When they occur close to the surface, they present major constraints for agricultural use. Petrocalcic horizons are rock hard and massive. The primary cementing agent is calcium carbonate. Other names for petrocalcic horizons are croute calcaire, calcrete and caliche. Petrogypsic horizons are similar to petrocalcic horizons except that the primary constituent is gypsum.
Petrogypsic horizons are common in North Africa and the Middle East.
Duripans are soil horizons cemented by secondary silica. Like petrocalcic and petrogypsic horizons, duripans present major constraints for agriculture because they limit root growth and water movement. This site constructed and maintained for the International Arid Lands Consortium by the Office of Arid Lands Studies .