Soil compaction: tyres, tracks and smarter field traffic

Two kinds of compaction, two different causes

It helps to separate compaction into two layers, because they have different causes and different fixes. Topsoil compaction, roughly the top 30 cm, is driven mainly by contact pressure: how hard the tyre or track presses on the surface. As a rule of thumb, contact pressure is close to the tyre inflation pressure, so a tyre run at a road pressure in the field presses hard on a small area.

Subsoil compaction, below cultivation depth, is driven mainly by the total load on the wheel or axle. A big, soft tyre spreads the load at the surface, but a very heavy load still pushes stress deep into the profile. That is why a 16-tonne grain cart on flotation tyres can still damage the subsoil, and why fitting wider tyres to a heavier machine is not a complete answer.

The practical difference is that topsoil damage can be loosened by normal cultivation, frost and roots over a season or two. Deep subsoil damage largely stays put. Penn State Extension notes that subsoil compaction is not relieved by freeze-thaw or wetting-drying cycles on any soil type, and summarises long-term trials in which compaction from 10 to 12 ton axle loads cut yields by around 15% in the first year, with a loss of about 3% still present a decade later.

The numbers worth remembering

AHDB's guidance on soil compaction from machinery gives two useful rules. The first is its 8-8 rule: under wet conditions, the depth of compaction beyond 30 cm increases by about 8 cm for each extra tonne of wheel load above 1 tonne, and by about 8 cm for each doubling of tyre inflation pressure above 1 bar. The second is a warning that wheel loads above 3.5 tonnes can cause serious, permanent, deep-seated compaction even with large low-pressure tyres.

Penn State Extension puts the US version in axle terms: keep axle loads below 10 tons, and preferably below 6 tons, and keep tyres inflated to around 100 psi, such as truck road tyres, out of the field entirely. Note that a US ton is 2,000 lb (about 907 kg), so 10 US tons is roughly 9 tonnes per axle, or about 4.5 tonnes per wheel on a single-tyre axle. That is already above AHDB's 3.5 tonne wheel-load warning, which shows how easily modern kit crosses the line.

To see where your own machines sit, you need real weights, not brochure figures. Weigh loaded trailers and carts on a weighbridge at least once, and remember that a full grain tank or slurry tanker can double the axle load compared with running empty.

• Wheel load above 3.5 t: AHDB flags a risk of permanent deep compaction.
• Axle load: keep below 10 US tons (about 9 t), preferably below 6 tons (about 5.4 t), per Penn State.
• Inflation pressure: each doubling above 1 bar adds roughly 8 cm to compaction depth in wet soil (AHDB 8-8 rule).
• Timing: AHDB advises staying off fields for 48 hours after heavy rain.

Soil moisture decides how much damage a pass does

The same tractor can do little harm on a dry, firm field and serious harm on a wet one. Wet soil loses strength, so the same load pushes deeper. AHDB advises avoiding traffic where any layer in the top 40 cm is above its plastic limit. A quick field test is to roll a sample from the working depth between your palms: if it forms a thin worm about 3 mm across without crumbling, it is too wet to travel on without risk.

This is where timing decisions become machinery decisions. A farm that owns enough capacity to wait for the right conditions will do less damage than one that must go on the first possible day because the drill or harvester is booked elsewhere. When you size kit, think about how many days of genuinely suitable conditions you usually get, not the number of calendar days in the window.

Tyres: pressure, construction and what VF actually means

Correct pressure is the cheapest improvement available. Tyre makers publish load and pressure tables that give the minimum pressure for a given load and speed. Running at that minimum in the field, rather than a single year-round road pressure, lengthens the footprint and lowers contact pressure. Our companion guide to tyre pressure and ballast covers how to work this out for your own tractor.

IF (Improved Flexion) and VF (Very High Flexion) tyres use stronger sidewalls so they can run lower pressures. Michelin describes VF tyres as able to carry up to 40% more load at the same pressure than a non-VF radial of the same size; put the other way, they can carry the same load at a lower pressure. That helps topsoil, but it does not reduce the weight on the axle, so the subsoil still sees the full load.

Central tyre inflation systems let the operator switch between road and field pressures from the cab. They make most sense on machines that swap between road and field several times a day, such as tractors on trailer or tanker work. On a tractor that stays in one field all day, a good gauge and a routine can deliver most of the benefit.

Tracks versus tyres

Rubber tracks spread weight over a long footprint and usually give lower average ground pressure than a single tyre. They are not a cure-all. Pressure under a track is not even: it peaks under the bogie wheels and idlers, so a poorly tensioned or lightly-wheeled track can still produce pressure spikes. Tracked machines are also often heavier overall, and the total axle-equivalent load is what reaches the subsoil.

When comparing a tracked and a wheeled option, ask the dealer for the machine's operating weight, the number of mid-rollers per track, and any independent ground-pressure measurements. Then compare against a wheeled machine on correctly set VF tyres, not against the same machine at road pressure. You can check operating weights and tyre sizes for shortlisted models through FarmFleets search or the tractor category.

Controlled traffic: limiting the damaged area

Controlled traffic farming (CTF) accepts that some soil will be compacted and confines it to permanent wheelways, leaving the rest of the field untrafficked. AHDB describes it as isolating the compaction exerted by vehicles into narrow ribbons within cropped fields. It relies on matching implement widths (for example drill, sprayer and combine header on multiples of the same base width) and on accurate guidance, usually RTK-level auto-steer, so wheels follow the same lines every year.

Full CTF is hard where the combine header, trailer track width and drill do not match, which is common on mixed farms. A partial approach still helps: use permanent tramlines for the sprayer and spreader, keep grain trailers and chaser bins on headlands or fixed lanes, and avoid diagonal runs across the field. Every pass you can move onto an existing wheeling is a pass that no longer damages new ground.

Finding and fixing compaction

Before buying a subsoiler, dig. Open several test pits to about 50 cm in good and poor areas of the field and compare structure, root growth and any plates or smeared layers. A penetrometer helps map resistance, but readings depend heavily on moisture, so take them when the field is near field capacity and compare like with like.

If you find a compacted layer, loosen only to just below it. AHDB suggests using targeted cultivations in suitable conditions and checking the result by digging again. On grassland, a sward lifter can remove compaction in sub-surface layers below about 10 cm. Subsoiling wet soil, or going far deeper than the problem, wastes fuel and can leave the soil weaker and easier to re-compact. Deep loosening is also no substitute for lighter loads: the next heavy pass on wet ground will undo it.

A practical checklist before the next season

Use the list below as a starting point for a compaction review of your own fleet.

• Weigh your heaviest loaded machines (grain cart, slurry tanker, combine with a full tank) and work out wheel and axle loads.
• Compare each against the 3.5 t wheel-load and 10 ton (preferably 6 ton) axle-load guidance above.
• Check tyre sizes against the manufacturer's load and pressure table and set field pressures accordingly.
• Agree a simple rule for when soil is too wet to travel, such as the 3 mm roll test, and give operators authority to stop.
• Plan fixed routes for trailers and carts, and keep them off the cropped area where you can.
• Dig test pits before and after any subsoiling to confirm it is needed and that it worked.

Sources

Sources checked 2026-09-25

• AHDB: Soil compaction from machinery — checked 2026-09-25
• Penn State Extension: Effects of Soil Compaction — checked 2026-09-25
• Michelin: UltraFlex (IF/VF) tyre technology — checked 2026-09-25

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