Measuring Acidity and Alkalinity
Soil pH is a measurement of the hydrogen ion concentration (H+) in solution. The pH is measured on a scale from 1 to 14 in solution (pH is reported as the negative logarithm of the hydrogen ion activity). At a pH of 7.0, there is an equal balance of H+ ions and hydroxyl (OH–) ions. Therefore, the soil-water suspension is considered neutral. Because the pH measurement is logarithmic, each unit change in pH represents a tenfold increase in the amount of acidity or basicity. That is, a soil solution with a pH of 6.0 has 10 times as much active H+ as one with a pH of 7.0. Pure water with no minerals has a H+ activity of 10-7 grams/liter (0.0000001 g/L), explaining why a pH of 7.0 is considered neutral.
The pH of any solution is one of the easiest laboratory measurements to take using a pH meter and an electrode specifically designed to measure hydrogen (pH electrode). Color indicators and litmus paper are a quick alternative for less precise measurements. Soil testing labs mix a quantity of soil with demineralized or distilled water (usually a 1:1 mixture) to determine soil pH.
Most Alabama soils have a pH ranging from 4 to 8. Most crops do best in a slightly acidic soil with a pH between 5.8 and 6.5, with the exception of some native vegetation, such as pine trees and a few acid-loving plants, such as azaleas, blueberries, and centipede grass.
Acid Soil Infertility
When pH falls below 6.0, the availability of most macronutrients, including phosphorous, potassium, calcium, and magnesium, decreases. However, the availability of the metallic micronutrients zinc, manganese, copper, and iron increases as pH decreases. If the pH drops much below 5.5, the availability of manganese and aluminum increases to the point where they could become toxic to plants. Aluminum toxicity is a major concern in our acid soils.
Factors Affecting Soil pH
Soils are not homogenous, and pH varies considerably throughout fields and at different soil depths. Soils in different geographic regions also have differing pHs because of different forming factors.
Soils of the Piedmont and Sandstone Plateau regions of Alabama are acidic due to the nature of native rocks, such as granites and sandstones, forming these soils. Limestone Valley soils formed from basic rocks, such as limestones. These soils may be acidic on the surface because of time and weathering. Most Black Belt Prairie soils are alkaline because the Selma chalk forming the soils is alkaline.
Rainfall and Leaching
Rainfall also affects soil pH. Water passing through the soil leaches basic cations such as Ca2+, Mg2+, and K+, into drainage water. Acidic cations replace basic cations, such as Al3+ and H+. Soils formed under high rainfall conditions are more acidic than those formed under arid conditions.
Both chemical and organic fertilizers can eventually make the soil more acid. Hydrogen is added in the form of ammonia-based fertilizers (NH3), urea-based fertilizers [CO(NH2)2], and proteins (amino acids) in organic fertilizers. Transformations of these sources of nitrogen into nitrate (NO3 –) release H+, creating greater soil acidity. Therefore, using fertilizers containing ammonium, or even adding large quantities of organic matter to a soil, will ultimately increase the soil acidity, lowering pH.
2NH3 + 3o2 bacteria 2NO3– + 6H+
Plants take up basic cations, such as K+, Ca2+, and Mg2+. When these are removed from the soil, they are replaced with an H+ to maintain electrical neutrality. Again, this creates a more acid solution.
Raising Soil pH (Liming)
Adding lime reduces the harmful effects of low pH and provides calcium and magnesium to the soil. The amount of lime needed to achieve a desired pH depends on the current pH and the buffering capacity of the soil.
Lime reduces soil acidity, or increases pH, by changing some of the free hydrogen ions into the water and carbon dioxide (CO2). A Ca2+ ion from the lime replaces two H+ ions on the cation exchange complex. The carbonate (CO32-) reacts with water to form bicarbonate (HCO3–), which reacts with H+ to form water and CO2. The pH increases because the H+ concentration has been reduced.
The reverse of this process can also occur, forming an acid soil. This occurs as basic cations, such as Ca2+, Mg2+, and K+, are removed, usually by crop uptake or leaching, and replaced by H+.
The most common liming materials are calcitic and dolomitic agricultural limestone, natural products made from finely ground natural limestone. Natural limestone is fairly insoluble in water, but when finely ground it can be thoroughly mixed with the soil, allowing a reaction with the soil’s acidity. Either will neutralize soil acidity.
- Calcitic limestone is mostly calcium carbonate (CaCO3).
- Dolomitic limestone must have at least 6 percent magnesium, according to Alabama state law. It is made from rocks that contain a mixture of calcium and magnesium carbonates. Dolomitic limestone also provides magnesium.
Producers can apply lime at any time during the year. But, it may take several months for the lime to react with soil to change pH. Caustic liming materials, such as burned lime, or wood ashes, should not be applied to actively growing plants. Ground limestone (calcitic or dolomitic) will not harm actively-growing plants.
Lowering Soil pH
Lower soil pH by adding sulfur or aluminum sulfate [Al(SO4)3] to a soil or by heavy fertilization with ammonium fertilizers over several years. With the addition of elemental sulfur, soil bacteria oxidize the sulfur to sulfate, creating sulfuric acid, which lowers the pH. The aluminum in aluminum sulfate replaces the basic cations in the soil and hydrolyzes to release H+, thus lowering the soil pH.
The pH of soils naturally containing a large excess of alkaline materials, such as natural limestone or chalk (calcium carbonate), is not easy to lower. It takes 32 pounds of sulfur to neutralize 100 pounds of lime. Some Black Belt soils may be 50 percent calcium carbonate and cannot be easily acidified. Only soils that have been inadvertently overlimed and are to be used for blueberries, azaleas or other acid-loving plants can be practically acidified.