Soil Acidification

There are several variables influencing the outcome and production of any ornamental, cash crop or landscape. Weather and environment are the largest of variables, and unfortunately that is just the beginning of obstacles. One inevitable, and often overlooked, variable is soil acidity. Where most have been taught soil acidity is a product of using highly synthetic fertilizers or the result of acid rain there are several contributing factors beyond our control. What is acidity and where does it come from?

Acidity is considered one of the most important variables because it affects several biological and chemical soil properties. Soils with lower pH may even experience a higher fungi population. As a result, acidity can even change the physical properties of soil by promoting air and water movement throughout the soil and stabilize aggregate structures  “Acidity (or alkalinity) is usually quantified using the pH scale, which expresses the activity of concentration of H⁺ ions present in a solution.” (Brady and Weil et al. 2016) Where do hydrogen ions come from?

Carbonic acid is one of the most universal contributors to soil acidity  It is formed when carbon dioxide gas from soil air dissolves in water. This is caused by root respiration and the decomposition of soil organic matter by microorganisms. 

Organic acids are generated as microbes break down soil organic matter through biological metabolism.  These acids can range from citric and malic acids to much stronger types like carboxylic and phenolic groups in humic substances produced by litter breakdown. 

Accumulation of organic matter acidifies the soil for two reasons. During this process, cations are lost by leaching when organic matter forms soluble complexes with nonacid cations. The second is organic matter is a source of H⁺ because it contains many acid functional groups where ions can separate. 

The oxidation of nitrogen, or nitrification, is not solely a consequence of human overapplication.  Nitrogen can be oxidized from organic matter as well as fertilizers. The oxidation reaction produces hydrogen ions as a product which can be a result of bacterial and chemical processes. Two H⁺ ions are released for every NH⁺ ion oxidized.

Acids in precipitation is not always a result of pollution. It is a natural phenomenon and is a variety of acids that contribute hydrogen ions to the soil. As raindrops pass through the air, they dissolve CO² forming carbonic acid. This changes the pH of the water from approximately 7 to about 5.6. Pollution, lightning and natural disasters can contribute to a lower pH precipitation (acid rain). 

Plants must maintain a balance of positive and negative charges they take up from the soil. For every positive charge taken up by the plant it can either take up a negative charge or get rid of a positive charge. If the plant takes too many certain cations, it generally will release H⁺ ions into the soil to maintain balance. 

Soil acidification is inevitable. The good news is many growers can contradict these variables through liming practices. This is why it is always important to soil test and maintain a certain balance for crop/soil health and nutrient availability. 

Source: Brady, N. C., & Weil, R. R. (2016).  The Nature and Properties of Soils (13th ed.). Pearson.