Bioavailable tests: alternatives to standard soil extractions

Due to the complexity of the soil plant system and in spite of the enormous amount of information available, the search for better methods to determine the bioavailability of nutrients to plants continues to be important. The large number of methods of soil analysis available worldwide in routine soil testing is an unquestionable demonstration of the lack of agreement on best alternatives. The question discussed in this paper, based on a literature review, is if those methods, considered here as the "standard methods" of soil analysis, are the best possible alternatives for the evaluation of bioavailability of elements in soil. As alternative non-standard methods, electroultrafiltration (EUF), the extraction with 0.01 mol calcium chloride (CaCl2) L-1 solution, the extraction with ion-exchange resin and the adsorption of phosphorus (P) on iron oxide treated paper filter (Pi), were considered. These 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 the percent variation (100 r2) associated with the correlations of plant indexes of bioavailability, in most cases uptake by plants, and soil contents of the elements. For P, the extraction with ion-exchange resin presented considerable higher values of the percent variation, in comparison with the standard methods, EUF, and CaCl2. The resin extraction 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 better alternatives to the standard methods are not yet available. The extraction with resin seems to be feasible for other elements, including potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), nitrogen (N), molybdenum (Mo), copper (Cu), manganese (Mn), zinc (Zn), cadmium (Cd), and lead (Pb), but there is a need for a practical laboratory technique to determine all those elements and the biological validation with plant experiments. Thus only for P is there a more accurate alternative to evaluate bioavalilabity of nutrients in soils.