New diagnostic techniques, universal soil extractants

Abstract

Universal extractant is a term used to designate reagents or procedures 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 that supply plant roots. Most extractants in use fall short of these requirements and are in reality multinutrient extractants, given priority to the laboratorial convenience. The most commonly used “universal” extractants are known as Morgan, Mehlich No. 1, Mehlich No. 3, and AB‐DTPA, but there are other multinutrient extractants, such as 1 M neutral ammonium acetate, TEA‐DTPA, and ion exchange resin. None of these are able to extract all elements determined in soil testing laboratories, nor are they always efficient for all nutrients. Nitrogen, S, B, and Mo are usually not determined by these extractants and require single nutrient extractions. The elements commonly extracted for soil analysis are P, K, Ca, Mg, Na, Zn, Cu, Fe, Mn, and occasionally Cd, Ni, Cr, and Pb. Phosphorus is the most difficult extractable element since any extracting solution present lower correlations with the more complicated and not much used ion exchange resin extraction. The exchangeable cations, K, Ca, Mg, and Na are rather easily determined with most extractants. For Zn, Cu, Mn, Fe, Cr, Cd, Ni, and Pb, the most effective extractants are those containing DTPA. Among the non‐conventional soil test methods, the extraction with ion exchange resin is one of the most promising alternatives.