Alys Fowler has some instructions on how to make your own pH test:
To check your soil pH, you can buy a kit from the garden centre, but the easiest way is to make one yourself, using red cabbage. This contains a water-soluble pigment, called a flavin, that turns red in acid conditions, has a purplish tinge in neutral conditions, and shows bluish green to greenish yellow in alkaline conditions.
Bring two cups of chopped red cabbage to the boil and let them cool. Drain off the water – and eat the cabbage! Put about a tablespoon of soil in a cup and half-fill this with water. Stir the soil around so that it’s suspended in the water. Now add about 3mm of cabbage water.
The liquid will turn a purplish red with a slight tinge of blue for a neutral soil. Greenish-yellow is very alkaline and very red is acid. You should check soil at various locations around your garden. Be aware that builder’s rubble contains a lot of lime which will give alkaline results, so make sure you take a few different readings.
– by Alys Fowlder