There are several different extracting solutions used to determine
extractable cations in soils. A 0.05N HCl in 0.025N H2SO4 solution
(Double Acid) or a 0.1N HCl solution is common. These are suitable for
several elements, including calcium, copper, iron, magnesium,
manganese, potassium, sodium and zinc, and may be applicable to other
elements.