The pH measurement is based on a scale from 1 to 14 (pH is reported as the negative logarithm of the hydrogen ion activity). At a pH of 7.0, there is an equal balance of hydrogen (H⁺) ions and hydoxyl (OH^-) ions, and the soil (actually the soil-water suspension) is said to be neutral. Because the pH measurement is logarithmic, each unit change in pH represents a ten-fold 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.

Most Southeastern soils have a pH ranging from 4 to 8. With the exception of some native vegetation (e.g. pine trees) and a few acid-loving plants such as azaleas, blueberries, gardenias, and centipede grass, most plants do best in a slightly acid soil with a pH between 6.0 and 7.0.

Plants don’t need aluminum to grow. It’s not an essential plant nutrient. Aluminum, however, is one of the prominent mineral components of silt and clay. Therefore, the earth’s crust is naturally high in aluminum. Like zinc, manganese , copper and iron, the more acid the soil, the more aluminum will be dissolved into the soil solution. If the pH is allowed to drop much below 5.5, the availability of manganese and aluminum is increased to the point that they could become toxic to plants. Aluminum toxicity to plants is the main concern we have with acid soils in our region.

Some plants such as alfalfa, spinach, and lettuce require high levels of calcium and potassium and can tolerate high salt levels that may occur in near neutral to alkaline soils. White potatoes will do well in near neutral to slightly acid soils but are usually grown in more acid soils (pH less than 6.0) because of “scab”, a bacterial disease. As already noted, bacteria don’t thrive in very acid soils.

Parent material. Soils of the Piedmont and Sandstone Plateau regions of Alabama are very acid because of the acid nature of the rocks (granites and sandstones, respectively) which formed these soils. Limestone valley soils were formed from basic rocks (limestones) but may be acid on the surface because of time and weathering. Some Black Belt Prairie soils may be alkaline because the Selma chalk (soft limestone) which formed the soils is alkaline.

Rainfall/leaching. Rainfall also affects soil pH. Water passing through the soil leaches basic cations such as calcium (Ca²⁺), magnesium (Mg²⁺), and potassium (K⁺) into drainage water. These basic cations are replaced by acidic cations such as aluminum (Al³⁺) and hydrogen (H⁺). For this reason, soils formed under high rainfall conditions are more acid than those formed under arid conditions.

Fertilizers. Both chemical and organic fertilizers may eventually make the soil more acid. Hydrogen is added in the form of ammonia-based fertilizers (NH₄⁺) , urea-based fertilizers [CO(NH₂)₂], and as proteins (amino acids) in organic fertilizers. Transformations of these sources of N into nitrate (NO₃^-) releases H⁺ to create soil acidity. Therefore, fertilization with fertilizers containing ammonium or even adding large quantities of organic matter to a soil will ultimately increase the soil acidity and lower the pH.

NH4+ + 2O2 bacteria NO3-+ 2H+ + H2O

Plant uptake. Plants take up basic cations such as K⁺, Ca⁺⁺, and Mg⁺⁺. When these are removed from the soil, they are replaced with H⁺ in order to maintain electrical neutrality.

Lime reduces soil acidity (increases pH) by changing some of the hydrogen ions into water and carbon dioxide (CO₂). A Ca⁺⁺ ion from the lime replaces two H⁺ ions on the cation exchange complex. The carbonate (CO₃^-) reacts with water to form bicarbonate (HCO₃^-). These react with H⁺ to form H₂O and CO₂. The pH increases because the H⁺ concentration has been reduced.

H+
Soil Colloid + CaCO3 Soil Colloid-Ca++ + H2O + CO2
H+

Remember, the reverse of the above process can also occur. An acid soil can become more acid as basic cations such as Ca²⁺, Mg²⁺, and K⁺ are removed, usually by crop uptake or leaching, and replaced by H⁺.

Because high quality, finely ground limestone is very dusty and difficult to spread, some companies market a “prilled” or “granular” limestone for home use. A small amount of clay or a polymer is added to the ground limestone so small prills are formed instead of dust. This makes it easier to spread by hand or with a garden fertilizer spreader. Once applied, soil moisture will cause the “prills” or “granules” to dissociate and disperse the limestone particles. The limestone can then react with exchangeable acidity in the following manner.