Soil test kits are simple to use although they tend to show the pH as a colour not a number. Acid soil shows as yellow-orange, neutral soil turns the solution green and alkaline soil turns it dark green. The kits range in price from a few dollars up to forty dollars or more.

Soil can gradually become acidic due to the decomposition of organic matter or if nitrogen fertilizers are used. Hydrogen ions are produced and these cause calcium, magnesium and potassium to be dislodged from the soil allowing them to be washed away and lost. As the hydrogen ions increase soil becomes more acid.

The essential nutrients available for plant growth alter dramatically as soil gets more acidic. Phosphorus availability is maximal at pH 6.5. Decreasing the pH to 5.5 reduces available phosphorus down to 50 percent and other essential elements, such as molybdenum and calcium are also reduced quite drastically.

If pH values drop below 5.5 aluminium and manganese levels can build up to toxic quantities. The bacteria that converts ammonium nitrogen into nitrate function best at soil pH levels above 6.0 so a decrease in pH will reduce their activity as well as the activity of other beneficial fungi. Many disease producing fungi are a lot more active in acidic soils too.

See this (Link to page 3) for more information on the most common types of soil and how to tell what kind of soil you have in your garden.