Sandy, poor soils: how to improve them without creating an isolated planting hole

Sandy soils have large pores, drain quickly and store less water and fewer nutrients. These characteristics do not make them unproductive, but they call for different preparation and watering from a clay soil.

The improvement should cover a wide area and keep a transition with the existing soil. Creating a small rich pocket within poor sand concentrates the roots and does not solve the future volume.

A pale, loose soil may not be uniform sand.

Decomposed granite, made ground and construction mixes can have a free-draining surface layer over compact material. Water passes through the first horizon and is held below.

The profile should be opened up to look at depth, stones, organic matter and transitions.

A texture analysis confirms the proportions of sand, silt and clay.

Rapid infiltration reduces the time available for uptake.

Watering too fast can drive water below the root zone. A single emitter creates a narrow column and leaves much of the rootball dry.

It may be necessary to split the application into cycles, use several points and water more often than on a heavy soil.

Soil cover reduces evaporation from the surface, but it does not completely change the low retention of the profile.

Mature compost increases water retention, exchange capacity and biological activity.

It is incorporated in moderation into the prepared volume. A concentrated layer at the bottom, or a pit filled with compost alone, causes settlement and behaves differently.

The improvement should continue through mulching and top-ups over the years.

Manures or immature materials should not be placed in contact with roots.

Woodchip, bark or another organic mulch protects against heat, reduces evaporation and feeds the soil as it breaks down.

The layer is kept away from trunks and stems.

On banks, the material needs to be stable so that it is not washed away. Ground-cover plants can complement it and protect the surface.

Aggregates reduce evaporation, but they can heat up more and add no organic matter.

Nutrients move and leach more easily.

Feeding should be based on analysis and, when needed, split into moderate applications. A high dose increases losses and salinity without guaranteeing uptake.

Organic matter helps to hold nutrients, but it is no substitute for choosing adapted species.

Pale leaves can be the result of water, pH or roots, and not just a lack of fertiliser.

Mediterranean species and those from free-draining environments can respond well, provided that hardiness and exposure are compatible.

A plant that is drought-tolerant once established still needs water while it establishes.

Large trees with high transpiration need more volume and aftercare. In some places, the size planned should be reduced.

The selection should also consider pH, which can vary a good deal in sands of different origin.

The improvement extends beyond the rootball or the pot.

Organic matter is mixed with the existing soil, avoiding an abrupt boundary. The roots should find conditions that change only gradually.

The base stays free-draining and stable. The pit is not deepened without checking the layer below.

Over large areas, it can be more effective to prepare whole beds or strips than individual pits.

The first watering needs to wet the whole rootball. Over the following days, check whether the centre is holding moisture.

Several emitters spread around the tree improve uniformity. As the roots grow, they are moved away from the base.

The programme is adjusted to heat and wind. In sand, a short spell without water can quickly cause stress.

Small plants have shallow rootballs and dry out fast. In the first weeks, they need watering more often.

The reduction is gradual, encouraging deeper rooting without setting back establishment.

Density and ground cover help to shade the soil once the planting closes up.

Water can soak in quickly on level ground and carry particles away on a bank.

Ground modelling, the rate of watering and surface protection should be coordinated.

On slopes, planting in tiers, small basins and ground-cover vegetation reduce losses.

Structure and organic matter change over time.

Water use, growth and the breakdown of the mulch are observed. The maintenance plan tops up the cover and adjusts the feeding.

The aim is not to turn sand into clay. It is to increase the capacity of the soil and to choose plants that can live within the limits that remain.

• How to assess the soil
• Organic mulch or aggregates
• How to programme irrigation

1. European Soil Data Centre.
2. Literature on the management of sandy soils and organic matter.
3. Principles of irrigation efficiency in landscaping.