Soil pH and Liming

Soil is comprised of organic matter, minerals, water, air, and organisms on the Earth’s surface that enable farmers to plant, manage, and harvest high-yielding crops every year. Soil is the medium that allows seeds to germinate, emerge, take root, and develop into plants. Productive soil has appropriate soil tilth, drainage, microbial activity, and nutrient availability to support plant growth. Nutrient availability is mainly dependent on soil pH, shown in the figure below. Soil pH affects the solubility of nutrients and the amount of nutrients available in the soil solution, thereby affecting nutrient availability. Fourteen of the 17 essential elements for plant growth are derived from the soil. Chlorine and nickel are relatively unaffected by soil pH, and carbon, hydrogen, and oxygen are obtained from the air and water.

pH is an abbreviation for the potential (p) for hydrogen ions (H+) in water and is presented on a 14-point logarithmic scale (a pH of 6 is 10 times more acidic than pH of 7). A value of 7.0 is neutral, while values lower than 7.0 indicate an increase in the H+ ion concentration and acidity. Values higher than 7.0 increase the hydroxyl (OH-) ion concentration and alkalinity. Alkaline soils are also called basic soils. The pH is determined through soil sampling. Most soils in Midwest fields become acidic over time through environmental and farming practices.

Soil acidification is a natural process as soils age, which is accelerated by agriculture. The parent material of the soil in question is a major driving factor of soil pH.

Many different liming materials are available to combat acidic soils, including limestone, calcium oxide, calcium hydroxide, marl, slags, liquid lime, and pelletized lime. Limestone is the most common product used to correct acidic soils or raise soil pH. Limestone exists in two different forms: calcitic (CaCO3) and dolomitic (CaMg(CO3)2), and is quarried in different areas throughout the world. Liming applications are dependent on numerous things:

Liming applications are typically made every three to four years and can be as little as 1 ton/A. on sandy soils or as high as 4 tons/A. on clay soils.

The functions of liming materials are to supply a basic cation that will replace exchangeable H+ and inactivate the H+ released into the soil solution. Liming materials consist of calcium (Ca2+) and/or magnesium (Mg2+) that are capable of neutralizing soil pH. These functions are accomplished as Ca2+ and Mg2+ are released from the liming material and then act as a base, replacing the H+ in the clay particles. The H+ is then transformed into a neutral molecule, water, resulting in a reduction in soil acidity. Not all products containing Ca and/or Mg are liming materials. Gypsum (CaSO4) does not have a neutralizing capability because when the Ca is released from the gypsum and replaces the H+ in the clay particles, the H+ remains in the soil solution, resulting in no change in pH. Examples of effective and ineffective liming materials are presented below.