How to assess the soil before designing or planting

Assessing the soil does not start in the laboratory. It starts with learning the history of the site, opening profiles and watching how water moves.

Chemical analysis answers important questions about pH, organic matter and nutrients, but on its own it will not show a compacted layer, old fill or an area where water stands.

The design needs to combine observation, sampling and real behaviour.

Farmland, an old garden and a building platform have different profiles.

Ask where machinery has run, where soil has been tipped, what buildings once stood there and whether there are old drains.

Photographs of the building work, surveys and information from the owner help to explain differences the surface no longer shows.

An area that looks natural may be sitting on rubble or imported soil.

Self-sown vegetation, puddles, cracks, erosion and differences in colour all give clues.

Plants that are vigorous in one spot and weak in another may point to water, depth or compaction.

The surface also shows where people walk and where water collects.

These observations guide where it is worth opening up the soil.

The pits should represent the zones of the design and reach the depth that matters for the planting.

Look at colour, texture, aggregation, stones, roots, moisture, smell and transitions.

Abrupt layers may indicate fill. Greyish tones and rusty mottling suggest periodic saturation. A smooth, resistant face may reveal compaction.

A single profile does not necessarily represent the whole site.

A small moist sample can be worked between the fingers.

Sand feels gritty and will not hold a shape. Clay is plastic and forms a ribbon. A loam behaves somewhere in between.

This test is a guide, but it does not replace particle-size analysis when the design needs precision.

Organic matter, stones and structure change the behaviour over and above texture.

A rod, spade or auger lets you compare resistance. A penetrometer gives a more systematic reading where needed.

The aim is not just to get a number. What matters is the depth and continuity of the layer.

Dry soil offers more resistance than moist soil, so the conditions of the test should be recorded.

Compaction in a paving sub-base is intentional; in a root zone, it is a limitation.

A simple test is to fill a hole with water and time how long the level takes to drop.

The initial moisture affects the first measurement. It can be useful to saturate the hole and repeat.

Compare several points and note whether the water drains away downwards or through cracks in the sides.

The test does not replace a hydrogeological study, but it identifies areas that need closer attention.

Water may appear only in winter or after prolonged rain.

Trial holes dug on a dry day may not reveal the seasonal condition. Local knowledge, vegetation and mottling in the profile help to reconstruct it.

Also look for springs, run-off from higher ground and discharges from roofs.

Each sample should represent a uniform zone. Mixing soil from a dry bank with soil from a saturated low spot gives an average of little use.

Take several subsamples at the set depth, mix them and label them.

The laboratory can analyse pH, organic matter, conductivity, texture and nutrients, depending on the aim.

Interpretation needs to consider species and use, not just compare against a generic table.

pH influences nutrient availability and biological activity.

Correction is possible in some situations, but permanently changing large volumes of soil can be difficult.

Where the ground is very alkaline or acid, selecting compatible plants can be more sustainable than continual correction.

Irrigation water can also change the condition over time.

A low percentage may justify mature compost and mulch. A high figure does not guarantee good structure if the soil is compacted.

The source and maturity of the material matter. Unstable compost uses up oxygen and can cause problems.

Incorporation should be matched to the volume, not concentrated at the bottom of the planting hole.

The assessment defines zones, preparation, drainage, species and irrigation.

A sandy bank can take adapted plants and mulch. A low spot can be regraded or planted with moisture-tolerant species, provided there is no damaging waterlogging.

Mature trees are positioned where there is volume and access to prepare the soil.

The design does not need to eliminate all variation. It can use it.

Excavation reveals conditions the trial holes did not find.

The team should have criteria for stopping, recording and deciding on changes. Replacing soil, installing drainage or moving a tree should not be an informal decision made without sign-off.

Soil assessment continues during construction and through the first years of maintenance.

• Soil, water and drainage
• Clay soil and compaction
• Sandy and poor soils

1. European Soil Data Centre.
2. LUCAS Soil — framework and sampling methods.
3. SoilGrids — supporting map information, no substitute for the local profile.
4. Standard methods of agronomic and landscape soil analysis.