Soil acidification is an increase in the acidity (i.e., a decrease in the pH) of soil due to the accumulation of hydrogen ions. It is a large and increasingly severe threat both to agriculture and to various ecosystems.
Soil acidification can occur naturally through the weathering of rocks and soil minerals and the decomposition of organic matter. But it is often accelerated by human activity, particularly agriculture and pollution, and especially the use of ammonia- and ammonium-based nitrogen fertilizers, which produce hydrogen ions as they convert to nitrate. It is also caused by the removal of plant material, which otherwise would return alkaline substances to the soil, as well as by acid rain.
Soil acidification can lead to a decreased availability of essential nutrients such as phosphorus, calcium, magnesium and molybdenum. It can also increase the solubility of aluminum and manganese and thus increase their toxicity, which can harm plant roots and soil microorganisms, such as nitrogen-fixing bacteria and earthworms, consequently reducing soil fertility. The result of these nutrient imbalances and toxicities is diminished plant growth and crop yields as well as reduced nutritional values of foodstuffs.
Acidification can also degrade the soil structure and thereby increase the soil's vulnerability to erosion. Soil structure is the arrangement of sand, silt, clay, organic matter and microorganisms into tiny clumps which vary in size and are held together by organic matter, microbial activity and mineral cements, creating a porous system that allows the flow of water and air and provides habitat for soil-dwelling organisms.
Soil acidification is just one of several ways in which the earth's soil is becoming increasingly degraded and irretrievably lost. Among the others are chemical pollution (such as from synthetic fertilizers and pesticides and from mining), salinization, some agricultural practices (e.g., intensive tilling and mono-crops), climate change, changes in land use (including urbanization and deforestation), and erosion. The results are higher costs for foodstuffs and other agricultural crops, lower nutritional content of plant foods, and a growing likelihood of eventual regional malnutrition and famines.
Ways in which soil acidification can be mitigated include using less acidifying fertilizers or alternative nitrogen sources to reduce acid input, crop rotation, leaving crop residues in the fields, using calcium carbonate to reduce the acidity, and the regular monitoring of soil pH in order to allow prompt corrective measures.