Sesquioxide clays

Sesquioxide clays are a product of heavy rainfall that has leached most of the silica and alumina from alumino-silica clay, leaving the less soluble oxides of iron Fe2O3 and iron hydroxide (Fe(OH)3) and aluminium hydroxides (Al(OH)3). It takes hundreds of thousands of years of leaching to create sesquioxide clays. Sesqui is Latin for “one and one-half”: there are three parts oxygen to two parts iron or aluminium; hence the ratio is one and one-half. They are hydrated and act as either amorphous or crystalline. They are not sticky and do not swell, and soils high in them behave much like sand and can rapidly pass water. They are able to hold large quantities of phosphates. Sesquioxides have low CEC. Such soils range from yellow to red in color. Such clays tend to hold phosphorus tightly rendering them unavailable for absorption by plants.