AbstractRye‐grass was grown in six different soils with four additions of75Se‐labelled selenite through six cutting in a pot experiment. Following the last cutting, the soils were extracted with a series of solvents to fractionate the residual soil selenium. The plant content of added and total selenium was determined. The results showed that equilibrium between added and native selenium was obtained before the third cutting, giving a nearly constant LSe‐value for the last four cuttings. The LSe‐values differed between the soils, and there was a positive correlation between the LSe‐values and the plant uptake of native selenium. In accordance with this, the best correlation with the total selenium concentration in the plants was obtained with the isotopically exchangeable selenium in the soil after the last cutting. It is concluded that isotopically exchangeable selenium approximates the fraction of soil selenium from which plant uptake of seleni