Comparison of different methods for the determination of the potassium availability of soilsExtractable K was determined in 148 soil samples by using three different lactate solutions (AL, DL, CAL), 0.1 m HCl and 0.0125 m CaCl2as extracting agents.The extractable portion of the total exchangeable K (K(NH4)) was found to decrease with increasing content of the<6,3 μm particle size fraction in the samples.Correlation coefficients were calculated for characterizing the relationship between extracted amounts of K (K values) and the „K numbers”︁. The latter serve as a basis for fertilizer recommendations in the Netherlands, where they have been checked in a great number of field trials. The K number depicts the K supplying power of soils as determined by HCl soluble K, content of the<16 μm size fraction, and pH.The K (CaCl2) values were found to correlate more closely with the K numbers than the amounts of K extracted with lactate solutions. This is because the influence of particle size on K(CaCl2) is eliminated as shown by multiple regression analysis.The strength of bonding of the easily exchangeable K is reflected much better by the K(CaCl2) values than by the lactate values. Improvement of K fertilizer recommendations by taking K(CaCl2) data as a basis can thus be expected.The different methods of recommendation for K fertilization in the Fed. Republic of Germany and in the Netherlands are illustrated for the soils derived fro