Soil pH is a value that will quickly tell you if your soil is acid, alkaline or neutral. Why is this important?
Because while soils with pH values ranging from slightly alkaline through neutral to slightly acid will sustain a wide variety of plants, soils that fall outside this range require plants that are specially adapted to either very alkaline or very acidic conditions.
These plants are quite fussy about soil pH values and will not flourish in the 'wrong' conditions.
From a gardener's point of view, this means wasting your money buying plants that are never going to do well, and then wasting your time and effort planting them only to watch them die a lingering death... and, believe me, this is pretty dispiriting!
So, down to the nitty gritty...
The pH value of soil is determined by the amount of calcium or lime it contains. pH values run from 1 (very acid) to 14 (very alkaline) and pH7 is neutral. Most soils fall in the pH range 4 to 8, and the majority of plants prefer a pH in the range 5.5 to 7.5.
However, some plants, such as rhododendrons, are ericaceous, or acid loving, and need an acid soil to thrive, whilst others, including saxifrages, are calcicoles, or lime lovers, and need an alkaline soil to do well. (Just to add to the confusion, note that ericaceous plants are also sometimes known as calcifuges, or lime haters!)
Such plants are adapted to extreme soil pH ranges and their growth will suffer if they are planted in soil with the wrong pH level.
It is possible to raise the pH of acid soils by adding lime, or to lower the pH of alkaline soils by adding sulphur, but this is expensive over a large area and the results are temporary. In fact, I think as solutions go it's unsustainable.
If you have set your heart on a particular plant that needs different soil conditions, grow it in a container filled with an appropriate compost or soil mix.
As a rough guide, chalk soils and those in limestone areas will be alkaline whilst more peaty soils will usually be acidic. However, it is not always obvious and it is certainly worth buying and using either a soil pH test kit or pH meter to find out for sure.
Both DIY pH test kits and meters are inexpensive and give good results. The pH meters can also be used time and time again making them very cost effective. There are a number of reasonably priced soil test kits and meters on the market and, as well as telling you the pH, some will also test for the levels of important nutrients in your soil.
Once you have used your pH test kit or pH meter, you will know the pH value of your soil and can use this information at the garden centre or nursery to choose the right plants; most plants are labelled with information about pH and soil type.
If you need more information about soil you can get an overview of the subject, together with links to other soil-related pages, at healthy garden soil.