STUDIES ON THE POTASSIUM-SUPPLYING CAPACITIES OF SOUTHERN ONTARIO SOILS. III. MEASUREMENT OF AVAILABLE K

Abstract

This study was conducted to determine which of eight different extractants best described the K-supplying capacities of nine southern Ontario soils. The total amount of K extracted by eight crops of alfalfa grown in the greenhouse was related to soil K by regression techniques. The following procedures extracted increasingly higher average amounts of soil K: water; electro-ultrafiltration (EUF) at 20 °C (EUF-K-20) (37 μg K g −1 ); EUF at 80 °C (EUF-K-80) (83 μg K g −1 ); 2M NaCl (89 μg K g −1 ); 1M NH 4 OAc (131 μg K g −1 ); 0.1 M HNO 3 (163 μg K g −1 ); seven sequential 7-min extractions with boiling 1 M HNO 3 (Mactotal) (940 μg K g −1 ); and 0.2 M sodium tetra-phenyl boron (NaTPhB) (3248 μg K g −1 ). Of the eight extractants tested, the amount of K removed by EUF was the most closely associated to total K uptake; a multiple regression model with the logarithm of EUF-K-20 and the logarithm of EUF-K-80 as independent variables explained 97% of the total variation in K uptake. The K extracted by 2 M NaCl and by 0.1 M HNO 3 also explained more variation in total K uptake (r 2 = 0.86, and 0.92, respectively) than did 1 M NH 4 OAc (r 2 = 0.61), which is currently used in the Ontario soil test program. The other extractants did not offer an improvement over NH 4 OAc, with the exception of the most soluble fraction of Mactotal (StepK). Extraction of soil K with electro-ultrafiltration may offer a more precise estimation of the K supplying capacities of southern Ontario soils than the currently used extractant. Key words: Electro-ultrafiltration, nonexchangeable K, ammonium acetate, soil test