Soil acidification

Acid rain

Rainfall is naturally acidic due to carbonic acid forming from carbon dioxide in the atmosphere.^[3] This compound causes rainfall pH to be around 5.0–5.5. When rainfall has a lower pH than natural levels, it can cause rapid acidification of soil. Sulfur dioxide and nitrogen oxides are precursors of stronger acids that can lead to acid rain production when they react with water in the atmosphere. These gases may be present in the atmosphere due to natural sources such as lightning and volcanic eruptions, or from anthropogenic emissions.^[4] Basic cations like calcium are leached from the soil as acidic rainfall flows, which allows aluminum and proton levels to increase.^[5][6]

Nitric and sulfuric acids in acid rain and snow can have different effects on the acidification of forest soils, particularly seasonally in regions where a snow pack may accumulate during the winter.^[7] Snow tends to contain more nitric acid than sulfuric acid, and as a result, a pulse of nitric acid-rich snow meltwater may leach through high elevation forest soils during a short time in the spring.^[8] This volume of water may comprise as much as 50% of the annual precipitation. The nitric acid flush of meltwater may cause a sharp, short term, decrease in the drainage water pH entering groundwater and surface waters.^[9] The decrease in pH can solubilize Al³⁺ that is toxic to fish,^[10] especially newly-hatched fry with immature gill systems through which they pass large volumes of water to obtain O₂ for respiration. As the snow meltwater flush passes, water temperatures rise, and lakes and streams produce more dissolved organic matter; the Al concentration in drainage water decreases and is bound to organic acids, making it less toxic to fish. In rain, the ratio of nitric-to-sulfuric acids decreases to approximately 1:2. The higher sulfuric acid content of rain also may not release as much Al³⁺ from soils as does nitric acid, in part due to the retention (adsorption) of SO₄^2- by soils. This process releases OH⁻ into soil solution and buffers the pH decrease caused by the added H⁺ from both acids. The forest floor organic soil horizons that are high in organic matter also buffer pH, and decrease the load of H+ that subsequently leaches through underlying mineral horizons.^[11][12]

Biological weathering

Plant roots acidify soil by releasing protons and organic acids so as to chemically weather soil minerals.^[13] Decaying remains of dead plants on soil may also form organic acids which contribute to soil acidification.^[14] Acidification from leaf litter on the O-horizon is more pronounced under coniferous trees such as pine, spruce and fir, which return fewer base cations to the soil, rather than under deciduous trees; however, soil pH differences attributed to vegetation often preexisted that vegetation, and help select for species which tolerate them. Calcium accumulation in existing biomass also strongly affects soil pH - a factor which can vary from species to species.^[15]

Parent materials

Certain parent materials also contribute to soil acidification. Granites and their allied igneous rocks are called "acidic" because they have a lot of free quartz, which produces silicic acid on weathering.^[16] Also, they have relatively low amounts of calcium and magnesium. Some sedimentary rocks such as shale and coal are rich in sulfides, which, when hydrated and oxidized, produce sulfuric acid which is much stronger than silicic acid. Many coal soils are too acidic to support vigorous plant growth, and coal gives off strong precursors to acid rain when it is burned. Marine clays are also sulfide-rich in many cases, and such clays become very acidic if they are drained to an oxidizing state.

Soil amendments

Soil amendments such as chemical fertilizers can cause soil acidification. Sulfur based fertilizers can be highly acidifying, examples include elemental sulfur and iron sulfate while others like potassium sulfate have no significant effect on soil pH. While most nitrogen fertilizers have an acidifying effect, ammonium-based nitrogen fertilizers are more acidifying than other nitrogen sources.^[17] Ammonia-based nitrogen fertilizers include ammonium sulfate, diammonium phosphate, monoammonium phosphate, and ammonium nitrate. Organic nitrogen sources, such as urea and compost, are less acidifying. Nitrate sources which have little or no ammonium, such as calcium nitrate, magnesium nitrate, potassium nitrate, and sodium nitrate, are not acidifying.^[18][19][20]

Pollution

Acidification may also occur from nitrogen emissions into the air, as the nitrogen may end up deposited into the soil.^[21] Animal livestock is responsible for nearly 65 percent of man-made ammonia emissions.^[22]

Anthropogenic sources of sulfur dioxides and nitrogen oxides play a major role in increase of acid rain production.^[23][24] The use of fossil fuels and motor exhaust are the largest anthropogenic contributors to sulfuric gases and nitrogen oxides, respectively.^[25]

Aluminum is one of the few elements capable of making soil more acidic.^[26] This is achieved by aluminum taking hydroxide ions out of water, leaving hydrogen ions behind.^[27] As a result, the soil is more acidic, which makes it unlivable for many plants. Another consequence of aluminum in soils is aluminum toxicity, which inhibits root growth.^[28]

Agriculture management practices

Agriculture managements approaches such as monoculture and chemical fertilization often leads to soil problems such as soil acidification, degradation, and soil-borne diseases, which ultimately have a negative impact on agricultural productivity and sustainability.^[29][30]

Pesticides are a common agricultural practice that can have a negative impact on microbial organisms and plant health. Most pesticides have a pH between 5.5 and 7 which is acidic, so after long term use it can decrease the pH making the soil more acidic.^[31] After pesticide overuse, various factors such as sunlight, microbial activity, and water break down the pesticides to release an excess amount of hydrogen ions in the soil, lowering the pH. Without these nutrients the soil can't support the survival of living organisms which inhibits them from assisting plant and soil health. Ammonia-oxidizing microorganisms (AOM) are sensitive to pesticides and have an abundance of the amoA gene which help promote nitrogen cycling and maintaining soil health.^[32] Nitrification done by these organisms benefits plants but releases a byproduct of hydrogen ions and nitric acid which continue to acidify the soil. Some nitrogen-containing pesticides release high amounts of ammonium, which is a reactant for the nitrification process.

It is essential to check the label on the pesticide and perform calculations for the size of application to achieve efficient pesticide. Other factors such as water temperature, soil type, persistence, pesticide type, and legal restrictions should be considered and have research done before application.^[33]