How to test your soil pH level

Testing and correcting your soil’s pH is something you’ve probably heard of but, if you’re anything like me, it’s something you don’t really understand and have been too shy to admit.

But help is at hand in the form of Mrs Boyce, our house’s resident gardening expert who sat me down to explain exactly what it means, how to check it, and what to do with the results.

What is pH?

A soil’s pH level is a measure of how acid (‘ericaceous’ is another commonly used term) or alkaline (or ‘loamy’) it is. You might remember from chemistry lessons at school that a substance is said to be neutral if it has a pH figure of 7, while an acidic substance has a pH figure of 6.9 and below, with acidity growing as the numbers drop. Alkalinity, on the other hand, starts at 7.1 and rises to a high of 14.

Common acids include vinegar (with a pH of around 2-3) and coffee (5-6). Common alkalines include baking soda (8-9) and Milk of Magnesia (10-11). As you can see, the range is wide and only usually problematic for us at the extreme ends of the scale.

Plants, on the other hand, are much less tolerant; most soils span the range 4 to 8, with acidic soil being the most common. Soil with a pH of 6.5 is considered to be suitable for most, but not all, plants and vegetables.

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What affects the soil's pH level?

A soil’s acidity or alkalinity is usually caused by the presence or absence of minerals and salts from the underlying rocks and parent material, the level and purity of the rainfall that falls on it, the plants that are growing there (both natural and those introduced as crops), and the fertiliser and organic material that have been introduced to it.

How do I test soil pH?

While we used pink and blue litmus paper to test for pH in the school laboratory, establishing your soil’s pH means using a dedicated soil testing kit. 

A simple DIY soil testing kit from your local garden centre or Amazon starts at around £10, and should be enough for 10-15 individual tests. Which you might need, because your soil’s pH will vary from place to place. So it’s a good idea to test areas throughout the whole garden, marking the results on a simple map of your plot to help you figure out what to plant where, and what action, you need to take to correct your soil’s pH.

Alternatively, the Royal Horticultural Society will undertake a comprehensive analysis of your soil, which includes testing its pH level, for £25 for members, and £30 for non-members.

In acidic soil (below pH 6) hydrangea flowers turn blue. In alkaline soil (above pH 7) hydrangeas turn pink. Between pH 6 and 7 hydrangeas will be purple or a mix of blue and pink flowers.

Are there any other ways to check for pH?

Yes, there are. While none of them are as easy to interpret or use as a testing kit, the following will give you clues as to your soil’s acidity or alkalinity:

• If your hydrangeas are nicely blue and your rhododendrons are flourishing then then it’s highly likely that the pH of the soil in that area is less than seven.
• If your tap water is soft then you probably have acidic soil, while hard water can be an indicator of a soil that is mildly alkaline.

How do I change my soil’s pH?

Most soils are modestly acidic at between 6-7, a level that works well for most plants, so if that’s what you have then consider yourself lucky!

Increasing acidity

If your soil is too alkaline, attempting to increase its acidity will be a long-winded and expensive process; luckily, this is hardly ever the case. 

If you must make it more ericaceous because you want to grow an acid-loving plant, then filling a raised bed or a series of containers is the only practical way to be able to add enough leaf mould, sawdust, wood chips, or peat to the base soil to make an appreciable difference. 

Coffee grounds, which can be picked up free from almost every coffee shop, can also be used to acidify soil in smallish containers.

Increasing alkalinity

Increasing a soil’s alkalinity, on the other hand, is relatively easy and can be done by applying ground limestone, often marketed as ‘garden lime’. Hydrated Lime, sold in builders’ merchants, can also be used but needs careful handling as it is as fine as talcum powder and can irritate skin and eyes if care isn’t taken, so gloves and tight-fitting eye protection are essential.

As for quantities, a rough-and-ready guide is that approximately 0.7-1.0kg of lime per metre per square metre will raise the pH by one full point if it is dug into the top 6-8 inches of soil but applying and re-testing is the only accurate way to gauge it.

Wood ash can also be used to increase a soil’s alkalinity, and can be especially effective where club root among brassicas is a problem, in which case you’ll probably need to raise the pH as high as 7.5 to try and bring it under control.

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