Scottish growing tool
Soil Amendment Calculator
Tell us your soil's tested pH, your soil type, and what you're growing, and this works out how much lime to add, and whether you need to at all.
Sources
Lime rates for clay, loam and sandy soil:
Peaty soil's lower target pH:
Sulphur rates for lowering pH:
Telling your soil type by feel, clay and sandy:
Telling your soil type by feel, loam and peaty:
What very high or low pH readings mean:
Per-crop pH preference: linked from its result above, to that crop's own guide page.
How it works
Give it a pH from a soil test, your soil type and what you're growing. For clay, loam or sandy soil below pH 6.5, it uses the RHS lime table. That gives kilograms of ground limestone per square metre, worked into the top 20cm of soil, to bring it up to pH 6.5, the general target for most garden plants. Heavier soil needs more. Between pH 6.5 and 7.0 it suggests neither lime nor sulphur. At pH 5.5, clay takes 1.0kg per square metre, loam 0.8kg and sandy soil 0.7kg.
A worked example
A loam bed testing at pH 5.5 needs about 0.8kg of ground limestone per square metre. For a raised bed of 3m by 1.2m, 3.6 square metres, that's about 2.9kg in all. The Bed Area Calculator works out the total for your own bed.
When the crop changes the answer
Some crops push the advice one way or the other. Cabbages, broccoli, cauliflower, kohlrabi and Brussels sprouts benefit from liming acid soil, usually the winter before, which helps deter clubroot. Potatoes go the other way: liming raises the risk of scab, so for potatoes the calculator won't suggest lime at all. On soil above pH 7.0 it gives a rate for sulphur instead: 135 to 270g per square metre to bring pH 7.0 to 7.5 down to 6.0 to 6.5, sandy soil needing the least and clay the most. Sulphur works slowly and is easy to overdo, so several small doses are safer than one big one, and on chalky soil it often doesn't work at all.
Peaty soil
Peaty soil doesn't need liming as far. Scotland's Farm Advisory Service sets its farm target for peaty soil about 0.7 lower than for mineral soil, pH 5.3 to 5.5 against 6.0 to 6.2, because organic soil holds less of the soluble aluminium that makes low pH harmful to roots. No garden-scale lime rate for peaty soil has been published, so for peat the calculator gives that target rather than a dose.