In 2016, there are about 500,000 acres of rice planted in the Sacramento Valley of California, valued at more than $700 million. Productivity per hectare in this region has historically been higher than national average, but yield growth has slowed in recent years. One important factor related to rice productivity is soil potassium (K) concentrations. As the soils in this area have historically been high in K, growers have not needed to apply it to their fields. However, after years of harvest without K replenishment, growers are beginning to observe K deficiencies. After attempts to correct for K deficiencies with K fertilizers, some growers observe little or no crop response. The reason for this lack of response to K amendments is likely K fixation, in which K ions are trapped in between soil particle interlayers and unavailable to plants. Our preliminary data of soils sampled from rice fields across the Sacramento Valley show that there is no clear relationship between a field’s K budget and the extractable potassium in the soil or with the concentration of K in the plant tissues, indicating that there is likely K fixation occurring. The distribution of K fixation in the Sacramento Valley is suspected to be related to soil mineralogy, as the two sides of the valley contain different soil minerals, each with different potassium fixation capacities. Our research aims to better the understanding of these relationships between soil mineralogy and soil potassium. We will be conducting soil mineralogy analysis to compare the relationships between soil mineralogy and potassium fixation.  An improved understanding of this soil-nutrient relationship will help growers better manage their fertility and increase yields.