Organic Matter

• The breakdown of organic matter in soil, which can occur naturally or as a result of compost being added, can cause increased acid levels. As the organic material deteriorates, it releases additional hydrogen ions into the soil. Composting is generally a good gardening practice, since the majority of plants thrive in soil that is just slightly acidic, with a pH of six to seven, but it is important to maintain a balance.
  
  

Nutrient Leaching

• Acidic soil is more commonly found in wet areas than dry. Rain leaches out alkaline elements, primarily calcium, potassium, sodium and magnesium, from the soil, decreasing the soil's pH. Leaching only occurs when rainfall exceeds the amount of water that can be removed from the soil by evapotranspiration. Evapotranspiration is the process by which plants absorb water from the soil and then release it back into the air through their leaves.
  
  

Over-Cultivation

• Plants remove calcium and magnesium from the soil and replace it with hydrogen, which is easily ionized to make soil acid. Some plants, particularly those considered "high yield" for the agricultural industry, remove more of these nutrients than others do. The use of nitrogen-containing fertilizers can also increase acidity; while use of fertilizers is usually beneficial for plants short term, long-term use can be problematic.
  
  

Clay

• Clay soil often has a high degree of exchangeable acidity. Most often, the pH readings from at home soil tests only reflect the active acidity of soil. Exchangeable acidity is caused by hydrogen ions that have bonded chemically to other substances in the soil, such as clay. When active acidity is decreased, such as by adding lime to the soil, these ions are released and the soil is made acid again.
  
  

Addressing Acidity

• Acid soil is most often treated by the addition of liming agents. It can often take time for soil treatment to impact acid levels and exchangeable acidity adds an additional level of complication. The best approach to reducing acidity in soil is to take samples from several areas of the garden, and send them to a soil lab through your local extension service for recommendations on type and quantity of lime treatment to use.