Soil Acidity
Causes of Soil Acidity
Some of the more important factors influencing soil pH are parent material, crop production, acid-forming fertilizers, precipitation, and organic matter.
Parent Material
TRocks from which parent material originally formed vary from acidic to alkaline in reaction. Soils that developed from granite material are likely to be more acidic than soils developed from calcareous shale or limestone.
Acid Forming Fertilizers
Nitrogen fertilizers have a greater acidifying effect on soils than other fertilizers. Two processes are involved. First, commonly used nitrogen fertilizers contain ammonium nitrogen (urea is an ammonium forming material). Soil bacteria convert ammonium (NH4⁺) to nitrate (NO3¯) through a biochemical process called nitrification. Hydrogen (H⁺) ions are released in this process, and free hydrogen ions cause an increase in acidity. The second acidifying effect comes from nitrate that is not taken up by the growing crop. Nitrates are very soluble and, if not taken up by plants, will move downward with soil water and may be carried below the root zone.
Crop Production
Plant roots take up nutrients as either cations, which are positively charged (such as NH4⁺, Ca2⁺, Mg2⁺, or K⁺) or as anions, which are negatively charged (such as, NO3¯, PO43¯, or SO24¯). Plants absorb more cations than anions, which means that most plant material is slightly alkaline. In a natural system, when plants die, they are decomposed and returned to the soil, balancing the acidity caused by the hydrogen ions. In agriculture, if plant material is removed by grazing or harvest, rather than being returned to the soil, some of the basic cations responsible for counteracting the acidity is lost and residual hydrogen ions remain in the soil contributing to soil acidity.
Precipitation
Soils can become acid even in the absence of crop removal or fertilizer applications. In the U.S. there is a fairly strong correlation between excessive precipitation and pH, with soils receiving more than about 30 inches (76cm) of annual precipitation. Rainfall is considered a natural cause of acidity because as water moves down through the soil profile, it has a slow but persistent acidifying affect. Weak acids are produced in the soil when plant residues and organic matter decompose.
Organic Matter
While increasing organic matter has many benefits, including improvement of soil structure, it also increases soil acidity. The accumulation of organic matter tends to acidify the soil for two reasons. First, organic matter forms soluble complexes with such cations such as calcium (Ca2+) and magnesium (Mg2⁺), thus facilitating the loss of these cations by leaching.
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