You may need to buy in topsoil if the existing soil is poor, damaged or if the space to be gardened has no natural soil, such as a courtyard or patio garden.

Many gardens have poor soil, such as those found behind new-build homes, where the natural topsoil has been stripped away during the construction process. Topsoil can be used to cover the ground, to create new beds, borders or to provide a base for turf laying or sowing grass seed.

In paved gardens where there is no access to soil, topsoil can be used in raised beds for growing many plants, including vegetables.

Topsoil is generally available to buy in three different grades: premium, general-purpose grade and economy grade.

• Premium grade: This can be expensive but should not bring in weed seeds. High in fertility, this loamy soil has good structure and is commonly used to build up flower borders or create new beds, while nurseries use it in compost mixes for container grown plants
• General-purpose grade: This can be bought in differing screen size grades. Also good for making new beds or borders, or as a base for sowing new lawns. Coarser grades are particularly useful for turf laying while finer grades are good for top dressing lawns
• Economy grade: This tends to be supplied ‘as dug’ (unscreened) and is commonly used to build up areas where volume rather than quality is required

To avoid later problems, it is best to check the topsoil before buying it. Look out for high stone content, thick fibrous roots, weeds and contaminants such as glass and brick. Inspection will enable any concerns you may have to be discussed before purchasing it.

Ask the supplier where it is coming from and whether it all has the same origin (building sites are often used as a source and the quality can be variable). If the supplier has had soil analysis undertaken, request a copy. If they claim it is good and state facts and figures request these details in writing.

The following criteria (taken from BS3882: 2007 Specification for Topsoil) can be used as a guide against any analysis report a supplier offers you;

Premium grade

• pH = 5.5-7.8
• Phosphorus (P) Index min = 2
• Potassium (K) Index min = 2
• Magnesium (Mg) Index min = 1

General purpose grade

• pH = 5.0-8.2
• Phosphorus (P) Index min = 2
• Potassium (K) Index min = 2
• Magnesium (Mg) Index min = 1

Economy grade

• pH = 5.0-8.2
• Phosphorus (P) Index min = N/A
• Potassium (K) Index min = N/A
• Magnesium (Mg) Index min = N/A

Note that imported topsoil can sometimes introduce invasive plants, such as Japanese knotweed and couch grass to the garden. Inspect the soil for signs of weed roots or shoots. Weed roots are often white and fleshy, sometimes with a brown covering, unlike the brown, dry, fibrous roots of trees and shrubs.