Raising soil pH means adding lime, worked into the top few inches and given time to react. The amount required cannot be worked out from the current pH reading alone. It also depends on how strongly your soil resists change, which is why two gardens both reading the same pH can need very different quantities of lime.
That resistance is what a buffer pH test measures, and it is an important part of deciding on a lime rate.
Do you actually need to raise it?
Check what you are growing before you decide.
Many vegetables and garden plants do well in mildly acidic to neutral soil, and a bed sitting comfortably inside that band does not need liming. Our guide to the best soil pH for vegetables lists where individual crops sit, and a few, including potatoes, prefer the more acidic end.
Extension guidance on liming home gardens recommends lime when a soil test shows pH below the target for the intended crop, with the amount determined by a buffer test rather than by the pH figure on its own. If your reading is heading the other way, our guide on how to lower soil pH covers that.
Confirm the reading first
Liming a bed that never needed it is both wasted effort and hard to undo, since bringing pH back down is slower than pushing it up.
Take readings from several representative locations, at a moisture level you can reproduce, and to the depth stated in your meter's manual. If those readings vary substantially, sample the areas separately rather than liming a whole bed to one figure.
Why do two beds with the same pH need different amounts of lime?
Soil pH describes the current acidity, while buffer pH indicates how resistant the soil is to change.
Sandy soils hold few reserves and shift with relatively little material. Clay soils and soils rich in organic matter hold a large reserve of acidity that has to be neutralised before the measured pH moves at all, so the same target can take several times as much lime and considerably longer.
This is why extension lime recommendations are calculated from a buffer test, and why a bag label can only give you a rough starting point. For a whole plot, a laboratory test that reports buffer pH is worth the small cost, because it comes back with a rate calculated for your soil.
What kind of lime should you use?
Ground limestone is the standard choice for garden beds. Finer grades react faster.
Pelletised lime is the same material bound into pellets. It spreads more cleanly and is far less dusty, which is why many gardeners prefer it.
Dolomitic lime adds magnesium as well as calcium. Choose it when a soil test shows magnesium is low, rather than by default, because magnesium can build up over repeated applications.
Wood ash can raise pH and is sometimes used as a substitute for limestone. Extension guidance on using wood ash as a liming material suggests applying it at roughly one and a half times the recommended limestone rate, and advises against applying it once soil pH is above about 6.8. Its neutralising value and nutrient content vary considerably with the material burned, and it also adds potassium and other salts, so use it with a known rate and a soil test rather than as a direct one-for-one replacement for limestone.
How do you apply it?
Spread it evenly and work it into the top few inches.
Lime moves through soil very slowly, so material left on the surface takes far longer to have any effect. Water it in afterwards. Autumn, or well ahead of planting, is the usual timing, which gives the reaction time to happen before roots are in the ground.
Follow the fertiliser and lime labels when scheduling applications. In particular, avoid combining lime directly with ammonium-based nitrogen fertilisers unless the product guidance allows it.
When do you retest?
Follow the recommended retesting interval.
Retest at the interval given by the soil testing laboratory or on the lime label. This may be several months later, or in the following season, rather than after only a few weeks. Lime reacts gradually, and a reading taken soon after application may not reflect how much of it has acted.
Any of our soil pH meters will track this, provided you use it the same way each time: same spots, similar moisture, readings written down with dates. Overshooting into alkaline soil is harder to correct than being slightly under target, which is the main argument for adding less and checking again.
Quick reference
Confirm the reading first, from several representative spots rather than one.
Lime is called for when a soil test shows pH below the target for the crop you are growing. There is no single threshold that applies to every garden.
The amount depends on buffer pH, not on the pH reading alone: clay and high-organic soils need considerably more than sand.
Ground or pelletised limestone is the standard choice; dolomitic lime only when magnesium is low.
Wood ash varies in strength and composition, adds potassium and salts, and is not a one-for-one replacement for limestone.
Work lime into the top few inches rather than leaving it on the surface, and retest at the interval on the label or soil test report.
About Yieryi
Yieryi has built pH, EC and TDS instruments for more than ten years. Applying lime conservatively and retesting is safer than making one large application based on a handheld reading alone, and that approach depends on being able to repeat the measurement under the same conditions.
*Written by Sunny Feng, Yieryi.*
FAQ
How long does lime take to raise soil pH? Often several months, and sometimes into the following season. Finer grades react faster, and lime worked into the soil acts much faster than lime left on the surface.
Why do two gardens with the same pH need different amounts of lime? Because they differ in buffering capacity, which describes how much reserve acidity the soil holds. That is what a buffer pH test measures, and it is why lime rates come from that test rather than from pH alone.
What if I add too much lime? Bringing pH back down is slower than raising it, so add less than you think you need and retest at the recommended interval before applying any more.
