Soil pH, how to test it and what to do about it

Why the pH matters more than the fertiliser

You can put the best feed in the world on a bed and watch the plants stay yellow and stunted, and when that happens it is usually not a shortage of nutrients at all, it is that the pH has locked the nutrients up where the roots cannot reach them. Soil pH runs from 0 to 14 with 7 in the middle, and most vegetables are happiest somewhere between 6 and 7, which is where the widest range of nutrients stays available. Push far outside that and the iron or the phosphorus is still sitting in the soil, it is just chemically stuck.

Which plants want what

Most of what goes in a veggie patch will cope anywhere from 6.0 to 7.0 without complaint, so you are not aiming at a single number. The exceptions are worth knowing. Blueberries, azaleas and camellias want it properly acidic, down around 4.5 to 5.5, and they will go yellow between the leaf veins if the soil is anywhere near neutral. Brassicas like cabbage, cauliflower and broccoli go the other way and do better nudged up towards 7.0, partly because clubroot struggles in limed soil, so a bed that has had clubroot is worth liming before you plant brassicas in it again.

Test it before you do anything

Soils change from one end of a street to the other and sometimes from one end of a backyard to the other, so guessing is genuinely a waste of a bag of lime. A colour-change test kit costs less than a punnet of seedlings and gives you a number in about five minutes, which is all the accuracy a home garden needs. Take a small sample from several spots in the bed, take it from about 100mm down rather than off the surface, and mix them together before you test so you get an average rather than one odd corner. If you are going to grow something fussy in a big way it is worth sending a sample off for a proper lab analysis, and for a normal veggie patch the kit does the job.

Raising the pH, which means lime

Garden lime and dolomite both raise pH, with dolomite adding magnesium as well, which is handy if your soil is short of it and unnecessary if it is not. It works slowly, so spread it on the surface, water it in and expect to wait 2 to 3 months before it has shifted much, which means it wants doing well ahead of planting rather than the day before. Do not lime and apply manure or sulphate of ammonia at the same time, because the reaction between them drives the nitrogen off as ammonia gas and you lose most of what you paid for. Leave a few weeks between the two. Wood ash will also raise pH and it moves quicker than lime, so it is easy to overdo and better used in small amounts.

Lowering the pH, which is harder

Elemental sulphur is the honest answer for bringing an alkaline soil down, and it works by being eaten by soil bacteria, which means it needs warmth and moisture and several months to do anything. Powdered sulphur sold for lowering pH is the same stuff. Peat moss and plenty of organic matter will help hold an acidic patch where you want it, and if you only need one acid-loving plant it is far easier to grow it in a pot of azalea and camellia mix than to fight the whole bed.

Two things you will read constantly that do not really work. Used coffee grounds are close to neutral by the time they have been through a machine, so they will improve the soil but they will not acidify it. Pine needles shift pH so slowly and so slightly that you would need a forest floor's worth over years to measure it. Neither will hurt anything, they just will not do the job you wanted them for.

Keeping it where you put it

Test again the season after you make a change rather than assuming it landed, because both lime and sulphur are slow and it is easy to keep adding more while the first lot is still working. Organic matter buffers the soil, so a bed with plenty of compost through it swings around far less than a bare sandy one. And if the native soil is a long way from what you want, a raised bed filled with the mix you actually need is less work than years of trying to argue with the ground.

Shop related products

• Manutec Soil pH Test Kit. The colour-change kit, enough tests for years of a home garden.
• Brunnings Garden Lime 3kg. For bringing an acidic bed up before you plant.
• Brunnings Azalea and Camellia Planting Mix 30L. The easy way to give one acid-loving plant what it wants.
• Manutec Azalea and Camellia Fertiliser 2.5kg. Feed for the acid lovers without pushing the pH up.

Did you know we also stock sulphur for lowering pH, dolomite in 8kg and 20kg, garden lime in 10kg and iron chelate for yellowing leaves, and not all of that is on the website, so give us a ring on (08) 8536 4333 to check what is on the shelf.

More from the garden centre

The test kits and everything you would use to shift a reading are in the garden centre, and if you bring your test result in we will tell you how much of what to put on.

Keep reading

• How to grow vegetables that are perfect for the South Australian climate
• Seven secrets to growing crunchy carrots in any soil
• The ultimate guide to growing climbing beans with ease

I test my own soil about half as often as I tell everybody else to, which is not a great look for somebody who sells the kits.

Candeece