Should I lime my soil, and when should I not?

Liming was once an annual ritual on vegetable plots and it caused a great deal of quiet damage, because most British garden soil is nowhere near acid enough to need it and lime cannot easily be taken back out. It is a genuine tool for one job — raising a measured, too-low pH — and a mistake in almost every other use.

Safety

What you'll need

• Soil pH test kit
• Ground limestone or dolomitic limestone
• Goggles and a dust mask
• Gloves
• Kitchen scales or a marked scoop (optional)

Step by step

1. Test before you buy anything.Take five or six samples across the bed at 100mm depth, mix them, and test the mixture. A single reading describes one square metre. Lime applied without a test is guesswork that you cannot undo.
2. Decide whether the reading actually justifies it.Below 5.5 and you are growing vegetables: yes. Between 5.5 and 6.0 and brassicas keep failing: probably. Above 6.5: no, whatever the plant. Above 7.0: absolutely not, and if you have been liming, stop.
3. Choose ground limestone.Calcium carbonate, sold as garden lime or ground limestone. It is slow, safe to handle with basic protection, and hard to overdose badly. Dolomitic limestone adds magnesium as well and is the better choice on sandy soil, which is often short of it.
4. Match the dose to the soil type, not just the pH.Clay needs several times as much lime as sand to shift the same distance, because clay resists pH change. The pack states rates by soil type — follow them, and use the lower figure if you are unsure.
5. Move it in stages.Never try to jump a full point in one application. Apply at the stated rate, wait a season, retest, and apply again if it is still short. Overshooting into alkalinity is much harder to correct than being slightly too low.
6. Apply in autumn or early winter, on bare soil.It needs months of weather to react. Scatter evenly on a still day, rake it in lightly, and let rain do the rest. Applying in spring and planting immediately achieves very little that season.
7. Keep it away from manure and nitrogen fertiliser.Lime plus ammonium nitrogen releases ammonia gas, which is a real waste of the nitrogen you paid for and unpleasant to breathe. Separate them by a season: lime in winter, manure the following autumn.
8. Retest the following year.Do not assume it worked. Ground limestone typically shows most of its effect over six to twelve months, and the number tells you whether to repeat.

Tips

• Brassicas are the usual honest reason to lime a vegetable bed. Club root is suppressed — not cured — by pH above about 7.0, so lime the brassica bed of a rotation and let it drift back down over the following years.
• Wood ash raises pH too, and much faster than limestone because it is more soluble. If you spread ash regularly you may be liming without realising it.
• Mushroom compost is alkaline. Using it as an annual mulch on a bed you also lime will push the pH up faster than you intend.
• Lime does not break up clay. That is a separate and largely mistaken claim — see the guide on clay improvers.

Common mistakes

• Liming every year as a routine — The old advice assumed heavy rain, no organic matter and continuous cropping. On a modern garden bed getting an annual compost mulch, it steadily pushes soil into alkalinity and locks up iron and manganese.
• Liming a lawn because it has moss — Moss in a lawn is caused by shade, compaction, wet and close mowing far more often than by acidity. Lawn grasses are perfectly happy at pH 5.5 to 6.5, and liming a lawn favours weeds and worm casts.
• Using hydrated lime or builders' lime — Both are far more caustic than ground limestone, act violently, and are easy to overdose. There is no gardening job that needs them.

Questions people ask