The pH is the level of acidity or alkalinity within the soil. This is brought about by chemical reactions within the soil, and results in a pH value between 1 and 14: 1 being highly acidic, 14 being very alkaline. The middle point (7), is referred to as neutral. The chemical reactions when the soil is acidic are very different to those when it is alkaline.
An acidic soil is regularly high in aluminium, which is poisonous to plants in high enough doses. An alkaline soil is often deficient in iron and phosphorus, and both can be low in magnesium, which is one of the building blocks for chlorophyll, where photosynthesis takes place and creates food for the plant to survive. The pH of our soil is, therefore, significant for us as gardeners, as the success of the plants we grow is actively dependent upon this.
As you travel around the country, each area has its own soil type which is often either acidic or alkaline. The South Downs and the Chilterns are both known for having a chalk base rock which makes the soil alkaline, whereas the soil around Bracknell Forest (on the way to Heathrow Airport) is nearly all acidic. However, plants are well known for living in these challenging circumstances and many have adapted to thrive in these less-than-ideal areas.
So how do you determine the pH of your soil? The most reliable method is via a pH testing kit, which can be sourced reasonably cheaply from garden centres or on the internet. The ideal pH is around 6.5 (as most plants will live happily here and most nutrients required for growth are available), but if your soil falls in the range of 5.5 to 7.5, this can usually be easily remedied. In some areas of chalk soils, there may be loose lumps of chalk that have not broken down within the soil which will ensure a raised pH. You can test if this is the case in your garden by adding vinegar to a soil sample. If fizzing is witnessed, then free chalk is present.