How much grit do I need to improve clay soil?

For clay soil, add grit at 20-30% of the total soil volume being worked. Dig it into the top 20-30 cm for best results.

Does adding sand improve clay soil?

Sharp sand (not builders sand) can help, but grit is better. You need large quantities — at least 25% by volume — to make a real difference.

What is the best amendment for sandy soil?

Well-rotted compost or manure is best for sandy soil, adding organic matter that retains moisture and nutrients.

How deep should I work amendments in?

Dig amendments into the top 15-30 cm of soil. For established borders, top-dress with compost and let worms work it in.

How often should I amend my soil?

Annual organic matter additions in autumn or spring are ideal. Structural amendments like grit only need adding once. Test your soil every 2-3 years to check nutrient levels.

GardenCalcs

Soil Amendment Calculator — Improve Your Garden Soil

Calculate quantities of grit, sand, or organic matter to improve your soil.

Area

The plan area of the border or bed you are improving — length × width of the ground surface

Working Depth

How deep you will dig the amendment in — 15-30 cm for forking into a border, less for top-dressing

Amendment TypeGrit or sharp sand opens up heavy clay for drainage; compost or manure adds organic matter to sandy soil; perlite and vermiculite lighten potting and seed mixes

Mix Ratio

Percentage of amendment in total mix

How We Calculate This

This calculator works out how much amendment material you need based on the area, working depth, and desired mix ratio.

Formula: Amendment Volume = Area × Depth × (Mix Ratio / 100)

Weight uses typical loose bulk densities: horticultural grit and sharp sand ~1,500 kg/m³, perlite ~100 kg/m³, vermiculite ~120 kg/m³, garden compost ~500 kg/m³, well-rotted manure ~600 kg/m³.

Bag counts assume the usual UK retail pack sizes: grit and sand in 25 kg bags, perlite and vermiculite in 100 L horticultural bags, and compost and manure in 50 L bags.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.