Caliche (mineral)
Caliche is a hardened deposit of calcium carbonate.  This calcium carbonate cements together other materials, including gravel, sand, clay, and silt.  It is found in aridisol and mollisol soil orders.  Caliche occurs worldwide, generally in arid or semi-arid regions, including in central and western Australia, in the Kalahari Desert, in the High Plains of the western USA, and in the Sonoran Desert.  Caliche is also known as hardpan, calcrete, kankar (in India), or duricrust.  The term caliche is Spanish and is originally from the Latin calx, meaning lime.
Caliche is generally light colored but can range from white to light pink to reddish-brown, depending on the impurities present.  It is generally found on or near the surface, but it can be found in deeper subsoil deposits as well.  The layers can vary from a few inches to feet thick, and multiple layers can exist in a single location.
In northern Chile and Peru, caliche refers to the nitrate salt deposits of the Atacama Desert.  Caliche can also refer to various clayey deposits in Chile, Peru, Mexico, and Colombia.  In addition, it has been used to describe some forms of quartzite, bauxite, kaolinite, laterite, chalcedony, opal, and soda niter.
Similar material, but composed of calcium sulfate rather than calcium carbonate, is called gypcrust or gypcrete ("Gyp-Crete" is also a trademarked brand name).

# How it forms
Caliche generally forms when minerals are leached from the upper layer of the soil (the A horizon) and accumulate in the next layer (the B horizon), at depths of approximately 3 to 10 feet under the surface.  Caliche generally consists of carbonates in semiarid regions, while in arid regions, less soluble minerals will form caliche layers after all the carbonates have been leached from the soil.  The calcium carbonate that is deposited accumulates, first forming grains, then small clumps, then a discernible layer, and finally a thicker, solid bed.  As the caliche layer forms, the layer gradually becomes deeper, eventually moving into the parent material, which lies under the upper soil horizons.
However, caliche can also form in other ways.  It can form when water rises through capillary action.  In an arid region, rainwater will sink into the ground very quickly.  Later, as the surface dries out, the water below the surface will rise, carrying dissolved minerals from lower layers upward with it.  This water movement forms a caliche that tends to grow thinner and branch out as it nears the surface.  Plants can contribute to the formation of caliche as well.  The plant roots take up water through transpiration, leaving behind the dissolved calcium carbonate, which precipitates to form caliche.  Caliche can also form on outcrops of porous rocks or in rock fissures where water is trapped and evaporates.  In general, caliche deposition is a slow process, but if enough moisture is present in an otherwise arid site, it can accumulate fast enough to block a drain pipe.
While the formation of other caliches is relatively well understood, the origin of Chilean caliche is not known definitively.  One possibility is that the deposits were formed when a prehistoric inland sea evaporated.  Another theory is that it was deposited due to weathering of the Andes.

# Economic uses

## Building applications
Caliche is used in construction worldwide.  The caliche reserves in the Llano Estacado in Texas can be used in the manufacture of Portland cement; the caliche meets the chemical composition requirements and has been used as a principal raw material in Portland cement production in at least one Texas plant.  Where the calcium carbonate content is over 80 %, caliche can also be fired and used as a source of lime in areas, which can then be used for soil stabilization.
When mixed with small amounts of either pozzolan or Portland cement, caliche can also be used as a building material that will exceed the building code requirements for unfired masonry materials.  For example, caliche was used to build some of the Mayan buildings in the Yucatan Peninsula in Mexico.  A dormitory in Ingram, Texas and a demonstration building in Carrizo Springs, Texas for the United States Department of Energy were also built using caliche as part of studies by the Center for Maximum Potential Building Systems.
In many areas, caliche is also used for road construction, either as a surfacing material or, more commonly, as a base material.  It is one of the most common road materials used in southern Africa.  Caliche is widely used as a base material because it continues to accumulate and gain strength over time.

## Sugar refining
A nearly pure source of calcium carbonate is necessary to refine sugar.  It must contain at least 95 % calcium carbonate (CaCO3) and have a low magnesium content.  In addition, the material must meet certain physical requirements so it does not break down when burned.  Although caliche does not generally meet all of the requirements for sugar refining, it is used in areas where another source of calcium carbonate, such as limestone, is not present.  While the use of caliche necessitates beneficiation in order to meet the requirements, its use can still be significantly cheaper than shipping in the needed limestone.

## Chilean caliche
In Chile, the nitrate salt deposits in the Atacama Desert are also referred to as caliche.  These deposits are the largest known natural source of nitrates in the world, containing up to 25 % sodium nitrate and 3 % potassium nitrate, as well as , sodium chloride, sodium sulfate, and sodium borate (borax).  The caliche beds are from 0.2 to 5 meters thick, and they are mined and refined to produce a variety of products, including sodium nitrate (for agriculture or industry uses), potassium nitrate, potassium sodium nitrate, sodium sulfate, iodine, and iodine derivatives.

# Caliche and Gardening

## Problems caliche cause
Caliche beds can cause many problems when trying to grow plants.  First, an impermeable caliche layer prevents water from draining properly, which can keep the roots from getting enough oxygen.  Salts can also build up in the soil due to the lack of drainage.  Both of these situations are detrimental to plant growth.  Second, the impermeable nature of caliche beds also prevents plant roots from going through the bed, which means the roots have a limited supply of nutrients, water, and space, so they cannot develop normally.  Third, caliche beds can also cause the surrounding soil to be basic (have a high pH).  The basic soil, along with calcium carbonate from the caliche, can prevent plants from getting enough nutrients, especially iron.  An iron deficiency will cause the plants youngest leaves to become yellow.  Soil saturation above the caliche bed can make the condition worse.

## Fixing these problems
The best solution to these problems is to remove the layer of caliche and replace it with a mixture of organic material and soil.  The hole should be large enough to contain the plants mature root system and should go entirely through the caliche layer.  However, when the caliche layer is thick, dig a hole large enough for the mature root system.  Then, a smaller hole or holes can be dug through the remaining caliche to provide drainage.  To test drainage, the hole should be filled with water.  If the level drops by 1 inch per hour, the drainage is sufficient.
Caliche can be somewhat broken up by allowing beneficial weeds -- which tend to have stronger, deeper roots than garden plants (ergo their weed status) -- to grow in the garden as companion plants. They can force enough separation of the soil to allow other plants a better chance to breath, obtain nutrition, and have proper water circulation.