Brunnings Garden Lime, what it does and when to use it

What garden lime actually does to your soil

If a bed has gone quiet on you, and the plants are in and the water is going on and nothing much is happening, the first thing worth checking is not the fertiliser, it's the pH of the soil they're sitting in. Acid soil doesn't really starve a plant so much as lock the nutrients up where the roots can't get at them, so you can keep feeding a bed that already has plenty in it and get nowhere for your trouble. That's the job garden lime is for.

It's calcium carbonate, and that is the whole story

Garden lime is ground limestone, so what you're buying is calcium carbonate and not much else. It lifts the pH of an acid soil, and it puts calcium into the soil while it's doing it, and calcium is the one most people never think about because it doesn't get printed on the front of a bag the way nitrogen does.

What it doesn't give you is magnesium. That's dolomite lime, which is a different rock with magnesium carbonate in it as well, and the two get swapped around as though they're the same thing. If your soil is genuinely short of magnesium then garden lime won't do a thing about it, and if your soil already has plenty of magnesium then dolomite just pushes it further in a direction it didn't need to go. So it's worth knowing which of the two you actually want before you put either one in the trolley.

Test before you lime, and I mean that

Lime is one of the few garden products that's genuinely hard to undo. Putting the pH up is easy enough and bringing it back down again is slow and annoying, so a soil that was only a bit acidic and is now sitting too alkaline is a worse problem than the one you started with. A Manutec soil pH test kit costs less than the bag does and it takes about 5 minutes, and it will tell you whether you have an acid soil at all.

Bear in mind the kit is measuring pH and nothing else, so it tells you whether lime is the right call. It won't tell you a nitrogen level or a potassium level, and if that's the question you're asking then you want a proper lab nutrient test instead. Plenty of gardens around here sit somewhere near neutral already and don't need liming, and there's no prize for adding something the soil wasn't asking for.

What lime is good at

• Bringing an acid soil back up towards the range where phosphorus and the trace elements are available again.
• Feeding calcium into a sandy soil, where it tends to wash straight out over a few seasons.
• Peas, sweet peas, beans, brassicas and corn, all of which do better on sweeter ground.
• Roses, which will sit happily around neutral and sulk on ground that has gone sour.

What lime is not good at, and this is where it gets misused

Lime will not fix heavy clay. That claim gets made constantly and it's the wrong product for the job, because what makes a dispersive clay behave itself is the calcium ion holding the particles together, and gypsum does that without touching the pH at all. If your clay is heavy but the pH is already fine, liming it just moves the pH somewhere you didn't want and leaves the structure exactly where it was. For most clay beds the honest answer is organic matter and a lot of it, dug in year after year.

Lime also has no business anywhere near the acid lovers. Blueberries, azaleas, camellias, gardenias and most Australian natives all want the soil the way it already is, and liming those beds is a slow way of making them miserable. If the native bed is what's bothering you then that's a feeding question rather than a pH one, and Bush Tucker is the low phosphorus option made for it.

How to put it out

Spread it evenly over the surface at the rate on the bag and rake it lightly into the top few centimetres. Then water it in properly. It works by dissolving slowly, so the pH moves over months rather than overnight, and the sensible time to do it is well before you plant rather than on the morning you plant. Wear a dust mask while you're spreading it, because lime dust is an irritant and it gets everywhere on a breezy day.

One thing to keep apart: don't put lime out at the same time as a nitrogen fertiliser like sulphate of ammonia or urea. Lime plus an ammonium fertiliser drives nitrogen off as ammonia gas, so you lose a good part of the feed you just paid for. Leave a few weeks between the two and you keep both.

Lime in the compost heap

A sour, soggy heap that smells sharp will settle down with a light dusting of lime, and it does help the worms and the bacteria get back to work. The trick is restraint. Too much lime in a heap drives the nitrogen off as ammonia in exactly the same way it does in a bed, so you end up with a sweeter smelling heap and a weaker compost out the other end. A light scatter between layers is plenty, and if the heap has gone sour mostly because it's wet and packed down, turning it will do more than liming it will.

Shop related products

• Brunnings Garden Lime 3kg. Ground limestone for raising the pH of an acid bed.
• Manutec Soil pH Test Kit. Tells you whether you need lime before you buy any.
• Brunnings Garden Compost with Seaweed 25L. The organic matter answer for a heavy bed.
• Brunnings Sulphate of Ammonia 2.5kg. The nitrogen feed to keep a few weeks clear of your liming.

Did you know we also carry Garden Lime in a 10kg bag in the shop, along with gypsum in 10kg and 20kg for the clay job that lime won't do. They're not all on the website, so give us a ring on (08) 8536 4333 to check what's on the shelf. The rest of the range is in our garden centre, and you're welcome to bring a soil question in with you.

More on soil

• Soil pH, how to test it and what to do about it
• What dolomite lime does for soil and when to use it
• Soil amendments, and which one your garden actually needs

I'm happy to guess at plenty of things in my own garden, but pH isn't one of them, mostly because the test kit is cheaper than the bag and it's the only part of this I can actually be sure about.

Cheers,
Candeece

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