How to Change Your Soil pH

If your plants seem to be growing poorly with no obvious cause, working to change your soil pH may be the solution you need.

Good soil is the foundation for plants to enjoy healthy growth, delivering nutrition, moisture, and oxygen to the roots. But there’s another aspect of soil quality that is an essential factor in a plant’s chances of success. It’s known as the soil pH, and while it may seem to be one of the more technical parts of gardening, in fact it’s straightforward.

The pH of the soil is essentially a measure of how acidic or alkaline its chemistry is. The scale starts at 0 pH for highly acidic, passes through 7 pH for chemically neutral, and ends at 14 pH for highly alkaline. For most soils, the pH will be somewhere between 3 and 10 pH, with anything more extreme becoming increasingly toxic to most forms of life.

Few plants will survive in extreme soil conditions with high acid or high alkaline levels. But even in more moderate soils, different plants can have different preferences, depending largely on the conditions they’ve evolved to grow in.

But in general, most plants prefer slightly acidic soil with a pH of between 5.5 and 7.0. At this level, the nutrients in the soil are in a chemical form that’s most easily absorbed by the plant. Outside this range, many minerals become bound up into larger molecules and aren’t available to the roots, while others can be so abundant and easily absorbed that they become overpowering and actively harmful.

What’s more, soils outside the ideal pH range can be hostile to the many worms, bugs and microbes which are a key feature of a healthy, living growing environment.

Many gardeners go from season to season without worrying about soil pH. But if your plants simply fail to thrive and you can’t work out why, then checking the pH is a good first step to take in narrowing the problem down.

A straightforward pH measurement is easy using a home soil testing kit or pH meter, widely available at your local garden supplies store or online. The results these kits provide aren’t pinpoint accurate for scientific use, but are plenty close enough for most home gardeners.

If the test result shows your pH is outside the ideal scale, the basic method of fixing it is to add another substance to balance out the excessive acidity or alkalinity. The exact substance you add depends on the pH reading, but in all cases it’s sensible to use a light touch. It’s easy to go too far when trying to correct pH, and wind up unbalancing the soil in the opposite direction.

It’s natural for soil to grow slowly more acidic over time, as organic material within it decomposes and produces a weak carbonic acid. But if the pH falls too low, adding lime will push it back up to a more neutral level.

Lime is easily available in a powder form that’s designed specifically for adding to soils. When using it to make sour soil less acidic, follow the instructions carefully, and retest the pH frequently to ensure you don’t overshoot.

If you think a milder treatment is all that’s needed, sprinkling wood ash over mildly acidic soil will tend to make it more neutral, as well as add valuable nutrients including potassium.

Less commonly, you may need to add an acidic material to counteract too much natural alkalinity. For soils only slightly above 7 pH, simply adding organic mulches, composts and manures will tend to bring the alkaline levels down over time.

For slightly higher pH values up to around 8, a fertilizer that’s rich in ammonia will be effective and quick. But when more drastic correction is needed, sulfur is the most common substance used to acidify the soil. Most often available as aluminum sulfate or iron sulfate in a commercial soil treatment, sulfur will react with rainwater to produce a mild sulfuric acid to lower the pH and bring the chemistry back into balance.

While the pH of soil is extremely important, it’s not the only characteristic that matters. Read our article on Straightforward Tips for Soil Improvement to learn how to provide the best possible environment for your plants to thrive.