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Unformatted text preview: SSC 200 Soil Acidity and Adjusting pH Introduction Soil pH is of paramount importance to soil fertility, that is, it has a major impact on the productivity of soil by affecting nutrient availability. Many times soils, especially in the clay Ultisols of North Carolina, are highly acidic and needed to be limed in order to raise the soil pH to optimum levels. In order to accomplish this, one must first understand the nature and definition of soil acidity. Soil acidity is primarily the hydronium ion, H O , concentration in a soil. It is divided into three categories, exchangeable acidity, active acidity, and reserve acidity. Soil exchangeable acidity is the activity of hydrogen ions held near clay and humus surfaces, active acidity is the activity of the hydrogen ions in solution in the soil, and the soil reserve acidity is the activity of hydrogen ions bound to clay and humus colloids. The sources of soil acidity vary greatly, including the atmospheric deposition of sulfur from the burning of fossil fuels, the selective leaching and exchange of cations, the production of hydrogen ions from the reaction of carbon dioxide and water within the soil, the uptake of basic cations and the excretion of hydrogen ions, the decomposition of organic residues, the oxidation of ammonium ions to produce nitrates by microbes, and the hydrolysis of aluminum. In order to raise soil pH and lower soil acidity, lime, a group of naturally occurring minerals and substances derived from them that raise soil pH, is often added. Lime first reacts in the soil by replacing Al ions on the cation exchange complex. The carbonate ions in the lime then react with hydrogen ions in the soil, neutralizing them and producing water. The Al ions are then hydrolyzed to produce Al-hydroxides and hydrogen ions. The remaining hydrogen ions and react with carbonate ions to produce water. The purity, type, and texture of lime affect its effectiveness in raising soil pH. Different types of lime vary in purity, as the purer a lim ing substance is, the more effective it will be at raising soil pH. Also, the finer a lim ing substance is, the faster it will dissolve and react with the soil, raising pH. Different soils have different abilit ies to resist changes in soil chemistry and pH, a concept which is known as buffering capacity. Buffering capacity uses different mechanisms at different pHs to prevent sudden changes in soil chemistry. At very low pHs (2.0-4.0), the dissolution of m inerals buffers sudden change, at pHs of 4.0 to 5.5, alum inum compounds serve as buffers, at pHs of 5.5 to 6.8, cation exchange serves as a buffer, at pHs of 6.8 to 7.2, organic matter and m inerals buffer changes, and at pHs of 7.2 to 8.5, calcium and 7....
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This note was uploaded on 05/02/2008 for the course SSC 200 taught by Professor Crouse during the Spring '08 term at N.C. State.
- Spring '08