NEW DIAGNOSTIC-TECHNIQUES, UNIVERSAL SOIL EXTRACTANTS

Universal extractant is a term used to designate reagents or procedure s to extract several elements or ions to assess soil fertility status or levels of toxicity. Ideally, universal extractants should be rapid, reproducible, inexpensive, adaptable to soils from different regions, and the extraction of the nutrients should be from the labile forms t hat supply plant Toots. Most extractants in use fall short of these re quirements and are in reality multinutrient extractants, given priorit y to the laboratorial convenience. The most commonly used ''universal' ' extractants are known as Morgan, Mehlich No. 1, Mehlich No. 3, and A B-DTPA, but there are other multinutrient extractants, such as 1 M neu tral ammonium acetate, TEA-DTPA, and ion exchange resin. None of these are able to extract all elements determined in soil testing laborator ies, nor are they always efficient for all nutrients. Nitrogen, S, B, and Mo are usually not determined by these extractants and require sin gle nutrient extractions. The elements commonly extracted for soil ana lysis are P, K, Ca, Mg, Na, Zn, Cu, Fe, Mn, and occasionally Cd, Ni, C r, and Pb. Phosphorus is the most difficult extractable element since any extracting solution present lower correlations with the more compl icated and not much used ion exchange resin extraction. The exchangeab le cations, K, Ca, Mg, and Na are rather easily determined with most e xtractants. For Zn, Cu, Mn, Fe, Cr, Cd, Ni, and Pb, the most effective extractants are those containing DTPA. Among the non-conventional soi l test methods, the extraction with ion exchange resin is one of the m ost promising alternatives.