Soil Acidity: Development, Impacts, and Management

Soil acidity is a serious problem worldwide. Its causes can be both natural and anthropogenic. Natural processes involve (a) leaching losses of base cations such as calcium (Ca2+), magnesium (Mg2+), and potassium (K+) and replacing with proton (H+) and aluminum (Al3+) on the surface of soil particles wherever rainfall is substantial; (b) weathering of rock and soil minerals; (c) hydrolysis of Al3+; (d) differential uptake, i.e., more cations than anions are absorbed by plants; and (e) oxidation of soil organic matter and sulfide minerals. Human-induced processes include (a) the release of SO2 and NOx gases into the atmosphere by fossil fuel consumption that forms acid rain and (b) the excessive use of ammonium (NH4+)-containing fertilizers. Soil acidity reduces crop production, forest health, and aquatic lives. The main culprits are the toxicities of Al and/or manganese (Mn) and the deficiency of Ca and to a lesser extent of Mg, phosphorus (P), and molybdenum (Mo). Aluminum toxicity usually damages the root system first, whereas Mn toxicity adversely affects above-ground plant parts. Calcium deficiency impairs cell growth and integrity causing poor crop production and quality. To manage soil acidity, liming with OH−-producing materials (e.g., CaCO3, CaMg(CO3)2, or CaSiO3) is traditionally employed; alternatively, materials such as gypsum, animal and green manures, or biochar, if available, could be applied for "short-term" amelioration. Selecting and growing acidity-tolerant plants are also a viable strategy in dealing with acid soils that occupy nearly 30% of the ice-free land area of the world.