Soil aluminum toxicity in the Colombian coffee growing region: Sources of acidity and methods of determination.
Maria E. Ortiz Escobar, University of Hawaii, 3190 Maile Way, St. John 102, Honolulu, HI 96822, Raul D. Zapata, Universidad Nacional de Colombia, Calle 59 A # 63-20, Bloque 14, Medellin, Colombia, and Siavosh Sadeghian, Centro Nacional de Investigaciones de Café, CENICAFE, Plan Alto, Chinchiná, Colombia.
The coffee growing region in Colombia, is located on volcanic-ash derived soils, classified as Andisols, which are known for their acidic condition, rich in organic matter and associated to Al toxicity. Twenty soils were studied by using the first 30 cm from the A horizon, selected by the high Al-KCl extractable content (Al+3 > 1.5 cmolkg-1), and classified by the National Federation of Coffee Growers of Colombia in four Units of soil, Chinchiná, Montenegro, Timbío and Piendamó. Different determinations were used to evaluate their acidity: exchangeable Al with three KCl concentrations to estimate the effect of the extractant concentration on the Al content; soil solution, by centrifugation and leachate analysis; organic fraction and organic matter-Al complex capacity; and potentiometric and conductimetric titrations, to determine possible inorganic sources of acidity. The results indicate that 1) Al concentration can be affected by the extractant concentration, 2) hydroxy-Al species were not detected by titration methods, 3) Al-organic matter complexes were negligible, and the free Al determined, is probably from an inorganic source, which seems not be toxic for plants, and 4) the soil solution indicates that Al concentrations were not present in toxic levels, and disagree with the values reported by traditional methods reporting high concentration of Al in soil, without any damage in the root zone. We recommend that for ash-derived soils, pertaining to any of these four soils Units, and with exchangeable Al3+ > 1.5 cmol kg-1, additional analysis of soil solutions should be done, prior any recommendation to correct soil acidity.