Caliche

Caliche is a hardened sedimentary crust formed when calcium carbonate cements together soil, sand, gravel and clay into a tough, concrete-like layer. It typically appears as a whitish to cream or pinkish band within or atop the soil profile in dry climates.

Also known as calcrete or hardpan, caliche can range from soft, powdery nodules to massive, rock-hard sheets that resist digging and impede drainage and root growth. It is a familiar nuisance to gardeners and builders in desert regions.

Geologically it is a type of duricrust, recording long periods of arid soil development.

Caliche forms in arid and semi-arid climates where rainfall is low and evaporation high. Calcium carbonate dissolved in soil water is carried downward by occasional rains, then precipitates as the water evaporates or as carbon dioxide is lost, accumulating at a characteristic depth in the soil.

Over thousands of years repeated cycles cement the soil into progressively harder layers, eventually forming massive calcrete. Caliche develops widely in the southwestern United States, Mexico, Australia, India (where it is called kankar), and the Mediterranean and Middle East. The source calcium may come from dust, weathered rock, or underlying limestone.

Caliche is recognized as a whitish to tan, hard, cemented layer in otherwise loose soil, often appearing as nodules, crusts or massive hardpan. It fizzes in dilute acid because of its calcium carbonate content.

It is hard enough to require a pick or breaker to dig through, yet the carbonate cement itself is fairly soft to scratch. It can resemble concrete or weathered limestone; its setting within a soil profile, mixed grain content, and strong acid reaction help identify it. The whitish color, soil-layer occurrence and effervescence in acid are the most reliable clues.

Caliche has practical and economic uses despite often being seen as an obstacle. It is quarried as a low-grade construction material for road base, fill and rustic building, and historically was used to make adobe-style structures and as a source of agricultural lime.

In some regions caliche layers host nitrate and iodine deposits, notably in Chile's Atacama Desert, where caliche is mined for sodium nitrate fertilizer and iodine. For farmers and gardeners it usually requires breaking up or amending because it blocks drainage and root penetration. It has no gemstone or metaphysical use.