Soil and Irrigation Classification of Shallow Soils Overlying Gypsum Beds, Northern Iraq

Summary Recent reconnaissance soil surveys of proposed irrigation projects have revealed a belt of shallow soils overlying gypsum beds in the drier steppe zone of Northern Iraq. The soil profile consists of two sharply contrasting layers—upper layer of Mesopotamian Alluvium and a lower layer of highly gypseous material. The Mesopotamian Alluvium is a brown fine‐textured material, highly calcareous but very low in soluble salts, exchangeable sodium, and gypsum. The gypseous beds underlying this shallow soil mantle are relatively soft tertiary sediments (Fars beds) or their erosion products. Despite abundant evidence of the non‐toxicity of gypsum even in saturated solutions, observations on naturally occurring gypseous soil in steppe areas of Iraq and the U.S.S.R. show that root growth is inhibited where the gypsum content is 10 to 25 per cent, or higher, apparently because of poor transmission of air and water associated with bad structure. Agronomic and engineering problems were considered, together with U.S. data on rooting systems of climatically adapted irrigated crops, in arriving at a classification.