Chemical methods for determination of soil P available to plants do not take into account the effect of roots on P uptake by the plant. Moreover, the relative significance of root parameters, as compared to soil supply parameters in determining P uptake, is unknown. Simulation models have been verified for P uptake by corn (Zea mays L.) and soybeans (Glycine max L. Merr.). The objective of this paper was to use the Cushman simulation model, which has 11 plant and soil parameters, for a sensitivity analysis of the parameters involved in P uptake. Initial parameter values were those obtained from soybeans grown in Raub (Aquic Argiudoll) silt loam. Phosphorus uptake was simulated with each parameter changed independently, from 0.5 to 2.0 times initial value while all the other parameters remained constant. In addition, P uptake was simulated where interrelated parameters were changed according to their dependence. Root growth rate and root radius were the most sensitive parameters influencing P uptake. Soil P supply parameters were more sensitive than root physiological uptake parameters. Phosphorus concentration in soil solution affected P uptake more than the diffusion coefficient and buffer power. Reduction of root radius while root volume was maintained constant by increasing root length increased P uptake. Where both soil volume and root volume were kept constant, reduction of root radius to the size of root hairs or mycorrhizal hyphae gave the greatest P uptake.