Multinutrient Extraction of Soils by EDTA Ion Exchange Resin.
Bernardo van Raij, Aline R. Coscione, Heitor Cantarella, and Monica Ferreira Abreu. INstituto Agronômico, Av. Barao de Itapura 1481, Campinas (SP), Brazil
The use of multinutrient extractants for soil analysis is attractive from the laboratory point of view, because of the possibility of the determination of several elements by a single extraction procedure. However, the results obtained are often not the best possible alternatives for the determination of the bioavailability of all elements. As an example the ion exchange resin extraction, which provides the most reliable availability index for phosphorus, is seldom used because it is considered time consuming. Yet, an ion exchange resin extraction procedure for phosphorus and exchangeable calcium, magnesium and potassium was introduced in routine soil testing in Brazil in 1983 for the simultaneous extraction of P, Ca, Mg, and K; about 90 laboratories of 10 states use the method presently. In this study other elements were included in the resin procedure, not only in order to replace other extractants such as calcium phosphate (for S) and DTPA (for Cu, Fe, Mn, and Zn), but also to find alternatives that might possibly be better for the determination of the bioavailability of these elements. The most important changes introduced were: the use of ammonium bicarbonate instead of sodium bicarbonate to saturate the resin; addition of EDTA to the soil/resin suspension before the 16-hour overnight shaking period; reduction of the ratio of the final solution to soil from 20:1 to 5:1; and the use of ammonium acetate at pH 4.8 instead of acidified ammonium chloride to extract the elements from the resin. All elements in the extracts were in a range that could be easily determined by ordinary laboratory methods. With these changes, the results of the four elements already determined by the resin method varied little, as shown by the correlation coefficients: P – 0.98**; Ca – 0.98**; Mg – 0.98**; K – 0.99**. For S, compared to calcium phosphate extraction, the value of r was 0,98**. Compared with the DTPA, the results of metallic micronutrients presented the following correlations: Cu – 0,98**; Fe – 0,85**; Mn – 0,65*; Zn – 0,59*. The procedure developed indicates that the resin-EDTA extraction has the potential to be used as a multinutrient extractant, thus eliminating the need of separate extractions for S and for Cu, Fe, Mn, and Zn. Keywords: ion exchange resin, EDTA, multinutrient extraction