Ground pressure, and what it does to soil

Who this is for

• Owners whose ground will be grazed, cultivated, or planted afterwards
• Anyone clearing on soft, wet, or heavy soil
• People deciding whether to wait for drier conditions

The short answer

Ground pressure is machine weight spread over the area actually in contact with the ground. A heavy machine on wide tracks can exert less pressure than a lighter one on narrow tyres.

Compaction happens when that pressure squeezes air out of the soil. The pore space that held air and water collapses, and what remains conducts water poorly and resists root growth.

Moisture is the multiplier. Wet soil has water in the pores acting as a lubricant, so particles slide past each other and rearrange under load. The same machine on the same ground does far more damage wet than dry.

Compaction in the upper layers can be relieved by cultivation. Deeper compaction is expensive to address and sometimes practically permanent.

What changes the answer

• Machine weight and contact area
• Soil moisture at the time of work
• Soil texture — clays compact differently from sands
• Number of passes over the same ground
• What the ground will be used for afterwards

What raises and lowers ground pressure

Two machines of the same weight can affect your soil completely differently, and the reason is contact area.

Why the first pass does most of the damage

This is the most useful thing to know about compaction, because it changes how you manage traffic rather than which machine you choose.

The largest change in soil density happens on the first pass over undisturbed ground. Subsequent passes over the same line add progressively less, because much of the available pore space has already gone.

The practical consequence is that concentrating traffic on designated routes is far better than spreading it evenly. Ten passes along one line damages one line badly; one pass each along ten lines damages ten lines substantially.

This is why a contractor who establishes a working route and sticks to it is protecting your soil, even though the route itself will look worse than if traffic had been spread around.

The residue layer as a cushion

There is a genuine interaction between the material left on the surface and the damage the machine does through it.

A layer of processed woody material distributes load and separates the tracks from the soil surface. On ground that has just been mulched, the machine is often driving on its own residue rather than on soil, which reduces both compaction and rutting.

This is one of the less obvious reasons in-place processing is gentler on soil than conventional clearing. Conventional clearing removes the material and then drives loaded vehicles over the exposed surface, which is the worst combination available.

It is not unlimited protection. A thin layer on saturated ground under a heavy machine will not prevent rutting, and the layer itself gets pressed into the soil.

When to wait, and what waiting is worth

Timing is the cheapest compaction control there is, and the hardest to arrange because it conflicts with schedules.

Working when soil is dry, or frozen where that occurs, dramatically reduces damage for the same machine and the same work. Working saturated ground maximises it.

The decision is a trade between schedule and soil. Where the ground will be grazed, cultivated, or planted, the soil usually deserves to win, because remediating compaction across an area costs more than a delay. Where the ground is becoming a building pad that will be excavated and engineered anyway, compaction matters much less.

This is worth raising explicitly rather than leaving to the contractor, whose incentive is to keep the machine working.

What compaction costs afterwards

The reason to care is that the consequences are agronomic and drainage-related rather than cosmetic.

Compacted soil conducts water poorly, so water sits on the surface or runs off rather than infiltrating. Roots struggle to penetrate it, so plants root shallowly and are more vulnerable to drought. Both effects reduce what the ground can grow for years.

Shallow compaction can be relieved mechanically, at a cost, where the surface can be cultivated. Deeper compaction is much harder to reach and in practice is often lived with.

Whether your soil is at risk and what would relieve it are local soil questions. A soil test and your state extension service are the appropriate route, not a national figure.

What to ask for, in practical terms

Ground pressure is a specification, and specifications are only useful once translated into requests you can actually make.

• Ask for the figure. A contractor who knows their machine will have it, and one who does not has told you something.
• Ask for designated traffic routes, and walk them before work starts so you agree where they run.
• Ask for exclusion zones around anything whose root zone matters, and mark them physically.
• Ask what happens if the ground is wetter than expected on the day, and whether there is a threshold at which they stop.
• Ask what the crossed surface will look like, and who is responsible for returning it to that condition.

What drives the cost

Common mistakes

• Judging a machine by weight alone rather than by ground pressure
• Allowing traffic to spread across ground that will be grazed or cultivated
• Working saturated soil to keep a schedule that was not critical
• Assuming a residue layer prevents all rutting

Questions to ask a contractor

• What is the ground pressure of the machine you are bringing?
• Where will machines travel, and can we fix those routes?
• Would waiting for drier ground change the result here?
• What will the soil surface be like where you have driven?

What to do next

1. Decide what the ground has to grow or carry afterwards
2. Ask for ground pressure figures and designated routes
3. Consider timing the work for drier conditions if soil matters
4. Get a soil assessment through your extension service if you intend to establish pasture

Questions people actually ask