The use of certain types of fertilizer for food crops slowly deteriorates the condition of the soil. Calcium hydroxide added to standard ammonium fertilizers provides an answer, but the U.S. government has decided that its negative side effects outweigh its potential benefits.
Fertilizers that contain ammonium are used commercially to provide crops with nitrate, an important source of nitrogen for plants. The ammonium reacts with water to form nitrate, but the reaction slightly acidifies the soil.
Soil that is too acidic presents two different problems for crops. First, the amount of necessary minerals in the soil such as potassium and calcium decreases. Second, aluminum and manganese levels increase to potentially toxic levels below a pH of roughly 5.5.
Calcium hydroxide, which is produced by adding water to calcium oxide, addresses the problem presented by ammonium fertilizer. It reacts in water to release calcium ions and hydroxide ions, which raise the pH of the soil.
Calcium hydroxide is toxic to humans; it can chemically burn skin and can damage the respiratory system if it is inhaled. Also, the National Organic Standards Board states that safer alternatives exist, such as limestone and gypsum. Thus, it is illegal to use calcium hydroxide on food crops in the United States.
Between 2005 and 2008, Hawaii declared that calcium hydroxide could be used as a contact pesticide against coqui and greenhouse frogs. Even during that time, application to food crops was not allowed.