Understanding Soil Compaction in Ontario
Soil compaction occurs when soil particles are pressed together, reducing pore space between them. This reduces the soil's capacity to hold air and water — the two essential components that roots need to function. In ideal growth conditions, topsoil should be approximately half solids (mineral and organic fractions) and half pore space, with the pore space roughly equally divided between air and water. Compaction disrupts this balance, creating dense layers that restrict root penetration, limit water infiltration, and reduce biological activity year after year.
Common Problems We See
Most Ontario soil types are at risk of compaction if they are not in good condition. The risk is highest when soils are trafficked or tilled under wet conditions — a common scenario in Ontario's spring seasons. Fine-textured soils (clay loams, silty clay loams, and clays) are particularly susceptible because their smaller particles can be packed more tightly than sandy soils.
How Compaction Develops
Subsurface compaction on Ontario farms typically develops through two primary mechanisms:
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Tillage-induced compaction (plow pan) — Repeated tillage to the same depth creates a dense layer at the bottom of the tillage zone. On fields that have been moldboard plowed at 15 cm (6 inches) depth for years, a compaction layer forms at exactly that depth. The plow sole becomes progressively denser with each tillage pass, particularly when tillage occurs under borderline moisture conditions.
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Traffic-induced compaction — Modern farm equipment is significantly heavier than the machinery of previous generations. Axle loads of 10–20 tonnes are common during harvest, manure application, and grain cart operations. This weight compresses soil well below seedbed depth, creating compaction that conventional tillage cannot reach or remediate.
Common Problems We See
Evidence of compaction includes: stunted plants along traffic lanes, restricted root development visible when plants are pulled from the soil, horizontal root deflection at the compaction layer, standing water in wheel tracks after rain events, and increased tire slip during field operations. This issue is commonly uncovered during a farmland health checkup.
Measuring Compaction in the FHCU
The Farmland Health Check-Up uses penetrometer testing to measure soil resistance at multiple depths across each field. A penetrometer is a pointed probe pushed into the soil that measures the force required to penetrate — expressed in pounds per square inch (psi).
Key Takeaway: Generally, root growth becomes restricted above 300 psi and severely limited above 400 psi.
The FHCU also evaluates compaction through visual soil profile examination. A shallow pit or spade test reveals the depth, thickness, and severity of compaction layers, as well as root penetration patterns. On compacted soils, roots grow horizontally along the top of the compacted layer rather than penetrating downward — a clear indicator that the crop is unable to access moisture and nutrients in the lower soil profile.
Compaction and Yield Loss
Documented Yield Impact
Research across Ontario's major soil types consistently demonstrates that subsurface compaction reduces crop yields by 10–30%, depending on severity, soil type, and seasonal moisture conditions. The yield impact is most severe in dry years, when compacted soils prevent roots from accessing subsoil moisture reserves.
In wet years, compacted soils drain slowly, delaying planting and creating saturated conditions that suppress root function and promote denitrification of applied nitrogen.
Why This Matters for Yield
On a 200 bu/ac corn field, a 15% yield reduction from compaction represents 30 bu/ac — or approximately $165/ac in lost revenue at $5.50/bu. Across 100 acres of compacted field, that's $16,500 in annual lost income from a single, addressable problem. See the full analysis in our compaction yield loss page.
Remediation Strategies
Compaction remediation depends on the depth and cause of the problem:
Surface (0-10 cm)
Often self-correcting through freeze-thaw cycles in Ontario's climate, provided the soil is not re-compacted in spring
Plow Pan (10–25 cm)
Can be addressed through strategic deep tillage (subsoiling) to a depth below the compacted layer, ideally under dry soil conditions in late summer or early fall
Deep (>25 cm)
Caused by heavy axle loads, this compaction is below the reach of most tillage tools and requires a long-term approach combining controlled traffic patterns, deep-rooted cover crops, and reduced axle loads
