Sub-soiling

Subsoiling to break up compacted ground

For fields and paddocks where water sits on the surface, grass is shallow-rooted and patchy, or years of machinery and hooves have packed the soil hard. A subsoiler breaks up the compacted layer below the surface so water and roots can get through again.

What’s involved

A subsoiler is a frame carrying one or more heavy steel legs that run through the ground at depth, commonly somewhere around 30 to 45 cm, set just below the compacted layer. As each leg passes it lifts and cracks the soil, shattering the pan without turning the topsoil over.

On grassland a low-disturbance subsoiler or sward lifter is used, with narrow legs and a roller behind, so the turf is barely marked and can go straight back into use. On arable ground a heavier, more aggressive machine may be used ahead of cultivation.

Before the job, dig a few holes with a spade. Compaction shows as a hard, dense layer where roots turn sideways and the soil smells sour or looks grey and mottled. Its depth decides how deep the legs need to run – going deeper than necessary wastes fuel and brings up poor subsoil.

When to do it

Subsoiling only works when the soil is dry enough at working depth to crack and shatter. In wet soil the leg just cuts a slot and smears the sides, which can make drainage worse. That usually means late summer or early autumn after a dry spell, before the winter rain.

On grassland, allow time for the grass to recover before heavy use or winter grazing, and ideally follow on with rolling to firm the surface.

What affects the price

Area of the field and the working depth needed

Soil type – heavy clay and stony ground need more power and time

Grassland machine or arable subsoiler

Slopes, field shape and obstacles such as trees and troughs

Known land drains, pipes or cables that need working around

Follow-on work such as rolling or overseeding

Good to know

Tell the contractor about any land drains, water pipes or buried cables. Subsoiler legs run deep enough to break them.

If you have land drains, subsoiling across them at an angle can help water reach the drains, but only if the drains themselves still work.

Subsoiling fixes compaction; it does not fix the cause. Keep machinery and horses off saturated ground, or the pan will re-form.

Not every wet field is compacted. If the problem is high groundwater or blocked drains, subsoiling alone will not solve it.

Sub-soiling: common questions

What does subsoiling do?

It breaks up a compacted layer, or pan, below the surface by pulling steel legs through the soil. The cracks let water drain away and roots grow deeper. It does this without turning the topsoil over.

When is the best time to subsoil?

When the soil is dry at working depth, usually late summer or early autumn. In wet conditions the legs smear rather than shatter the soil and can make things worse.

How do I know if my field is compacted?

Look for standing water after rain, shallow-rooted or patchy grass, and ground that is hard to push a spade into. Dig a hole: a dense layer where roots grow sideways, often with a grey or mottled colour, is a sign of a pan.