A petrocalcic horizon is a diagnostic horizon in the USDA soil taxonomy (ST) and in the World Reference Base for Soil Resources (WRB). They are formed when secondary calcium carbonate or other carbonates accumulate in the subsoil to the extent that the soil becomes cemented into a hardpan. Petrocalcic horizons are similar to a duripan (ST – WRB: petroduric horizon) and a petrogypsic horizon (WRB) in how they affect land-use limitations. They can occur in conjunction with duripans where the conditions are right and there are enough free carbonates in the soil. Calcium carbonates are found in alkaline soils, which are typical of arid and semiarid climates. A common field test for the presence of carbonates is application of hydrochloric acid to the soil, which indicates by fizzing and bubbling the presence of calcium carbonates.