Aluminum Readings in Soil

Aluminum Forms

• Aluminum does not exist as a free metal in nature, even though it makes up so much of the earth's crust. It combines with other elements making compounds like potassium aluminum sulfate, aluminum oxide and aluminum hydroxide. Cryolite or sodium fluoaluminate is a limited occurrence in nature. Pure aluminum was not extracted until 1825 by Hans Oersted. The soluble aluminum cation, positively charged ion, Al+++ occurring in soil is called soluble aluminum. The highest readings for it are found in acid soils below pH 5.5. Soil-exchangeable aluminum consists of Al+++ adhered to organic matter or clay. It becomes soluble aluminum when it breaks away and is free in the soil.

Al+++ Readings

• Soil readings for soluble aluminum are expressed in mg/kg in Australia and in ppm in the United States. Spectrum Analytic Inc. reports that most U.S. crop plants can't grow when soluble Al+++ is present in the soil in readings of 1.0 ppm, and that some crops cannot grow well when test readings are 0.5 ppm. In Australia, aluminum levels of 15 mg/kg may present problems for the crop lucerne, and levels above 50 mg/kg are toxic to it.

Factors Affecting Al+++ Availability

• The presence of organic material, the types and quantities of clays present, and the concentrations of other cations and anions, or negatively charged ions, and other salts all affect how much available Al+++ is present in a soil. These factors affect readings.

Effects on Plants at Different Soil Readings

• Emmanuel Delhaize and Peter Ryan of the Division of Plant Industry, Commonwealth Scientific and Industrial Research Organization, Canberra, Australia, discussed the mode of action of Al+++ on plants. Soluble aluminum affects root growth and inhibits the general growth and health of plants as a result. Research done at Clemson University showed that cotton plants grown at zero ppm aluminum had roots 16 cm long. At 0.25 ppm, cotton roots were 11 cm long. At 0.50 ppm, cotton roots were 8 cm.

Effect of Soil pH on Aluminum Readings

• Acidic soils between pH 4.0 and 5.0 have high percentages of soluble aluminum present. Clemson University research showed a decreased corn yields as pH went down and Al+++ soil percentages went up. At pH of 5.0 and below, high Al+++ levels don't allow uptake of essential cations of calcium and manganese, and deficiencies of these elements suppress plant growth. At lower pH values, there is less soil exchangeable Al+++ and much more soluble Al+++.