Increasing soil bulk density has been shown to reduce root growth and decrease K uptake by soybeans (Glycine maxL. Merrill). Changing soil bulk density also affects soil buffer power, b, and effective diffusion coefficient, De, which affect K influx. The relative decrease in K uptake due to reduced root growth as compared to reduced K influx is not known. Addition of P may affect root growth and P influx properties of plant roots. The objectives of this paper were (1) to use the Cushman mechanistic model to simulate the effect of changing soil bulk density and soil P on K uptake by soybeans, and (2) to determine the parameters that are changed by changes in bulk density and added P and their effect on K uptake. Plant and soil data of an experiment where Williams soybeans were grown for 21 days in pots of Raub (Aquic Argiudoll) silt loam with factorial treatments of two rates of K (0 and 100 mg K kg‐1soil), two rates of P (0 and 100 mg P kg‐1soil), and two bulk densities (1.25 and 1.45 g cm‐3) were used to verify the model. Plant and soil parameters for the model were measured independently of the verification experiment. Predicted K (y) uptake agreed with observed uptake (x) (y = 1.09x‐0.19; r = 0.97) for the P x K factorial and (y = 1.19X‐0.22; r = 0.90) for the K x soil bulk density factorial treatments. In a sensitivity analysis, the model predicted a maximal K influx at a soil bulk density of 1.38 g cm‐3. The greatest effect of soil bulk density on K uptake was due to reduction of root growth. Increased K uptake as a result of P addition was because of the effect on root growth.