Soil pH and Your Lawn: Lime vs Gypsum Explained

You can feed, water and mow your lawn perfectly and still end up with thin, patchy grass if one thing underneath is out of balance, your soil pH. Soil pH controls how well your lawn can actually take up the nutrients you give it, so when it is wrong, even the best fertiliser barely works. The good news is that soil pH is easy to test and, in most cases, easy to fix. This guide explains what soil pH is, how to test it, and the difference between lime and gypsum so you know exactly which one your lawn needs.

Soil pH is a measure of how acidic or alkaline your soil is, on a scale from 0 to 14. A pH of 7 is neutral, anything below 7 is acidic and anything above 7 is alkaline. Most lawns are happiest in slightly acidic to neutral soil, roughly a pH of 6.0 to 7.0.

Why does it matter so much? Because pH decides how available nutrients are to the grass. When it drifts too low or too high, nutrients get locked up in the soil where the roots cannot reach them. A lawn with the wrong soil pH stays hungry and thin no matter how much you fertilise. Get the soil pH right and everything else you do starts working properly.

You cannot fix what you have not measured, so the first step is always to test. Checking your soil pH is quick, cheap and well worth doing before you reach for any product.

• Grab a test kit. A basic soil pH test kit or a pH meter from a garden centre is all you need, and both are inexpensive.

• Sample properly. Take a small amount of soil from a few centimetres down, not just the surface thatch, so you test the root zone.

• Test more than one spot. Soil pH can vary across a yard, so test several areas and note any that read very differently.

• Follow the kit. Most colour kits mix soil with a powder and liquid, then you match the colour to a chart to read the pH.

• Retest after treating. If you apply lime or sulfur, retest after a couple of months to check the soil pH has actually moved.

TIP: Do not guess your soil pH from how the lawn looks. Moss, yellowing and a poor response to feeding can all point to a pH problem, but only a test tells you whether the soil is too acidic, too alkaline, or actually fine.

If your test shows acidic soil, below about 5.5 to 6.0, lime is the fix. Lime, usually garden lime or dolomite, is a form of calcium that slowly raises soil pH back toward neutral and unlocks the nutrients that acidity had tied up.

Spread it evenly and water it in, and remember that heavier soils need more. As a rough guide, sandy soils need around 100 grams per square metre, loam around 200, and heavy clay 300 to 400, because clay resists a change in soil pH more than sand. Lime acts slowly over weeks and months, so apply, wait, and retest rather than piling on more. Dolomite lime is a good choice where the soil is also low in magnesium.

Here is the mix up that catches a lot of people out: gypsum does not change your soil pH. Despite the common myth, gypsum is not a liming product and will not fix acidic soil. What gypsum does is improve the structure of heavy, compacted clay.

Gypsum, or calcium sulfate, works by breaking up tight, sodic clay so water, air and roots can move through it. It adds calcium and sulfur and improves drainage without shifting the soil pH one way or the other. So if your real problem is rock hard clay rather than acidity, gypsum is your product. If your problem is a low soil pH reading, gypsum will not help and you want lime instead.

The simplest way to choose is to match the product to the problem your test revealed. Lime and gypsum solve completely different issues, and using the wrong one is a waste of time and money.

Reach for lime when your soil pH is too low and the soil is acidic, since lime is the only one of the two that actually raises pH. Reach for gypsum when the soil is heavy, sticky clay that drains poorly and compacts, whatever the pH. And yes, you can use both on the same lawn if it has both problems, acidic and clay, since they do different jobs. When in doubt, test your soil pH first and let the result decide.

Soil is one of those behind the scenes factors that quietly makes or breaks a lawn. Test it before you spend money on fertiliser or amendments, use lime to lift an acidic soil pH and gypsum to loosen heavy clay, and give either one time to work. Sort out the soil underneath and everything you do on top, from feeding to mowing, finally pays off.