soil acidity

Origins of acidity

There are a multiple origins of soil acidity. The following is a list of causes which are common in Hawaii:

• Release of hydrogen atoms under natural chemical processes in the soil
  • Atmospheric carbon dioxide reacts with water to form carbonic acid
  • Organic molecules react with water and cause acid dissociation
  • Oxidation of ammonium nitrogen, sulfur, and iron
    
• Accumulation of organic matter and subsequent release of fulvic and humic acid, products of decomposition
  
• Reaction of aluminum cations with water (a process known as hydrolysis)
  
• Natural Deposition
  • Lightning deposits acidic HNO₃
  • Volcanic activity deposits acidic H₂SO₄
    
• Human Factors
  • Oxidation of applied synthetic ammonium based fertilizers
  • Oxidation of nitrogen compounds in applied animal manures and/or sewage sludge
  • Deposition of acid rain (HNO₃ and H₂SO₄) caused by industrial pollution

Pools of Soil Acidity

There are three general pools, or sources, of acidity: active, exchangeable or residual.

• Active acidity is the quantity of hydrogen ions that are present in the soil water solution. The active pool of hydrogen ions is in equilibrium with the exchangeable hydrogen ions that are held on the soil’s cation exchange complex. This pool most readily affects plant growth. Active acidity may be directly determined using a pH meter, such as an electron probe.
• The second pool, exchangeable acidity, refers to the amount of acid cations, aluminum and hydrogen, occupied on the CEC. When the CEC of a soil is high but has a low base saturation, the soil becomes more resistant to pH changes. As a result, it will require larger additions of lime to neutralize the acidity. The soil is then buffered against pH change. (See base saturation discussion.)
• Residual acidity comprises of all bound aluminum and hydrogen in soil minerals. Out of all pools, residual acidity is least available.