Soil reactions that alter soil pH

Soil pH is an important determinant of crop nutrient availability, the efficiency of soil biological transformations, and the behavior of soil-active herbicides. In the humid Midwest, South, and East, the general tendency of soils is to acidify (lower pH) over time.

In the arid West, the opposite is true, as soil pH can be naturally high and unlikely to be lowered naturally. It all comes down to historical rainfall (over thousands of years in some cases), current climate, and the chemistry of the original soil parent material. The five soil-forming factors (parent material, climate, organisms (both plant and microbial), topography, and time) come into play as they control the original soil pH (parent material) and the loss or addition of basic cations that are associated with higher pH. A useful rule of thumb is to think of regions with annual precipitation in excess of 30" a year to be acid-favoring and those with less rainfall as preserving higher pH levels.

📸: @Sasiistock via Canva

High-pH soils are created or maintained when bases (Ca+2, Mg+2, K+, Na+) are favored over acids (H+, Al+3) on cation exchange sites. Acidic soils result from processes that generate hydrogen (H+) and/or remove bases from the soil profile via plant uptake or leaching.

Natural processes in a humid environment

A humid environment is defined as a region where incoming precipitation exceeds total plant water use. Humid regions experience both soil leaching and surface runoff and the landscape typically contains streams and water bodies. The net movement of soluble elements is downward over time in these regions.

Acidifying processes and soil reactions

Rainfall and leaching: The downward movement of water through the soil profile has the tendency to deplete basic cations, none of which are abundant in rainfall. Rainfall has a pH of 5.0–5.5 in humid regions even in the absence of "acid rain," which has been lessened by reduced atmospheric sulfur additions by coal power plants that have either been taken offline or use cleaner technology to scrub emissions. Although rain contains acidity, the amount of acidity is low compared to the enormous amount of calcium and magnesium held in most fine-textured soils.

Nitrification of ammonium: In warm, moist soils, the tendency is for ammonium (NH4+) to be converted to nitrate (NO3-) by soil microorganisms, a process known as nitrification. When comparing the chemical formula for each nitrogen form, you can see that H+ is released as the nitrogen is oxidized (addition of oxygen) into nitrate. H+ is an acid and lowers pH. Ammonium-containing fertilizers include anhydrous ammonia, UAN (urea and ammonium nitrate solutions), ammonium sulfate, MAP and DAP (mono- and di-ammonium phosphate), and ammonium nitrate. Over time, all of these fertilizers will lower soil pH.

📸: BASF Contributing Writer/Featuring anhydrous ammonia – a nitrogen form that contains acidity, which over time will lower soil pH. (Sutter, IL)

Organic matter decay: Organic matter contains complex molecules that are eventually broken down by soil microbes. In this process, both H+ and CO₂ are released. CO₂ may react with water to form carbonic acid similar to that which arrives in rainfall. One of the last steps in organic matter breakdown is the conversion of NH₄⁺ to NO₃^-, an acid generator as described above with ammonium fertilizers.

Removal of bases by crop harvest: Harvested grain contains more bases than acids, so over time, the soil pH will lower faster with higher yields. Crops that remove the entire crop, such as hay or alfalfa, drop soil pH even more quickly as the stems and leaves in that harvest contain more bases than grain alone.

📸: BASF Contributing Writer/Featuring Soybean harvest (L) and fall tillage of a 4.5% soil organic matter (R). Harvested grain removes more bases than acids, and organic matter breakdown releases acidity.

High-soil pH scenarios

Low-rainfall areas: Higher soil pH levels are associated with lower rainfall regions where soil leaching is slight to nonexistent. In areas where net evaporation exceeds precipitation amounts, the net movement of water is upward, so leaching is virtually nonexistent. Bases in the soil profile are preserved and dominate soil chemistry, keeping pH high.

Soil parent material: Many high-pH soils have calcareous geologic material and the existence of visible calcium carbonate nodules or soil layers. This "free-calcium carbonate" acts like a permanent "lime" and keeps soil pH high. These soils dominate the Western US and the fringes of the Western Corn Belt. Some eroded soils in humid regions have exposed calcium carbonate layers that keep soil pH high. Some very unique soils (Harpster series) in the humid Midwest formed in glacial ponds at the feet of terminal moraines and contain remnant snail and mollusk shells that provide calcium carbonate and high soil pH 12,000 years after the disappearance of the glaciers.

Soil pH is an important determinant of herbicide behavior and plant nutrition and should be managed when low with the addition of agricultural limestone.

Coming next: Lime additions to correct soil pH

BASF provides the information in this article as a service to its customers; however, the views expressed by guest writers are their own and do not necessarily reflect the views of BASF.