In order to make fertilizer recommendations, multiple laboratory methods have been developed to determine the level of K in soils. These methods have been modified over time in an attempt to better estimate plant-available K. Extraction of K with 1 M NH₄OAc has had widespread use, and provides a value for exchangeable K. However, as some non-exchangeable K is also available to plants, a method using sodium tetraphenyl boron (NaTPB) as the extracting agent was developed by Scott et al (1960) and modified by Cox et al (1996). This method extracts both exchangeable and some non-exchangeable K, giving levels as high as 6 times the amount of extracted K than the NH₄OAc method in some K-fixing soils, and has been shown to more closely represent plant-available K. Additionally, the K-fixation potential of soils is critically important to determining K fertilizer recommendations. Two methods have been developed to measure K-fixation potential (Kfix) in the lab (Cassman et al, 1990; Murashkina et al, 2007). This study utilizes a combination of these methods to analyze the fate of K applied to K-fixing soils. Six soil profiles from the San Joaquin Valley of California representing a range of Kfix values were used. K was added in amounts equal to the initially measured Kfix of each sample. Samples were then incubated moist for 1, 2, 4, 8, or 16 days. Half were then air-dried and half were maintained in the moist condition. K availability was estimated by both the NH₄OAc and NaTPB methods, and Kfix was measured. Results showed that initial K application did not completely satiate the K-fixation potential of soils, as additional K continued to be fixed, though at reduced levels. Drying increased Kfix for all soils, but had mixed effects on NH₄OAc-extractable K. Kfix remained stable for all soils over the range of incubation times. NH₄OAc-extractable K and NaTPB-extractable K, however, were affected variably by incubation time. The results of this work will be useful for determining K fertilizer application requirements for K-fixing soils.