The pH of the Soil
We call the soil alcaline if its pH is above 7.0 and acid if it is below this. If it has a of 7.0 exactly, it is defined as neutral.
It is important to know the pH of your soil in order to know the nutritive needs of your plants. For example, heath plants like rhododendrons need a lot of iron which is a trace element whose presence increases with the acidity level of the soil. In an alcaline soil, rhododendrons sufffer from iron chlorosis.
Plants generally need a pH level of between 6.5 and 7.5 to grow properly.