Potassium Fixation in San Joaquin Valley Soils Derived from Granitic and Nongranitic Alluvium

Potassium fixation influences the effectiveness of fertilization in soil–plant systems. A rapid method for measuring K fixation could help clarify relationships with other soil properties, especially mineralogy. Our objectives were to compare some existing measurement techniques for soil K fixation and availability, develop an alternative K fixation test, and evaluate the utility of soil texture and parent material for predicting K fixation in soils derived from granitic Sierra Nevada (SN) and nongranitic Coast Range (CR) alluvium. Potassium pools were estimated by 1 mol L −1 NH 4 OAc and sodium tetraphenylboron (TPB) extractions. Our 1‐h fixation method correlated well ( R 2 = 0.95, P = 0.001) with a 7‐d procedure, so the 1‐h method was used for subsequent work. The SN soils fixed up to 740 mg K kg −1 ; CR soilsf ixed up to 263 mg K kg −1 There was no significant relationship between K fixation and soil clay or silt content for either parent material. The TPB test had a stronger correlation with NH 4 OAc‐extractable K in SN soils ( R 2 = 0.77, P = 0.001) than in CR ( R 2 = 0.49, P = 0.001). Plant‐available nonexchangeable K (PANK = TPB minus NH 4 OAc) did not correlate with K fixation potential for pooled data from all pedons ( R 2 < 0.11), and had negative correlation ( R 2 from 0.97 to 0.99, P = 0.01) for individual pedons. The PANK probably represents K that has already been fixed and satisfies some of the K fixation capacity. The 1‐h test is a reliable, rapid method for predicting K fixation potential. Together, the TPB and NH 4 OAc tests could be useful for identifying K already fixed by soils, thereby reducing K fixation potential.