Texture changes the answer several-fold
Almost every published figure for liming gives a single rate per unit area, and that figure is close to meaningless on its own. The amount of lime needed to move soil pH by one point depends overwhelmingly on the soil's buffering capacity, and buffering capacity varies by a factor of nearly three between sand and clay.
Sandy soil has little clay and little organic matter, so few sites are holding hydrogen ions and the pH moves easily — around 15 kg of lime per 100 m² per pH point. Clay is the opposite: a vast reservoir of exchange sites that resist the change, needing around 40 kg for the same movement.
Applying the sandy-soil rate to a clay plot produces almost no measurable change, and people conclude that liming does not work. Applying the clay rate to sand overshoots badly and can push the soil alkaline enough to lock up iron and manganese, which shows up as yellowing between the leaf veins.
| Soil | Lime per 100 m² per pH point | Sulphur to lower |
|---|---|---|
| Sandy | 15 kg | 8 kg |
| Loam | 25 kg | 13 kg |
| Clay | 40 kg | 20 kg |
Lowering pH is much harder than raising it
The two directions are not symmetrical, and anyone planning to grow blueberries in chalky ground should know it before buying sulphur.
Lime works chemically and directly. Elemental sulphur does not — it has to be oxidised by soil bacteria into sulphuric acid before it changes anything. Those bacteria need warmth and moisture, so sulphur applied to cold soil in winter does nothing at all until spring, and a full season can pass before the pH moves.
Worse, in soil over free chalk or limestone the change is temporary. The parent material keeps dissolving and pushing the pH back up, and you are committing to reapplying sulphur indefinitely. For acid-loving plants on chalk, containers of ericaceous compost are the honest answer rather than trying to acidify the ground.
The other rule: never apply lime and manure at the same time. The reaction releases ammonia, and the nitrogen you were adding leaves as gas. Separate them by a few months — lime in autumn, manure in spring, or the reverse.
Test before and after, and move slowly
A pH figure is only as good as the sample it came from, and pH varies across a garden more than people expect — a bed that has been limed for brassicas for years can sit a full point above the one next to it.
Take several small samples from across the plot, mix them, and test the mixture. A cheap chemical test kit is accurate to about half a point, which is enough for this decision. Probe meters pushed into the soil are generally not reliable enough to act on.
Move in steps of no more than one pH point per season. A larger correction applied at once overshoots, because the soil keeps reacting after you stop measuring. Apply half, wait a season, retest, then decide — the calculator flags this when the change you have asked for is more than 1.5 points.
Most vegetables want 6.0 to 7.0, and there is little to gain from precision within that band. The crops that genuinely care are brassicas, which prefer it toward 7.0 partly because clubroot is suppressed in alkaline soil, and potatoes, which prefer it lower because scab is worse in alkaline conditions. Those two preferences conflict, which is the real argument for rotating them.
Common questions
How much lime do I need to raise soil pH?
On loam, roughly 25 kg per 100 m² for each pH point. Sandy soil needs about 15 kg and clay about 40 kg for the same change, because clay buffers the shift far harder.
How long does lime take to work?
Months. Apply it in autumn for a spring crop. Elemental sulphur is slower still, because soil bacteria have to oxidise it first and they do nothing in cold ground.
What pH do vegetables want?
Most are content anywhere from 6.0 to 7.0. Brassicas prefer the alkaline end, partly because it suppresses clubroot; potatoes prefer the acid end, because scab is worse in alkaline soil.