pH Amendment Calculator — Lime & Sulphur for Soil pH

It depends on soil type. Using ground limestone (per the RHS/DEFRA table), a 1.0 pH rise from 5.5 to 6.5 needs roughly: sandy soil about 700 g/m² (0.7 kg/m²), loam about 800 g/m² (0.8 kg/m²), clay about 1,000 g/m² (1 kg/m²), and peat about 1,200 g/m². Clay and peat soils have higher buffering capacity and need more lime to shift pH. Always test first and re-test before re-applying.

Pelletised lime is easier to spread and less dusty, but slightly less effective weight-for-weight (about 80% as effective as powdered). To match the same neutralising effect you therefore need about 25% more by weight (1 ÷ 0.8 = 1.25), and the calculator adjusts quantities accordingly. Pelletised is better for small garden applications; powdered for large areas.

Avoid changing pH by more than 0.5 units at a time with lime, or 0.3 units with sulphur. Wait at least 3 months between applications and retest before adding more. Over-liming is difficult to reverse.

Autumn is ideal — lime has time to react over winter before spring planting. Avoid applying lime at the same time as manure or fertiliser, as it can cause nitrogen loss. Leave at least a month between lime and other amendments.

Use flowers of sulphur at the rates shown by the calculator. For quicker results on small areas, use sulphur chips or acidifying fertilisers. Ericaceous compost and pine needle mulch also help maintain lower pH over time.