How to Improve Soil Before Planting a New Border

Why Soil Improvement Matters

Plants spend their whole lives in the soil you give them. If the soil is poor, the plants will be poor. Improving the soil before planting is 10 times more effective than trying to improve it afterwards, when the plants are in the way. In the Chilterns, where chalky soils are often thin and low in organic matter, soil improvement is particularly important.

Step 1: Dig Over the Border

Dig the soil to a depth of 30cm (a spade's depth). Break up any compacted layers. Remove perennial weed roots (bindweed, couch grass, ground elder). Remove large stones. On chalky Chiltern soils, you may hit solid chalk within 20cm, dig as deep as you can and consider raised beds if the chalk is too close to the surface.

Step 2: Add Organic Matter

Add a 5cm layer of compost or well-rotted manure across the border and dig it in. This improves soil structure, water retention and nutrient content. Use garden compost, bagged compost, or well-rotted farmyard manure (not fresh, as it burns plants). On chalky soils around High Wycombe and Amersham, extra organic matter is essential to improve water retention.

Step 3: Add Grit on Clay Soils

On heavy clay soils in Aylesbury, Bicester and Didcot, add horticultural grit at 5kg per square metre and dig it in. This improves drainage and prevents the soil from becoming waterlogged in winter.

Step 4: Adjust pH if Needed

Test the soil pH. Most plants prefer pH 6.0 to 7.0. On chalky Chiltern soils (pH 7.5 to 8.5), do not try to acidify the soil, it does not work. Instead, choose chalk-tolerant plants or grow acid-loving plants in pots. On acidic soils, apply garden lime at 50g per square metre. See our chalk soil guide for detail on alkaline soils.

Step 5: Level and Consolidate

Rake the soil to a level surface. Gently firm it by walking on it on your heels. Leave it to settle for 2 weeks before planting. This allows the soil to find its natural level and prevents plants from sinking.

Step 6: Mulch After Planting

After planting, apply a 5cm mulch of bark or compost. This suppresses weeds, retains moisture and continues to improve the soil as it breaks down.

Soil Improvement by Soil Type in the Chilterns

Chalk soil (High Wycombe, Amersham, Princes Risborough, Wendover)

Add compost and grow in raised beds if shallow. Chalk soil is alkaline and free-draining, so add organic matter to improve water retention. See our chalk border guide.

Clay soil (Aylesbury, Bicester, Didcot)

Add compost and grit. Dig in 5cm of compost and 5kg of grit per square metre. Clay soil is fertile but needs opening up.

Sandy soil (some areas of Berkshire)

Add compost and manure. Sandy soil drains too freely and is low in nutrients. Add extra organic matter to improve water retention.

Cost of Soil Improvement

• Compost: 5 to 10 pounds per 100 litres
• Farmyard manure: 6 to 12 pounds per 50 litres
• Horticultural grit: 5 to 8 pounds per 25kg
• Total for 10 sqm border: 30 to 80 pounds in materials

Get Help with Soil Improvement

If you need help improving soil for a new border in Oxfordshire, Buckinghamshire, Berkshire or Bedfordshire, contact us on WhatsApp.

Further Reading

• Chalk soil in the Chilterns
• Topsoil vs compost
• Mushroom compost, manure or bark mulch?
• Building better borders on chalky ground

Sources and citations

This article draws on the following sources. Specific claims are cited inline where relevant.

1. CGM field records (2025-02. Site observations from gardens across Oxfordshire, Buckinghamshire, Berkshire and Bedfordshire.
2. Royal Horticultural Society (2024). RHS Gardening Advice and Plant Finder. Available at rhs.org.uk.
3. Met Office (2024). Regional climate averages, South East England. Available at metoffice.gov.uk.
4. British Geological Survey (2024). Soil parent material data for the Chilterns and Thames Valley. Available at bgs.ac.uk.