Overview
Acidity describes the pH of a solution. By definition, pH is a measure of the concentration of hydrogen ions (H+) on a scale of 1 to 14. Solutions with a pH between 1 and 6 have a higher concentration of H+, making them acidic, while those with a pH between 8 and 14 have lower concentrations of H+ and are basic. A pH of 7 is considered neutral. Measurements of pH are often useful in monitoring soil, plant, animal, and ecosystem health, as acidic environments harm or kill organisms.
Acidifying emissions are commonly caused by sulfur oxides (SOx), nitrogen oxides (NOx), ammonia (NH3), and carbon dioxide (CO2). Each of these molecules contribute differently to acidification. It is worthwhile to recognize that any molecule can impact multiple environmental indicators. CO2, for example, plays a significant role in global warming as described in the GHG emissions section above, but it is also responsible for certain types of acidification, mainly ocean acidification.
Sulfur dioxide (SO2) is also a contributor to ocean acidification, and, along with nitrogen oxides, is largely a consequence of burning fossil fuels. According to data collected by the NASA Ozone Monitoring Instrument Satellite, roughly two-thirds of global sulfur dioxide emissions are from oil and coal refineries and power plants. Freshwater acidification is less widely-researched than ocean acidification, but studies suggest that CO2 also decreases the pH levels of bodies of freshwater such as lakes, rivers, ponds, and streams. All types of acidification––ocean acidification, freshwater acidification, and soil acidification, which is detailed below––contribute to the larger, global problem of acidification.
Agricultural Acidification Potential
Soil acidification is mainly caused by agricultural processes. Harvesting crops and cutting down trees naturally increase soil acidity, as plants absorb basic nutrients like calcium (Ca2+), magnesium (Mg2+), and potassium (K+) from the soil. When crops are harvested, they take these compounds with them, leaving more-acidic soil behind. Additionally, ammonium-based fertilizers can impact soil and freshwater acidification, as soil converts ammonium nitrogen into nitrate (NO3-) and hydrogen ions (H+), which lower the soil’s pH levels. When applied in excess, these chemicals can leach through the soil and enter the water cycle.
This paper defines acidification potential as the sum of the acidification potential of the feed, farm, processing, transportation, storage, and retail for each individual commodity. We measure acidification potential in kilograms of sulfur dioxide equivalents (SO2 eq.) per kilogram of edible weight produced. A SO2 equivalent is a molecule whose acidification potential, when multiplied by a conversion factor, is comparable to that of SO2. For example, the acidification potential of 1 kilogram of NH3 is equivalent to 1.88 kilograms of SO2.
Impacts of Acidification
For agriculture and aquaculture, the consequences of acidification are great. Acidic soil is less fertile and can lead to significant losses in production due to the lack of basic nutrients or excessive application of chemical fertilizers, as detailed above. Decreased crop production can negatively impact livestock farmers, too, as animals like beef cattle and poultry rely heavily on crops for feed.
Acidifying emissions can also enter the water cycle through run-off, erosion, or leaching into the water table. This can decrease the pH levels of aquatic environments, disrupting the salinity of fish blood and increasing the risk of fish heart attack and suffocation. Moreover, evaporation from acidified bodies of water can create acid rain, which kills crops and trees.
Thus, acidification impacts commodity production, both for food and raw materials, and reducing emissions is ultimately in the best interest of producers, distributors, and consumers. The most widely-effective method for reducing the acidification of ecosystems is to eliminate acidifying emissions altogether. In extreme cases, however, neutralizing the pH of soil through the process of liming may be necessary to restore basic nutrients.