Agrochemicals

Agricultural soils are commonly too acidic or too alkaline for the optimal growth of many crop species. When this is the case, chemicals may be added to the soil to adjust its pH to a more appropriate range.

Acidic soils are an especially common problem in agriculture. Acidic soil can be caused by various factors, including the removal of acid-neutralizing bases contained in the biomass of harvested crops, the use of certain types of fertilizers, acid rain, the oxidation of sulfide minerals, and the presence of certain types of organic matter in soil. Because soil acidification is such a common occurrence, acid-neutralizing (or liming) materials are among the most important agrochemicals used, in terms of the quantities added to soil each year.

Acidic soils are commonly neutralized by adding calcium-containing minerals, usually as calcite (CaCO₃) in the form of powdered limestone or crushed oyster or mussel shells. Alternatively, soil acidity may be neutralized using faster-acting lime (Ca[OH]₂). The rate of application of acid-neutralizing substances in agriculture can vary greatly, from several hundred pounds per acre per year to more than 1,000 pounds per acre per year. The rates used depend on the acidity of the soil, the rate at which new acidity is generated, and the needs of specific crops.

Much less commonly, soils may be alkaline in reaction, and they may have to be acidified somewhat to bring them into a pH range suitable for the growth of most crops. This problem can be especially common in soils developed from parent materials having large amounts of limestone (CaCO₃) or dolomite (CaMg[CO₃]₂). Soil can be acidified by adding sulfur compounds, which generate acidity as they are oxidized, or by adding certain types of acidic organic matter, such as peat mined from bogs.