Silt and fine sand fractions dominate K fixation in soils derived from granitic alluvium of the San Joaquin Valley, California

Much of the previous work on potassium (K) fixation by soils and pure minerals has focused on K fixation and release by the clay size fraction. However, in soils containing considerable amounts of silt- and sand-size K-fixing minerals, such as vermiculite and hydrobiotite, K fixation capacity may be substantial in the coarser size fractions. We measured the contribution of clay, silt, very fine sand (VFS) and fine sand (FS) fractions to K fixation in five Sierra Nevada granitic alluvium soils. Mineralogy of size fractions was determined by X-ray diffraction. Potassium fixation of the whole soil and size fractions was measured by shaking samples for 1 h with 2 mmol KCl, followed by a 30-min extraction with NH4Cl. Smectite and mica were dominant clay-fraction minerals. Vermiculite and hydrobiotite dominated the silt fractions, while quartz, feldspars, amphiboles, mica and some vermiculite were present in the VFS and FS fractions. Soil materials from most soil horizons fixed from about 8 to 95% of added K. Depth-weighted average K fixation by clay fractions ranged from − 2.3 to − 6.5 mmol kg− 1 clay, meaning that K was released, not fixed. On a whole soil basis, calculated K fixation was highest in the silt fractions in all Ap horizons (0.5 to 3.1 mmol kg− 1 soil). In subsurface horizons FS fixed 0.04 to 4.6 mmol K kg− 1 soil and VFS fixed 0.1 to 3.4 mmol K kg− 1 soil, whereas silt fractions fixed 0.9 to 5.5 mmol K kg− 1 soil. Overall, the highest percentage of added K was fixed by the silt fractions, dominated by vermiculite and hydrobiotite. In pedons, where VFS and FS are dominant, these fractions constituted the main locus of K fixation.