Soil pH explained: what it is and how to change it

Sometimes a plant looks hungry even though you've fed it. The leaves go pale or yellow between the veins, growth stalls, and nothing you add seems to help. Often the problem isn't a lack of nutrients in the soil. It's the soil's pH locking them away.

What pH means

pH is a measure of how acidic or alkaline something is, on a scale from 0 to 14.

• 7 is neutral.
• Below 7 is acidic. The lower the number, the more acidic.
• Above 7 is alkaline.

The scale is logarithmic, so each whole step is a tenfold change. Soil at pH 5 is ten times more acidic than soil at pH 6. That's why small changes in the number matter, and why you should shift pH gradually.

Why plants care

pH changes how easily roots can take up nutrients. Most nutrients are easiest to reach when soil is slightly acidic to neutral, roughly pH 6 to 7. That's the sweet spot for most vegetables, herbs and flowers.

When soil is too alkaline, iron, manganese and some other trace elements become harder to absorb. That's when you see yellow leaves with green veins, especially on citrus, gardenias and blueberries. When soil is very acidic, other nutrients such as calcium and magnesium can run short, and some elements can build up to levels that harm roots.

Testing your soil

You don't need a lab. Garden centres and hardware stores sell cheap pH test kits that use a powder and a coloured dye. They're accurate enough for home gardens.

1. Take a small sample from root depth, about 10 cm down.
2. Put it on the white card in the kit and add a few drops of the dye.
3. Dust it with the white powder and wait a minute.
4. Match the colour to the chart.

Test a few spots, because pH can vary across a block. Beds that have had lots of compost or fertiliser over the years may read differently from the lawn next door. Retest once a year, or before you plant something fussy.

Who likes what

Potatoes are often grown on the acidic side of this range because it can reduce common scab. Brassicas grown in soil that's limed toward neutral are less troubled by clubroot.

Many Adelaide suburbs have alkaline soil, while the Hills tend to be more acidic. Working with what you've got is always easier than fighting it. If your soil is strongly alkaline, grow acid lovers like blueberries in pots of acid-loving potting mix rather than trying to change a whole bed.

Raising pH (making soil less acidic)

• Garden lime (calcium carbonate) is the standard choice.
• Dolomite also contains magnesium, which helps if your soil is short of it.

Spread it evenly, rake it into the top few centimetres and water it in. Follow the rate on the pack. Lime works slowly, over weeks to months, so apply it well before planting, ideally a few weeks ahead. Don't apply lime at the same time as manure or ammonia-based fertilisers, as it can cause nitrogen to be lost as gas. Leave a couple of weeks between them.

Lowering pH (making soil more acidic)

• Sulphur (often sold as flowers of sulphur or garden sulphur) is the main tool. Soil microbes slowly turn it into a mild acid, so it takes weeks to months to work.
• Organic matter such as compost, pine bark and leaf mould helps over time and also buffers the soil so pH swings less.
• Iron chelates won't change pH much, but they give plants iron in a form they can use. They're a handy quick fix for yellowing citrus or blueberries while you work on the soil.

Lowering pH is harder than raising it, especially where there's calcrete or limestone underneath. That buried lime keeps pushing the pH back up. In those gardens, pots and raised beds filled with suitable mix are the practical answer.

Go slowly

The most common mistake is doing too much at once. Change pH in small steps:

1. Test.
2. Apply a modest amount, following the pack rate.
3. Wait a couple of months.
4. Test again before adding more.

Overshooting is easy and hard to undo.

A word on gypsum

Gypsum (calcium sulphate) is often sold alongside lime, and people mix them up. Gypsum is for clay structure. It helps sticky clay particles clump into crumbs so water and air can get in. It has little effect on pH. If you want to change pH, use lime, dolomite or sulphur. If you want better-draining clay, use gypsum and compost.

In Patchmates

Log your pH readings as a garden note so you can see how each bed changes from year to year.