Clay soil and compaction: what changes at planting

A clay soil can be fertile, store water and support large trees. The problem arises when it loses structure, is worked wet or is compacted by machinery.

In that state, water infiltrates slowly, oxygen falls and roots meet great resistance. In summer, the same soil can dry out, harden and crack.

Preparation needs to improve continuity, and not just replace the soil inside a planting hole.

A clay texture means a high proportion of fine particles.

When these particles form stable aggregates, there are pores for water and air. The soil can have good fertility and holding capacity.

When the aggregates are destroyed, the mass becomes dense. The limitation lies in structure and drainage, not just in the amount of clay.

Two clays can behave very differently.

Clay holds water and moves it slowly.

After rain, the profile can stay wet for a long time. If the pores remain filled, the roots lose oxygen.

During drought, part of the water is tightly bound to the particles and becomes hard to take up.

The watering schedule needs to take this alternation into account.

Machinery passing over wet soil compresses and smears the profile.

A layer of topsoil spread at the end can hide the problem without correcting it.

Future planting areas should be protected before earthworks begin. Avoiding compaction is more effective than trying to undo it afterwards.

Where it has already happened, decompaction needs to reach the depth and the extent affected.

The spade goes in with difficulty, the sides of the excavation are left smooth and roots are concentrated in the cracks or at the surface.

Water stands in puddles or runs off without soaking in. Once dry, the soil forms hard clods.

Profiles may show a dense layer beneath looser soil.

Resistance should be compared with other areas and observed at a known moisture condition.

Filling a hole with light substrate inside compacted clay creates a container.

Water enters the loose material and is slow to leave. Roots meet a dense wall and stay in the prepared zone.

The tree can suffer from saturation in winter and dryness in summer, because the isolated volume is small.

The solution needs to open the ground up and connect sideways, not just swap what is inside.

The method depends on depth, moisture and the proximity of structures.

It may involve scarifying, subsoiling, tine work or hand work close to roots and services.

The soil should not be worked when it is plastic and saturated, because it loses structure again.

Afterwards, traffic that would undo the work is avoided.

Mature compost encourages aggregation, biological activity and infiltration.

It should be applied in moderation and worked into a wide volume. Excessive amounts cause settlement and differences in texture.

An organic mulch on the surface goes on feeding the soil and reduces the impact of rain.

Improvement is gradual; it does not turn compacted clay into a loose soil in a single day.

Before planting, it is established where the water will go.

A drain only works with a fall and an outlet. A layer of gravel at the bottom of the planting hole does not solve saturation.

In some places, raised ground modelling offers a better solution than digging deeper.

The position of the tree may need to change.

Several trees tolerate heavy soils when there is structure and drainage. Few tolerate a permanent lack of oxygen.

Selection should be based on the conditions that will remain after the works.

Choosing a species ‘for clay’ does not make up for a layer compacted by construction.

The ground is prepared over a wide area. The glazed sides are broken up and the base stays firm enough to support the rootball.

The root collar sits at the surface. The initial watering settles the soil, without turning it into a compressed mass.

Mulch reduces drying out and stops lawn competing directly with the tree.

Intervals tend to be longer than on sand, but each application needs to reach the rootball.

Before watering again, check that the profile has drained.

Automatic systems should allow the frequency to be changed without cutting the volume to a superficial application.

Clay soil does not need to be replaced in every case. It needs to be understood and protected so that it keeps the structure that allows roots to function.

• How to assess the soil
• Drainage at planting
• Waterlogging

1. European Soil Data Centre.
2. Literature on soil physics, structure and compaction.
3. European Arboricultural Standards. European Tree Planting Standard.