What to do when your soil is too acidic or alkaline

Test it before you add anything

This is the one where guessing costs you the most, because lime and sulphur both work and they work in opposite directions, so if you add the wrong one you've made the problem worse and you won't find out for a season. A test kit takes 5 minutes and tells you the number, and the number is the whole decision.

Most vegetables and most lawn want somewhere between 6.0 and 7.0. Under that and the plant starts struggling to pick up phosphorus and molybdenum, over about 7.5 and it stops getting iron, manganese, zinc and copper even though all of it is sitting right there in the soil. That's why a plant can look starved in ground you've been feeding all year.

Take the sample properly or don't bother

Scrape the top 20mm off, because that layer is mulch and fertiliser rather than soil, then take a small amount from about 100mm down in 5 or 6 spots across the bed and mix them together. One sample from one hole tells you about that hole. Beds can read differently from each other within the same yard, particularly where somebody has been watering with bore water or where an old concrete path used to sit.

If it's too acidic, lime

Garden lime is calcium carbonate and it raises pH. Spread it, dig it through, water it in, and then leave it alone, because it works slowly and the second application is the one that does the damage. Retest after a few months rather than adding more because nothing happened yet.

Dolomite does the same job and brings magnesium with it, which is worth knowing if your soil is short on magnesium and not much use if it isn't.

Wood ash out of the fireplace will also raise pH and it carries potassium, so it's genuinely useful. It's also variable and much faster acting than lime, so go light with it and keep it away from anything that wants acid soil.

If it's too alkaline, sulphur

Elemental sulphur lowers pH, and it works through soil bacteria converting it, which means it needs warmth and moisture and it takes its time. Spread it, water it in, and give it a couple of months before you judge it. Going heavy to speed it up doesn't speed it up, it just overshoots later.

Sulphate of ammonia will nudge it down as well while feeding nitrogen, which makes it handy where the plant is both hungry and sitting in alkaline ground.

Organic matter helps in both directions, and that's not a fudge. Compost buffers a soil, which means it resists swinging either way, so a bed you've been building up for years is much harder to knock out of range than a bare one.

The thing people confuse this with

Heavy clay that sets like concrete is usually a structure problem rather than a pH one, and gypsum is the answer to that rather than lime. Gypsum is calcium sulphate, it opens the clay up without moving the pH much at all, and reaching for lime on clay because somebody said calcium is a common way to end up with alkaline clay that's still hard.

Shop related products

• Manutec Soil pH Test Kit. Start here. It's the cheapest thing in this post and it decides everything else.
• Brunnings Garden Lime 3kg. For acidic soil, worked in and watered.
• Brunnings Sulphate of Ammonia 2.5kg. Nitrogen with a slight acidifying effect.
• Brunnings Peat Moss 5L. For potting up the acid lovers rather than trying to acidify a whole bed for them.
• Brunnings Garden Compost with Seaweed 25L. The buffer, and the one that makes the rest of it hold.

Did you know we also stock sulphur for lowering pH in a 500g pack, a pH meter if you'd rather a reading than a colour chart, garden lime in a 10kg bag as well as the 3kg, dolomite lime in 20kg, and bagged gypsum in 10kg and 20kg. None of that is on the website, so give us a ring on (08) 8536 4333 to check what is on the shelf.

It's all in the garden centre, and if you bring the test result in with you we can work out what to put on it and how much.

More from the garden

• What Dolomite Lime Does for Soil and When to Use It
• How to build better soil in any garden
• Do I need acid loving potting mix for blueberry bushes?

I keep a pH kit in the shed and I still tip things on the garden by feel, which is exactly the habit this post is telling you not to have.

Candeece

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