ABSTRACT
Aluminum (Al) is a light metal that makes up 7% of the Earth’s crust, where it is the third most abundant element after oxygen and silicon. Only at soil pH values below 5.0 (as positively charged ionic species) and above 8.0 (as AlðOHÞ4 ) is Al released to the soil solution in concentrations that may be biologically relevant. On acid mineral soils Al toxicity is the most important soil factor determining the composition of the natural vegetation and limiting crop yields. It has been estimated that worldwide on 30-40% of the arable land surface, crop productivity is limited by soil acidity (1). Aluminum may be present in the soil solution of acid soils in monomeric and polymeric forms. Although the potential specific role of the Al13 hydroxy polymer in Al toxicity is not yet fully understood, the consensus is that monomeric Al species, particularly Al3þ rather than Al(OH)2þ and AlðOHÞþ2 , are the most phytotoxic Al species (2). Organic and inorganic Al complexes are considered not or less phytotoxic (3).
