Why pH is important

It’s hard to find an article on gardening that doesn’t reference pH simply because it is important.  ‘It’s too acid for Azaleas’ or ‘You need to add dolomite’ are the kinds of soil talk you hear. Get the pH wrong and you will struggle growing plants. Let’s take a little dive into some chemistry and see how it applies to our patch.

In plain talk, PH is about acidity and alkalinity. A growing media with a ph of less than 7 is ‘acid’, above this is ‘alkaline’ and bang on 7 is neutral. You may hear the words ‘sour’ to describe very acid soils and this is because the media smells of sour cream or vinegar, which are acid. ‘Sweet’ smelling soils are slightly alkaline.

Technically speaking, pH refers to the concentration of hydrogen ions; the higher the concentration the lower the pH. Got it?

Low pH decreases plant growth because nutrients become less available, nutrients are held in the soil, the store of nutrients increases to toxic levels and microorganisms have a tough time living in very acid soils. It’s these little fellas that mine the rocks and convert minerals into a form usable by the plants, convert organic material into humus and convert atmospheric nitrogen to ammonia.

A wide range of plants grow happily in the range of pH 5 to 7 but when your media (soil in the garden or soilless media in the house plant pot) drifts outside these values you will run into trouble.

Coastal soils are generally acid (with considerable variations dependant on where you live) and lacking in nutrients. Thousands of years of rainfall leaching will do that. If you add dolomite (ground limestone) or better still Maglime fines, you will raise the soil pH. Start with 250gms for every square metre, spreading over the surface and water in gently. Warning! Don’t use builders lime /quicklime as they are much more concentrated and you will overshoot.

The lower the starting pH the more dolomite will be required.  The dolomite will be incorporated into the topsoil layers and over a period of time (a week to a month) will change the pH. Check again in 3 months to a year to see how the pH value changed. Don’t go silly now- pH is a logarithmic scale meaning that you need say a hundred grammes to go from pH5 to 6 but only ten percent, 10 gms, to raise it from 6 to 7. Once it’s right, only minor adjustments will be required.

The opposite of acid soils is alkaline ones. It follows that you need acid to bring the pH down to neutral or slightly acid. Here sulphur is used. There are a few areas that need this and specialist advice is required beyond the scope of a blog.

Good gardens happen because the soil structure is right, there is sufficient organic material, moisture and nutrients are freely available, microorganisms abound and the pH is right.

You are aiming for slightly acid soils measuring pH 5.5 to 7, get liming!