For some people soil is the dirt they get on the clothes or skin when they decide to get down and dirty. For farmers and gardeners, soil is the basis of their livelihoods.

That said, just as the driver of a vehicle does not have to understand all the workings of the internal combustion engine, neither does a gardener have to understand all the intricacies of the soil, to make it work. But a basic understanding can help greatly.

Soil pH and acidity

Science uses pH as a measure of the acidity of any substance and soil is no different.

For most plants and soils it is good to have the pH between 6.5 and 7, as this enhances the availability of the main nutrients and also provides a better environment for the soil bugs to thrive. While most species prefer soils to be close to neutral pH (7), others prefer acidic soils.

Much of the soil in this country tends to be naturally acidic with a lower pH. But soils overlying limestone rock tend to have naturally high pH. So, sometimes the origin of your soil will limit your choice of plant.

We can raise the pH of an acid soil by the addition of lime.

However, lowering a high pH soil is much less practical. But for those who really want to plant acid-loving plants like rhododendron or azalea, it is common to see pits dug out and filled with peat to produce local acidity. However, this will only give a certain time span of usefulness, as roots will grow out of the pit and soil organisms will mix other soil through it.

Other actions that might be considered to lower soil pH include the use of sulphate materials such as ammonium sulphate, aluminium sulphate or even iron sulphate. It can also be done more slowly over time through the incorporation of naturally acidic organic materials such as conifer needles, sawdust, peat moss and oak leaves. However, these effects can be shallow in the soil and it would take a long time to make a high pH soil suitable for an acid-loving plant.

Plants that can cope with either

A species like hydrangea can cope with lower or higher pH soils and it influences the colour of the flowers due to its pH. If you can prepare the ground to alter the pH before planting – great. But can you do anything to decrease the pH in the soil in which some of these species are already growing?

While the most correct answer is very little, a common action in the past was to place a number of rusty iron bars into the soil around the plant. The rusting process will accelerate in the soil and so lower the soil pH around the plant roots.

This can be seen to work as flower colour in the traditional varieties changes from pink (more alkaline) to blue (more acidic). Indeed, it is possible that this process would yield a mix of flower colours on the same plant.