Soil pH and liming

Soil pH is an important soil property that has a strong influence on plant nutrient availability and in some cases herbicide behavior. In the humid regions (defined as areas where annual precipitation exceeds evaporation) that includes most of the Eastern US, most soils tend to get more acidic with time. Leaching of basic cations such as Ca and Mg, the removal of those cations with harvested grains and most importantly use of N fertilizers containing ammonia all tend to lower soil pH. For those that like a science dive the letter "p" in the chemistry world stands for "negative logarithm" so "pH" is the negative logarithm of the hydrogen concentration. 1 pH unit means a 10x change in the amount of acidity. Lower pH means more acidity. Low soil pH can lead to aluminum or manganese toxicity, reduction in biological processes such as symbiotic N fixation and release of nitrogen and sulfur from organic matter.

Calcium carbonate limestone piled up at the field edge after soybean harvest

What is lime?

There are two very specific requirements that determine if a chemical compound can be classified as "lime". 1) the compound must raise soil pH, and 2) the compound must contain calcium (Ca) and/or magnesium (Mg). The pH raising part is obvious. There are three anions that are capable of neutralizing acidity; CO₃^2-, OH^-, and O. All of the lime materials contain one of these anions that combine with H⁺ in the soil to produce either H₂O or CO₂. Here is how they neutralize acidity (H⁺)

2 H+ and O form H₂O

H+ and OH^- form H₂O

2 H+ and CO₃^2- form H₂O and CO₂

The purpose of the cations (Ca or Mg) is to force H⁺ off of cation exchange sites where it can be neutralized by one of the anions shown in the reactions above. Both Ca and Mg have properties that promote soil structure and have the ability to drive H⁺ from cation-exchange sites into the soil solution.

The most common type of lime is CaCO₃ (calcium carbonate). Other less common lime forms include CaO (burnt lime), CaOH (calcium hydroxide), and CaMg(CO₃)₂

For readers with a lot of life experience (a soft term for older people) you may remember Rolaids or Tums old school remedies for heartburn. The reaction those products caused is similar to liming an acid soil. Those products were made with CaCO₃ with a hint of flavor. The carbonate anion neutralized acid and formed CO₂ which often caused a "burp"

Freshly spread lime on an Illinois field

Lime quality and application rate

Liming recommendations can be developed from soil testing either by direct soil-buffer tests or soil pH measurements and knowledge of the some critical soil characteristics. To make a given pH change, it takes more lime for soils with higher CEC values (more clay or organic matter) than sandy, low CEC soils. Your commercially available lime will have a known effective neutralizing value (ENV) that is derived from purity, chemical formula, and fineness of grind. That ENV value is used to calculate the actual amount of your limestone equivalent to 100% pure calcium carbonate rate that is recommended by your soil test.

Lime takes time to react in the soil. Fall application is optimal and the neutralization process begins as long as there is soil moisture. Tillage can be used to mix the lime into the soil and will speed the pH change. Lime is an excellent and necessary soil amendment in regions where soil acidifies over time.

Coming next week: Anhydrous Ammonia reactions in the soil