From geology, a term referring to a highly pedalfer soil occuring in areas of very heavy rainfall such as rain forests. Laterites are soils that have nearly all of their valuable and soluble minerals carried away by rainfall. Laterites are usually red because of the high proportions of iron compounds left behind after all the potassium, calcium, and phosphorus has journeyed off to the sea.

Laterites are severely impoverished soils, and due to their subceptability to runoff, even fertilizer does little to help. They are able to support rich ecosystems like rain forests due to the freely available water and the abilities of complicated webs of organisms to trap and store nutrients, but they are not at all suited to agriculture, despite what westernized society thinks.

This is what my father's college geology book*, dated 1939, has to say about laterites:

"The extreme example of the pedalfers are the laterite soils of the tropics, where the heavy rainfall causes intensive leaching. That is one reason why, in the equatorial rain forest belt, small clearings are cultivated for only a year or two and then allowed to revert to their natural condition."
Ah, for the halcyon days when we did not find it necessary to inflict on our children guilty nightmares of burning trees and black smoke . . .

*Dad's not really that old. The book is Geomorphology by A K Lobeck.

Laterite clay is used by home aquaria hobbyists who have planted tanks. There is an enthusiastic sub-set of the pastime which is as interested in live plants as in fish. Many of them consider laterite as a beneficial and desirable additive for plants, and it is added to the substrate of fresh water tanks in order to provide iron, and a trace element chelating location. Even after the available iron is consumed, the clay continues to serve a nutritive purpose in the tank. The fact that the clay does not contain nitrogen and phosphorous or indeed any organic matter, is a good thing as most aquaria have an excess of these nutrients due to the fish load. Thus, laterite will not cause algal blooms, or contribute to a spike in ammonia. Although, occurences may be indirectly linked to it if one neglects to siphon the tank gravel because one does not wish to stir up the laterite.

Laterite is a relatively expensive substrate additive, and is usually available as a powder, and sometimes in pre-formed balls. For new tanks, the powder is mixed with the base gravel or sand and then the mixture is top-dressed with a cosmetic as well as anchoring layer of gravel or sand. All of this occurs prior to the addition of water. One can also make balls out of the powder by mixing the clay with a quantity of water, shaping it into small pieces about 1 inch in diameter, and then drying them in the oven at a low temperature. The balls can be pushed into the existing substrate of an established tank or placed next to 'special' plants upon which one wishes to lavish extra care and attention.

The clay is very light and will cloud the water column and film surfaces inside the tank as it settles out, so one must be careful when disturbing the substrate or adding water to a new tank setup. It can take several days for heavy clouding to clear naturally, without additional filtration or a water polisher. It is not appropriate for use with under gravel filters (UGF).

With thanks to for teaching me my fish stuff.

Care must be taken to distinguish between a lateritic soil system and the duricrust material "laterite" which forms in such a soil.

Laterization is by far the most common soil-forming process in the Earth's tropics. If you ever took chemistry in high school, you probably had to perform a lab experiment demonstrating "chromatography". You were given a substance to analyze, and a solvent to dissolve it in. You were then directed to insert a piece of porous material (probably paper) into the solution (or drip the solution onto a piece of porous material) and watch the results. After a while, capillary action would cause the solution to soak into the material and spread out from the point where the solution was introduced. Molecules of the dissolved substance would travel with the solvent for a while, but different constituents of the solution would come out of solution and precipitate onto the paper in colored bands. A given band's distance from the point of indtroduction depended on the porosity of the material, the evaporation rate of the solvent, and the electrostatic and chemical properties of the molecules itself.

If you now imagine the pourous material being soil rich in iron minerals, and the solvent being water (plus a few acids resulting from combining with other soil minerals), you can get a fairly good picture of how laterization works. Rain leaches away all of the nutrients in the soil, but iron oxides tend to collect in a band between 1 and 3 meters below the surface (although this layer can sometimes be 15 meters thick). "Laterite" is this band of iron concentrated-soil.

Laterite that is still covered by its surface soil, or still wet after being recently dug up, is a plastic substance that is easily cut. But when the soil is stripped away and the laterite is allowed to dry out, it amply demonstrates why it is called a "duricrust": it turns into an exteremely hard substance, much harder than other sedimentary rocks. This is the source of its name: lateris is Latin for "brick". Often, the laterite will be pisolitic, breaking apart into a red gravel, but hardened laterite frequently forms a massive water-impervious surface. When a lateritic area undergoes desertification, the result can be a vast, hard surface that nothing can grow on. Large expanses of Western Australia's deserts are hardened laterite with a layer of sand on top.

Although laterite is not really a brick clay, it can harden enough to form a lasting building material. It has the dual advantage of being easy to cut while wet, and extremely durable while dry. Many historically important buildings in South and Southeast Asia, especially temples, are built of the stuff. Many of the impressive structures of Angkor have foundations or walls of laterite. The use of laterite in tropical countries, especially as a paving material, continues to this day.

Nature conveniently concentrates important metals in the process of laterization, and duricrusts can be used as ores. Although laterite can be used as an ore for iron, it is more important as a source of nickel and cobalt. Other duricrusts, particularly bauxite (which forms in an aluminum-rich soil), are the principal ores for some minerals.

Upon occasion, a stream will cut down through the soil to reach the laterite layer. When this happens, iron oxides will leach into the stream, making the stream bed appear to bleed. This brings to mind a story one of my professors once told me: A road cut being dug in Hong Kong in the early 20th century was interrupted suddenly when the workers hit a laterite layer. Believing that they had cut into the back of a dragon, the workers refused to finish the cut.

Lat"er*ite (?), n. [L. later brick, tile: cf. F. lat'erite.] Geol.

An argillaceous sandstone, of a red color, and much seamed; -- found in India.


© Webster 1913.

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