The most common test for available K+ is the exchangeable K+ obtained by extraction with 1M NH4Cl or NH4 acetate. This fraction contains mainly soil solution K+ plus K+ of the hydrated K+fraction and only a small part of the interlayer K+. Exchangeable K+ ranges between 40 and about 400 mg/kg soil and even more. Concentrations of <100 mg K/kg are frequently in the deficiency range; concentrations between 100 and 250 mg K/kg soil are in the range of sufficiently to well-supplied soils.
With the EUF technique, a differentiation between the nonhydrated exchangeable K+ and the interlayer K+ is possible, as shown in Figure 4.10. In the EUF, routine analysis extraction of the adsorbed hydrated K+ lasts 30 minutes (200V, 20�C); for the second fraction (400V, 80�C), the soil suspension is extracted for only 5 minutes. The K+ extracted during this 5-minute period is a reliable indicator of the availability of interlayer K+ and is taken into consideration for the recommendation of the potassium fertilization rates. This EUF technique is nowadays used on a broad scale in Germany and Austria with much success for the recommendation of K fertilizer rates, particularly to crops such as sugar beet (109). With the EUF extraction procedure, not only are values for available K+ obtained but the availability of other plant nutrients such as inorganic and organic nitrogen, phosphorus, magnesium, calcium, and micronutrients are also determined in one soil sample.
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