Diffusion theory was used to describe Fe uptake by sorghum plants in short-term uptake studies. Diffusion equations were used to calculate the concentration of Fe at the surface of roots as a function of time, the extent of depletion zones around roots, and Fe uptake rates.Calculated concentrations of chelated Fe at root surfaces decreased rapidly with time in chelated Fe treatments. However, soluble Fe concentrations decreased much less in nonchelated treatments. Calculated depletion zones were greatest for chelated systems with small capacity factors but decreased as the capacity factor increased. Predicted Fe uptake rates agreed fairly well with observed uptake values from a short-term uptake study.