EXTRACTION OF SOIL POTASSIUM WITH 0.01M CALCIUM-CHLORIDE COMPARED TO OFFICIAL SWEDISH METHODS

In Sweden the official method for estimating plant available potassium (K) is well calibrated to Swedish soils, but mainly provides informat ion about exchangeable K. The objective was to evaluate a 0.01 M calci um chloride (CaCl2) solution (K-CaCl2), with regard to its compatibili ty with standard laboratory procedures, and the relationship between t he K-CaCl2 method and official Swedish methods depending on soil and f ertilizing strategy. Soil was taken from long-term field experiments w ith no K application or with applications of 160-240 K kg ha(-1) y(-1) at Vojakkala, Ultuna, and Radde. The soil samples, air-dried or at fi eld moisture content, were extracted by 0.01 M CaCl2 at two different soil:extractant ratios (1:2 and 1:10), and analyzed by inductively cou pled plasma emission spectrometry (ICP) for K content. In Sweden, the official method for estimating plant available soil K is the ammonium lactate/acetic acid (AL) method and non-exchangeable K is estimated in 2M HCl (K-HCl). Amounts of K-CaCl2 from field-moist and air-dried soi l were significantly affected by soil:extractant ratio 1:2 or 1:10. Th is indicated that the same fraction of K in soil was not always extrac ted and that the amount depended on soil and fertilizing strategy, a s oil:extractant ratio of 1:10 was preferable in order to get clear extr acts. Air-dried soil samples were most suitable for Swedish laboratory procedures. Overall, CaCl2 extraction gave smaller amounts of K than AL and HCl extraction. However, at Vojakkala and Radde treatments with K application considerably increased amounts of K-CaCl2 and K-AL, com pared with no K application. In treatments with no K application, amou nts of K-CaCl2 and K-AL were largest at Ultuna. There was a clear diff erence among sites in amounts of K-HClI. There seemed to be a good rel ationship between the K-CaCl2 and the K-AL methods. However, the K-CaC l2 method extracted smaller amounts indicating that mostly K in soil s olution was extracted. In Sweden, CaCl2 could be a good complement to AL and HCI extractable K and give a more complete description of K ava ilability on different soil types and under different fertilizing stra tegies and could easily be incorporated into Swedish laboratory proced ures.