Soil pH Adjustment Calculator — Lime & Sulphur Rates

This calculator applies established amendment rates per pH unit for each soil type, then multiplies by your bed area: quantity (kg) = rate (g/m²) × pH change × area ÷ 1000.

To raise pH (garden lime / calcium carbonate): per pH unit, Sandy ~680 g/m², Loam ~800 g/m², Clay ~1000 g/m². These follow the RHS "Lime and liming" table (e.g. raising loam from pH 5.5 to 6.5 needs 0.8 kg/m²), assuming the lime is forked into the top 20 cm of soil. The RHS all-round target is pH 6.5.

To lower pH (elemental sulphur): per pH unit, Sandy ~25 g/m², Loam ~50 g/m², Clay ~75 g/m² (MSU/Oregon State extension per-pH-unit rates). The default amendment is sulphur chips, which act more slowly than sulphur powder — RHS rates sulphur powder at 135-270 g/m² (sandy to clay) for a roughly 1-1.5 unit drop — so allow for the slower reaction and retest before topping up.

Clay soils need more amendment because their higher clay and organic-matter content gives greater buffering capacity. Change pH gradually (no more than ~1 unit per year) and retest after a few months.