Soil Acidity
Effects of Soil Acidity
Soil pH controls a wide range of physical, chemical, and biological processes and properties that affect soil fertility and plant growth. Soil pH, which reflects the acidity level in soil, significantly influences the availability of plant nutrients, microbial activity, and even the stability of soil aggregates.
Nutrient Availability
The availability of some plant nutrients (elements) is directly affected by soil pH because pH affects the solubility of the compounds which contain plant elements. Most plant nutrients have a pH range that is optimal for their availability (Figure 14.4). In acidic soils, the availability of the major plant nutrients nitrogen, phosphorous, potassium, sulfur, calcium, magnesium and also the trace element molybdenum is reduced and may be insufficient for plant growth.
Aluminum and Manganese Toxicity
The actual amount of aluminum and manganese in solution in a soil at a given pH varies between soil type. When the soil pH is above about 5.5, the aluminum in soils remains in a solid combination with other elements and is not harmful to plants. As the pH drops below 5.5, aluminum containing materials begin to dissolve. Because of its nature as a cation (Al3⁺), the amount of dissolved aluminum is 1000 times greater at pH 4.5 than at 5.5, and 1000 times greater at 3.5 than at 4.5. For this reason, some crops may seem to do very well, but then fail completely with just a small change in soil pH. Wheat, for example, may do well even at pH 5.0, but usually will fail completely at a pH of 4.0.
Soil Microbial Activity
Most microbial processes are reduced in acidic soils because growth and reproduction of the soil microbes, primarily bacteria and fungi, are reduced. Breakdown of organic matter and cycling of nutrients is reduced. The rate of mineralization of nutrients by soil microbes into plant-available forms is slower, potentially limiting plant uptake. Nitrogen and sulfur, for example, are associated mainly with soil organic matter. Organic matter must decompose before nitrogen and sulfur become available to plants. Because microorganisms, which are influenced by pH, affect decomposition, the rate of soil organic matter decomposition is pH-dependent.
Soil Physical Properties
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.
Click on the following topics for more information on soil acidity.
