There is still a need to develop procedures for determining the K fertilizer requirements of K-fixing soils. A soil test for estimating K soil fixation potential that is suitable for commercial analytical laboratory usage was developed in our laboratory. However, this has not been correlated to K fertilization requirement. A second test that has been used by researchers to quantify the portion of fixed K that may be plant available is the sodium tetraphenyl boron (NaPh4B) test. We propose to use our large collection of well-characterized soils from San Joaquin Valley winegrape vineyards and cotton fields (>750 soil samples) to determine whether our regional model categories are informative with respect to K fertilizer requirement and whether the two analytical procedures mentioned here are predictive of the rate of K required to achieve sufficiency levels.