What Dolomite Lime Does for Soil and When to Use It

Dolomite lime is one of those garden products that people either use religiously or have never touched, and the honest answer is that it depends entirely on what your soil is doing, because putting lime on a garden that does not need it is worse than doing nothing at all.

So this one is about what it actually does, how to tell whether your patch wants it, and the plants that would much rather you left the bag in the shed.

What it is

Dolomite lime is ground dolomitic limestone, which is a rock that carries both calcium carbonate and magnesium carbonate. That second one is what separates it from ordinary garden lime, which is mostly calcium and very little else.

It does two jobs at once. It lifts the pH of acidic soil, and it puts calcium and magnesium in while it is doing it, and both of those are things most plants want and a lot of South Australian gardens are short of.

Why soil pH matters at all

Nutrients do not become unavailable because they are absent. They become unavailable because the soil chemistry has locked them up where the roots cannot reach them, and pH is the main thing deciding that.

Below about 5.5 the phosphorus starts getting tied up, calcium and magnesium leach out, and aluminium becomes more available than any plant wants it to be. So you can put fertiliser on an acidic bed all year and watch most of it sit there doing nothing, which is where the money goes on a lot of struggling gardens.

Most vegetables and lawn grasses are happiest somewhere between 6.0 and 7.0. Getting the pH into that range does more for a tired bed than another round of fertiliser will.

Test before you spread anything

This is the part people skip and it is the part that matters, because the symptoms of acidic soil look a lot like the symptoms of half a dozen other problems. Yellowing leaves, stunted growth and a lawn that will not thicken up can all be pH, and they can also be nitrogen, or compaction, or a watering habit.

A soil pH test kit takes about five minutes. You take a small sample, add the indicator solution and the powder, and read the colour off the chart. Take a few samples from different parts of the garden rather than one, since a bed under a gum tree and a bed near the house can read quite differently.

If the reading comes back at 6.5 then leave it alone and go and spend the money on something else.

How to put it on

Spread it evenly over the surface and water it in, or fork it lightly through the top 100mm of the bed if the bed is empty. It is a fine powder, so pick a still day or wear a mask, because a breeze will put most of it on you rather than on the garden.

Autumn and spring are the good windows, and the reason is that lime is slow. It takes months to work through the soil profile rather than days, so getting it on well before you plant gives it time to do something. Putting it down and sowing the next morning mostly wastes it.

Do not put lime and nitrogen fertiliser on together either, because the two react and you lose a good part of the nitrogen to the air as ammonia. Leave a few weeks between them.

Who should not use it

Plenty of plants actually want acidic soil and lime is the last thing they need. Blueberries, azaleas, rhododendrons, camellias, gardenias, magnolias and most of the natives sit in that group, and potatoes are worth a mention too, because limed ground makes scab worse.

Overliming is also a real thing and it is much harder to walk back than underliming. If you are unsure, use less than you think and test again in a few months, since you can always add more and you cannot easily take it out.

Lime, dolomite or gypsum

These three get mixed up constantly and they do different jobs.

• Garden lime raises pH and adds calcium. Use it when the pH is low and the magnesium is fine.
• Dolomite lime raises pH and adds calcium and magnesium. Use it when the pH is low on sandy soil, which is where magnesium leaches out first.
• Gypsum adds calcium and breaks up heavy clay without changing pH at all. Use it on a clay bed that already has the pH about right.

Getting that choice wrong is the commonest mistake with the lot of them, and it is an easy one to make because all three come in similar bags and all three are described as soil conditioners.

If you would rather just bring us a pH reading and tell us what you are growing, do that. We will tell you which of the three you want and roughly how much, and that is a shorter conversation than an hour of reading conflicting advice online.

Shop related products

• Brunnings Garden Lime 3kg. Straight lime for a small bed where the pH is low and the magnesium is not the issue.
• Naturally Good Mushroom Compost 25L. Goes in alongside lime to build the soil back up rather than only correcting it.
• Brunnings Blood and Bone Based Fertiliser 5kg. The feed that starts working properly once the pH lets the roots reach it.
• Martins Organic Sugar Cane Bale 11KG. Mulch over the top to hold the moisture in while the lime works through.

Did you know we also stock Neutrog Dolomite Lime 20kg, dolomite in an 8kg bag, garden lime in 10kg, gypsum in 10kg and 20kg, and soil pH test kits in the shop. Not all of it is on the website, so give us a ring on (08) 8536 4333 to check what is on the shelf.

The rest of the soil improvers and fertilisers sit in the garden centre at H, with the Neutrog range in its own section.

Keep reading

• What to do when your soil is too acidic or too alkaline
• How to build better soil in any garden
• The science behind soil pH and vegetable gardening

Cheers,

Candeece

I own a soil pH test kit and I still guess first and test afterwards, which is exactly the wrong order and I know it.