calcareous soil (kălkârˈēəs) [key], soil formed largely by the weathering of calcareous rocks and fossil shell beds. Different varieties usually contain chalk, marl, and limestone and frequently a large amount of phosphates. They are often very fertile, as in the case of the buckshot soils of the S United States. Sometimes calcareous soils are flinty, thin, and dry. They often form a large part of the soil of deserts, which may prove very fertile when sufficient moisture for crops is applied.