BIOAVAILABLE TESTS - ALTERNATIVES TO STANDARD SOIL EXTRACTIONS

Due to the complexity of the soil plant system and in spite of the eno rmous amount of information available, the search for better methods t o determine the bioavailability of nutrients to plants continues to be important. The large number of methods of soil analysis available wor ldwide in routine soil testing is an unquestionable demonstration of t he lack of agreement on best alternatives. The question discussed in t his paper, based on a literature review, is if those methods, consider ed here as the ''standard methods'' of soil analysis, are the best pos sible alternatives for the evaluation of bioavailability of elements i n soil. As alternative non-standard methods, electroultrafiltration (E UF), the extraction with 0.01 mol calcium chloride (CaCl2) L-1 solutio n, the extraction with ion-exchange resin and the adsorption of phosph orus (P) on iron oxide treated paper filter (Pi), were considered. The se four methods were compared with the standard methods Olsen, Mehlich 1, Bray 1, Mehlich 3, AB-DTPA, Egner, water, ammonium acetate (NH4OAc ), and DTPA. The criterion used to compare the different methods was t he percent variation (100 r(2)) associated with the correlations of pl ant indexes of bioavailability, in most cases uptake by plants, and so il contents of the elements. For P, the extraction with ion-exchange r esin presented considerable higher values of the percent variation, in comparison with the standard methods, EUF, and CaCl2, The resin extra ction is adaptable to all types of soils, including acid and alkaline, which is not the case for the standard methods. Comparisons for other elements was impaired by the limited amount of data available, but be tter alternatives to the standard methods are not yet available. The e xtraction with resin seems to be feasible for other elements, includin g potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), nitrogen (N ), molybdenum (Mo), copper (Cu), manganese (Mn), zinc (Zn), cadmium (C d), and lead (Pb), but there is a need for a practical laboratory tech nique to determine all those elements and the biological validation wi th plant experiments. Thus only for P is there a more accurate alterna tive to evaluate bioavalilabity of nutrients in soils.