ABSTRACT
About 26% of the global ice-free land surface is occupied by soils with pHw (pH determined in water) values of 6.5 or less, of which two-thirds have potential Al toxicity problems. Subsoil Al problems are estimated to encompass 16% of the ice-free land surface (Eswaran et al., 1997). Naturally, acid soils are found in regions where rainfall exceeds evapotranspiration (Jenny, 1941); H+ and eventually Al+3 replace basic cations lost from exchange sites through excess water transport. Most of the acid soils belong to the Alsols, Histosols, Oxisols, Spodosols, and Ultisols orders of the U.S. Soil Taxonomy (Buol et al., 1997).
