“…Over the years following the initial report, Al has become recognized as the most limiting factor in many acid soils throughout the world (Foy, 1988), possibly affecting about 40 °o and perhaps up to 70 °o of the world's arable land that is potentially usable for food and biomass production (Haug & Caldwell, 1985;and references tberein). Soil acidification resulting from either continuous use of ammonia-and amide-containing fertilizers (Mahler, Halvorson & Koehler, 1985) or the nitrogen-fixation by legumes (Bolan et al, 1991;Coventry & Slattery, 1991) aggravates Al toxicity and contributes to an increase in the soil area affected. In addition, the recent advent of acid precipitation (Johnson et al, 1981) has increased soil acidity (Kuylenstierna & Chadwick, 1991) and enhanced Al solubility in 'acid-sensitive' soils (FalkengrenGrerup, 1987), thus aggravating Al toxicity, especially in forest stands (Cronan et al, 1989).…”