pH is a soil characteristic that is affected by management, and is essential for optimal uptake of nutrients by plants.  While the concentration of hydrogen ions (H⁺) is what determines a soil's pH, other factors such as soil type, fertilization, etc. influence the concentration of H⁺, and thus the pH.  Liming soils to increase soil pH allows the plants to take up the proper nutrients.

Many liming materials are available commercially, the speed and strength of which can be calculated using Fineness Values, Calcium Carbonate Equivalents, and Effective Neutralizing Value.  These calculations are also essential to determining the rate of application.

Soil pH is the negative log of the H⁺ ion concentration: pH = -log H⁺ = log 1/H⁺.  The more H⁺ there is, the lower the pH and the greater the acidity.

• pH 7.0 = -log 0.0000001 = log 1/10^-7
• pH 6.0 = -log 0.000001 = log 1/10^-6
• Properties of pH
• The pH scale goes from 0 – 14.  pH < 7 is acidic, while pH = 7 is neutral (neither acid nor base), and pH > 7 is basic (alkaline).
• 1 pH unit represents a tenfold change in acidity.  Thus, pH 5 is ten times more acidic than pH 6, and 100 times more acidic than pH 7.
• Soils range between pH 3.5 and 9.  Hydrogen (H⁺) and aluminum ions (Al³⁺) and complexes are two primary sources of soil acidity.  Northeastern mineral soils generally have pH values between 4.5 – 8.2, while Northeastern muck soils tend to fall between 3.5 – 8.2.

Exchangeable acidity is a measure of the soil's ability to withstand a change in pH upon lime addition.  The higher the exchangeable acidity of a soil, the more lime is needed for a particular pH change.

Buffer pH is used to estimate a soil's exchangeable acidity; the degree of change in buffer pH is related to lime needs.  Buffering occurs when an acid (for instance, H⁺) and a base (for instance, OH^-) react and form a neutral product (in this case, H⁺ + OH^- → H₂O).

Alkalinity is the term used to describe the amount of base in a soil when the pH is above 7.  At higher alkalinity, there are fewer hydrogen ions (H⁺), and more hydroxyl ions (OH^-).

Basically, pH is a measure of active acidity, telling you whether you need lime or not.  Buffer pH and exchangeable acidity measure the buffer capacity, telling you how much lime you need to add.

There are several causes of soil acidity, including the leaching of basic cations (Ca²⁺, Mg²⁺, K⁺; leaving behind Al³⁺, which is acidic), crop uptake of basic cations and release of acids, decay of plant residues, acid rain, and the reaction of nitrogen fertilizer.

There are many benefits of liming: it prevents the toxic effects of aluminum, increases availability of essential nutrients, supplies plant needs for Ca and Mg, improves soil conditions for microorganisms, increases effectiveness of some key herbicides, and improves soil texture.

Nitrification or the conversion of ammonium N to nitrate N produces acidity:

2 NH₄⁺ + 4 O₂ → 2 NO₃^- + 4 H⁺ + H₂O

2 H⁺ are produced for every N in the ammonium-N form (NH₄⁺).  This reaction occurs regardless of the source of the NH₄⁺.  This acidity is often the largest single source of acidity in fertilized agricultural soils.

The net amount of acidity created when N fertilizer is applied depends on other reactions that occur with the fertilizer.  The acidity created by different fertilizer materials is summarized below, as is the lb CaCO₃ required to neutralize the acid.

This information can be used to calculate the amount of lime required to neutralize a given application of fertilizer. For example:

• Find the CaCO₃ equivalent (lime requirement) of 150 lb N/acre applied as urea: 150 lb N x 1.8 lb CaCO3/lb N = 270 lb lime requirement

• Find the CaCO₃ equivalent of 150 lb N/acre applied as ammonium sulfate: 150 lb N x 5.4 lb CaCO3/lb N = 810 lb lime requirement.

The following reactions explain why there are differences in the amount of acidity per pound of N for the different N fertilizer materials.

Lime requirements increase with CEC, and a soil's CEC increases with organic matter content and clay content.  Thus, clay soils with high organic matter content require more lime for a similar pH change than sandy soils with low organic matter.

Soil pH affects nutrient availability by changing the form of the nutrient in the soil.  Adjusting soil pH to a recommended value can increase the availability of important nutrients.  Plants usually grow well at pH values above 5.5.  Soil pH of 6.5 is usually considered optimum for nutrient availability.

Lower pH increases the solubility of Al, Mn, and Fe, which are toxic to plants in excess.  A critical effect of excess soluble Al is the slowing or stopping of root growth.

Extreme pH values decrease the availability of most nutrients.  Low pH reduces the availability of the macro- and secondary nutrients, while high pH reduces the availability of most micronutrients.  Microbial activity may also be reduced or changed.

1. Limestone is calcium carbonate and magnesium carbonate: CaCO₃ and MgCO₃
2. The limestone dissolves in water to form carbonic acid (H₂CO₃) and calcium hydroxide (Ca(OH)₂): CaCO₃ + H₂O ↔ H₂CO₃ + Ca(OH)₂
3. Carbonic acid is unstable and converts to carbon dioxide (CO₂) and water; the CO₂ gas escapes: H₂CO₃ ↔ CO₂ + H₂O
4. The remaining calcium hydroxide dissociates: Ca(OH)₂ ↔ Ca²⁺ + 2OH^-
5. The Ca²⁺ replaces 2H⁺ from the soil, increasing the soil base saturation
6. The hydroxide anion (OH^-) reacts with the soil acid cation (H⁺), forming water: OH^- + H⁺ ↔ H₂O