter in Ontario
Soil organic matter (SOM) is the fraction of the soil composed of decomposed plant and animal residues, microbial biomass, and humus — the dark, stable organic compounds that give productive soils their characteristic colour. In Ontario, SOM typically ranges from 1.5% to 6% in mineral soils, depending on soil type, landscape position, drainage class, and management history. Organic soils (muck soils) in areas like the Holland Marsh may contain 30–80% organic matter.
A Declining Resource
Virgin forest and prairie soils in southern Ontario typically contained 5–8% organic matter. After 150+ years of cultivation, many Ontario fields now contain 2–3.5% SOM — a 40–60% decline from original levels.
Key Takeaway: Each 1% increase in SOM in a silt loam soil increases plant-available water-holding capacity by approximately 15–20 mm in the root zone — the equivalent of one moderate rainfall event.
Components of Soil Organic Matter
Soil organic matter is not a single substance but a continuum of materials at different stages of decomposition:
Fresh Residues (0–10% of SOM)
Recently deposited crop residues, root exudates, and manure. These materials are actively decomposing and represent the primary food source for soil microorganisms. In Ontario's climate, surface residues from corn stalks may take 2–3 years to fully decompose, while soybean residues break down within one season.
Active Organic Matter (10–20% of SOM)
Partially decomposed materials and living microbial biomass. This fraction — measured as active carbon (POXC) or soil protein (ACE protein) — turns over on a scale of months to years and is the most responsive to management changes. Active organic matter drives nutrient mineralization, aggregate formation, and biological pest suppression.
Stable Humus (60–90% of SOM)
Highly decomposed, resistant organic compounds with turnover times of decades to centuries. Humus provides the long-term cation exchange capacity, water-holding capacity, and dark colour associated with productive soils. Building humus is a slow process — it takes consistent organic matter additions over many years.
Why Ontario Soils Are Losing Organic Matter
The decline of SOM on Ontario farms is driven by several interacting factors:
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Intensive tillage — Moldboard plowing inverts the soil, burying residues deep and exposing previously protected organic matter to rapid oxidation. Each tillage pass introduces oxygen into the soil profile, stimulating microbial decomposition of stored carbon. University of Guelph long-term trials show that continuous moldboard plowing reduces SOM by 0.02–0.05% per year compared to no-till management.
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Continuous row cropping — Corn and soybeans produce less root biomass and return fewer residues to the soil than perennial forages or small grains. A continuous corn-soybean rotation without cover crops returns approximately 3–5 tonnes of residue per hectare annually, compared to 8–12 tonnes from a rotation including hay or perennial forages.
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Erosion — Topsoil lost to water and tillage erosion carries the highest concentration of organic matter. On eroded knolls, where 20–40 cm of original topsoil has been displaced, SOM levels are often below 2% — compared to 3.5–4.5% on adjacent depositional areas.
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Removal of livestock — The historical decline of mixed farming in Ontario has reduced regular manure applications that once maintained organic matter levels.
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Drainage improvement — While essential for crop production on Ontario's clay soils, improved drainage lowers the water table, increases soil aeration, and accelerates organic matter decomposition.
Measuring Organic Matter in the FHCU
The Farmland Health Check-Up evaluates organic matter through several complementary approaches:
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Laboratory SOM analysis — Soil samples are analyzed for total organic matter content using the loss-on-ignition (LOI) method. This provides the baseline SOM percentage for each field and allows comparison between your three assessment fields and county averages.
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Active carbon (POXC) — Permanganate-oxidizable carbon measures the biologically active fraction of organic matter. Values above 600 mg/kg are generally considered good for Ontario mineral soils.
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Soil protein (ACE protein) — Autoclaved citrate-extractable protein measures the nitrogen-rich organic compounds in soil. Higher soil protein levels indicate a soil with greater capacity for self-sustaining fertility.
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Visual indicators — Soil colour, earthworm counts, residue decomposition rates, and aggregate stability all provide visual evidence of organic matter status.
What This Means on Your Farm
By comparing SOM and its active fractions across your three fields, the FHCU identifies whether organic matter depletion is contributing to yield differences and provides a baseline for tracking improvement over time. Book your free checkup to find out where your fields stand.
Organic Matter and Nutrient Supply
SOM is the primary reservoir of soil nitrogen (N), phosphorus (P), and sulphur (S) in organic forms. As microorganisms decompose organic matter, these nutrients are released in plant-available forms through a process called mineralization.
Free Nitrogen from Organic Matter
In Ontario, a soil with 3.5% SOM will mineralize approximately 40–60 kg N/ha annually under normal conditions — a significant contribution that reduces fertilizer requirements. Fields with higher SOM can receive a nitrogen credit of 10–20 kg N/ha.
Active carbon and soil protein provide a more precise estimate of mineralization potential than total SOM alone. Two fields with identical total SOM (e.g., 3.5%) may have very different active fractions — and therefore very different capacity to supply nitrogen to crops. The FHCU captures this distinction, helping explain why some fields respond to reduced nitrogen rates while others do not.
Building Organic Matter: OMAFRA Best Management Practices
Increasing SOM is a long-term process — typically requiring 5–15 years of consistent management changes to achieve measurable increases of 0.5–1% in total SOM. OMAFRA recommends the following strategies:
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Cover crops — Annual cover crops after cash crop harvest add 1–3 tonnes of root and shoot biomass per hectare. Cereal rye is the most reliable cover crop for Ontario, establishing well after corn or soybean harvest and providing winter ground cover.
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Reduced tillage — Transitioning to no-till or reduced-till slows organic matter decomposition. Long-term no-till fields in Ontario show 0.3–0.8% higher SOM in the surface 15 cm compared to conventionally tilled fields.
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Extended rotations — Including small grains and perennial forages in rotation increases the quantity and diversity of organic inputs.
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Manure and compost — Solid cattle manure applied at 10–15 tonnes/ha provides approximately 2–4 tonnes of organic matter. Composted materials contribute more to the stable humus fraction.
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Reducing erosion — Preventing topsoil loss is essential for maintaining the organic matter that has accumulated in the surface horizon.
Ontario-Specific Benchmarks
Expected SOM ranges for Ontario's primary agricultural soil types:
Sandy soils (Fox, Berrien)
1.5-2.5% SOM
Inherently low organic matter due to high aeration and rapid decomposition.
Loamy soils (Guelph, Harriston, Huron)
3.0–4.5% SOM
Best capacity for organic matter accumulation due to clay-organic matter complexes.
Clay soils (Brookston, Toledo, Haldimand)
3.5–5.5% SOM
Can maintain high SOM due to physical protection within clay aggregates.
Eroded knolls
1.5–2.5% SOM
Regardless of soil type. Erosion removes the organic-rich topsoil.
The FHCU compares your fields' SOM levels against these regional benchmarks and against each other, providing context for whether organic matter depletion is contributing to yield gaps between your best and worst performing fields.
