How Do I Raise the pH of My Acidic Garden Soil?

If your soil test has come back with a pH below 6.0 and you are growing vegetables, raising it is usually the highest-return action you can take. Acidic soil locks up phosphorus and leaches calcium and magnesium, leaving plants underfed even in ground that looks healthy. Raising pH is straightforward if you use the right material at the right rate and give it time to work.

Garden Lime: The Standard Choice

Ground limestone — also called garden lime or calcitic lime — is calcium carbonate (CaCO₃). It is the most widely available and safest material for raising pH in vegetable gardens. It works slowly over several months as soil water dissolves the particles, making it very difficult to over-correct in a single season. Apply it to the soil surface in autumn and rake lightly; frost, rain, and earthworm activity will incorporate it over winter. Avoid applying lime and manure at the same time, as the reaction releases ammonia nitrogen as gas and wastes both materials. Leave four to six weeks between lime and manure applications.

Dolomitic Lime: When to Choose It

Dolomitic lime contains both calcium carbonate and magnesium carbonate. It raises pH at roughly the same rate as calcitic lime but also adds magnesium, which is often co-deficient in acidic soils. If your soil test shows both low pH and low magnesium, dolomitic lime solves both problems at once. Avoid it if magnesium is already adequate — excess magnesium can compete with calcium and potassium uptake and worsen soil structure in clay soils.

Hydrated Lime and Wood Ash: Faster but Riskier

Hydrated lime (calcium hydroxide) acts much faster than ground limestone but is caustic and easy to over-apply. It is better suited to soil sterilisation between crops in commercial growing than to routine pH management in home gardens. Wood ash is a gentler fast-acting option, rich in calcium and potassium carbonate. It can raise pH noticeably within a few weeks and adds useful potassium. Apply sparingly — no more than 100 g per square metre — and not more than once per year, as it raises pH quickly and the effect is hard to reverse.

Application Rates by Soil Type

How much lime you need depends on three things: how far pH needs to move, soil texture, and organic matter content. Clay soils have high buffering capacity and need significantly more lime than sandy soils to shift the same number of pH units. As a general guide for raising pH by one unit using ground limestone: sandy soil needs roughly 150 to 250 g per square metre; loam needs 250 to 400 g; clay needs 400 to 600 g. These are starting points — always retest three months after application before adding more. Moving pH in stages of 0.5 units per season is safer than attempting large shifts in one go.

After Liming: Monitoring and Maintenance

Retest your soil at least three months after liming, ideally six. The numbers should have shifted toward your target. If you have not reached the target, apply a further half-dose and test again the following season. Ongoing maintenance liming — a light dressing of 100 to 150 g per square metre every three to four years — prevents pH drifting back down under the influence of rainfall and organic matter decomposition. Keep detailed notes of what you applied and when to avoid doubling up by accident.