Potassium fixation in soils derived from granitic and non-granitic alluvium of the San Joaquin Valley, California

Potassium (K) fixation plays a significant role in the soil-plant system by influencing the effectiveness of fertilization. Much of the previous work on K fixation by soils and pure minerals focused on K fixation and release by the clay size fraction. However, in soils containing considerable amounts of K-fixing minerals, such as vermiculite and hydrobiotite, K fixation capacity may be substantial in the coarser size fractions.; Soils were samples were collected from granitic Sierra Nevada (SN) and non-granitic Coast Range (CR) alluvium from the San Joaquin Valley, California. Potassium pools were estimated by 1 M NH4OAc and sodium tetraphenylboron (TPB) extractions. A rapid 1-hr method for measuring K fixation potential that produced results similar to a 7-day incubation method was developed. The contribution of clay, silt, very fine sand (VFS) and fine sand (FS) fractions to K fixation was measured in SN soils. Mineralogical composition of the various size fractions was determined by X-ray diffraction (XRD). A rubidium (Rb) fixation method with heating (Rb110°c) was evaluated as a method to quantify soil vermiculite contents.; SN soils fixed up to 740 mg K kg-1; in CR soils K fixation ranged from none to 263 mg K kg-1. There was no significant relationship between K fixation and clay or silt content in soils from either parent material. In the granitic SN soils smectite and mica were the dominant clay-fraction minerals. Vermiculite and hydrobiotite dominated the silt fractions, while some vermiculite was present in the VFS and FS fractions. Rather than fix K, the clay fraction of SN pedons released K to solution and had negative K fixation capacities (-0.4 to -1.2 mmol kg-1). In fine-loamy and finer-textured SN soils, silt was the main K fixing fraction. In coarse-loamy soils, fine sand was the dominant particle-size fraction, and this fraction fixed the most K.; Rb110°c fixation was highly correlated with clay content (R2= 0.85***). However, XRD data showed that clay fractions were dominated by smectite and chlorite. The data suggest that Rb fixation with heating to 110°C method overestimates the amount of vermiculite if tetrahedrally substituted smectites are present.