Soil testing and pH for lawns: how to test, read and fix your soil

When a lawn struggles, it’s tempting to reach for fertiliser first. But if the real problem is soil that’s too acid, too alkaline, too sandy to hold nutrients, or already full of phosphorus, more fertiliser won’t help. It can even make things worse.

A soil test tells you what’s actually going on. A home pH kit takes ten minutes and costs little. A laboratory test costs more but tells you far more. This guide covers how to take a good sample, which test to use, what the results mean, and how to correct soil that’s too acid or too alkaline.

When to test

• Before laying a new lawn. It’s the only time you can mix amendments right through the soil.
• When a problem won’t go away, such as yellowing that doesn’t respond to feeding, or a lawn that thins despite good care.
• Before buying lime, sulfur or other amendments, so you know whether you need them.
• Before a feeding program, if you don’t know what your soil needs. The Fertilise Wise brochure produced for Perth councils recommends understanding your soil before you fertilise.
• Every few years on an established lawn, to see whether anything is changing.

What pH means

Soil pH measures how acid or alkaline the soil is, on a scale where 7 is neutral, lower numbers are acid and higher numbers are alkaline. Each whole number is a tenfold change, so a pH of 5 is ten times more acid than a pH of 6.

pH matters because it changes how easily grass roots can take up nutrients. In strongly acid or strongly alkaline soil, some nutrients become hard to reach even when they’re present, and others can build up to harmful levels. That’s why a lawn in the wrong pH can look hungry however much you feed it.

Water Corporation’s advice on establishing a new lawn puts the ideal for most lawn grasses at fairly neutral: a pH of 6 to 7.5.

Water or calcium chloride?

Laboratories measure pH in one of two ways, and the results differ. NSW Department of Primary Industries explains that pH measured in calcium chloride (CaCl₂) reads lower than pH measured in water, as a rule of thumb by about 0.7 of a unit. Check which method your result uses before comparing it with a target. Home kits generally give a result closer to the water method.

Taking a good sample

The result is only as good as the sample. Soil varies across a lawn, so one scoop from one spot can mislead you. Soil-testing guidance for farmers, from NSW and Victoria, gives the principles; they work just as well for a lawn.

1. Use clean tools and a clean bucket. Traces of fertiliser, lime or garden soil on a trowel will change the result.
2. Decide what you’re testing. Test the front and back lawns separately, and a problem patch separately from healthy lawn. Don’t mix them.
3. Take many small samples. NSW Government guidance for paddocks suggests 20 or more. A lawn is smaller, but the principle is the same: lots of small samples from across the area, mixed together, give a fair average.
4. Sample the top 10 cm. That’s where most lawn roots are. Push the trowel or corer to the same depth every time, and leave out the grass and thatch on top.
5. Avoid unusual spots: under trees, near buildings and paths, dog urine patches, places where fertiliser or lime has been spilt, and wet hollows.
6. Mix the samples thoroughly in the bucket, breaking up lumps.
7. Bag the amount the lab asks for, label it with the area and date, and send it promptly.

Victorian guidance for farmers suggests not sampling for 6 to 8 weeks after the last fertiliser application, so the test reflects the soil rather than the last feed. Sending samples the day you collect them, or drying them at a low temperature first, helps keep the result accurate.

Home pH kits

Home kits are cheap and quick. Gardening Australia describes how most work: you put a little soil on a white card, add a few drops of indicator dye, dust it with a white powder, and compare the colour with a chart.

• Test several spots, because pH varies across a garden.
• Use your mixed sample for a fair average, and test problem areas separately.
• Follow the kit’s instructions exactly, and check the use-by date on the dye.

A home kit tells you whether soil is strongly acid, near neutral or strongly alkaline. It doesn’t measure nutrients, and it’s less precise than a laboratory.

Laboratory tests

A laboratory test usually reports pH and a range of other measurements, such as:

Ask the laboratory for a home garden or turf test, and ask how to interpret the results. [CONFIRM: reviewer to confirm target nutrient ranges LawnMan should publish for home lawns]

If your soil is too acid

Acid soil is common in many parts of Australia. Agriculture Victoria explains that soils become more acid over time through natural processes and farming and gardening practices, and that lime is used to correct it.

• Use garden lime or dolomite. Gardening Australia recommends either to raise pH. Dolomite also adds magnesium.
• Use the rate on the product, adjusted for your soil test and your soil type. Clay soils need more than sandy soils to make the same change.
• Spread it evenly with a calibrated spreader, and water it in. On an established lawn, lime can only go on the surface, so it works its way down slowly.
• Don’t try to fix it all at once. Several smaller applications, months apart, are gentler on the lawn.
• Retest after several months before adding more.

If you’re laying a new lawn, mix lime through the soil as you prepare it, so it reaches the root zone straight away.

If your soil is too alkaline

Alkaline soil is harder to change. Colorado State University Extension explains that elemental sulfur can lower pH, but slowly: it depends on soil moisture, temperature and soil microbes, and may take several months to a year to work, with repeat applications needed. Soils that contain free lime are impractical to acidify at all.

For most alkaline lawns, the practical approach is to:

• choose fertilisers suited to alkaline soils, following their labels;
• add organic matter through topdressing after coring;
• choose a grass that copes with your soil if you’re replanting;
• ask a soil laboratory or local expert before using sulfur, and follow the product’s directions exactly.

A worked example

A back lawn has been pale and thin for two summers, despite regular feeding. The owner takes 20 small samples from the top 10 cm, avoiding the dog’s favourite spot and the area under the gum tree, mixes them and sends the sample to a laboratory.

• The result: pH 5.2 measured in calcium chloride. Using NSW DPI’s rule of thumb, that’s roughly 5.9 in water, just below the 6 to 7.5 range Water Corporation describes as ideal.
• Phosphorus: high, so there’s no need to add more.
• The plan: spread garden lime at the rate on the product for a sandy soil, water it in, and switch to a low-phosphorus lawn fertiliser at the label rate.
• The check: retest the same way the following year before adding more lime.

The numbers here only illustrate how to read a report. Your own results, and the advice of the laboratory, decide what you do.

Other things a test can reveal

• High phosphorus. If your soil already has plenty, choose a low-phosphorus fertiliser. Excess phosphorus can wash into waterways and feed algal blooms.
• Salty soil. High EC can come from salty bore water, greywater or overuse of some fertilisers.
• Low organic matter. Common in sandy soils, which also hold few nutrients. Topdressing with compost after coring helps.
• Sandy soil that won’t wet. A test won’t measure repellency, but a water drop test will. See the hydrophobic soil guide.

Before laying a new lawn

Testing before you lay turf or sow seed is the best value test you’ll ever do.

1. Sample the prepared area once the old lawn and weeds are gone.
2. Correct pH by mixing lime through the soil if it’s acid.
3. Mix in organic matter or clay-based amendments if the soil is very sandy.
4. Retest if you made big changes.
5. Lay the turf or sow the seed.

See the laying new turf and starting from seed guide.

Common mistakes

• Testing one spot and treating the whole lawn on the result.
• Mixing samples from very different areas.
• Using a dirty trowel or bucket.
• Comparing a CaCl₂ result with a water-based target, or the other way round.
• Adding lime “just in case” without testing.
• Adding more lime too soon, before the first application has had time to work.
• Expecting quick results from sulfur on alkaline soil.

Log it

Record each test in your Lawn Journal: the area, the date, the method, the results and anything you applied afterwards. When you retest, you’ll be able to see whether your changes worked.