diffusive gradients in thin-films (dgt), a new technique in predicting soil bio-availability of mineral elements for plants

total concentrations of mineral elements in soil bear little relation to their availability for plants. the dgt (diffusive gradients in thin-films) technique has been found to be a good predictor of trace metals, micronutrients, phosphorous and potassium bio-availability and uptake. in the dgt technique, elements are accumulated on a binding gel after their diffusive transport through a hydrogel. in this review paper, we explore in a more detail why and under which conditions dgt correlates with plant uptake. the theoretical considerations will be illustrated and some of  experimental results in relation to metal uptake and toxicity, phosphorus and potassium will be reported. strong correlations between dgt and plant uptake are predicted if the diffusive transport of the element from soil to the plant roots is rate-limiting for its uptake.   if uptake is not limited by diffusive transport, dgt-fluxes and plant uptake may still show correlation. of course, this will be true in the condition which plant uptake is not saturated. however, competitive cations may affect the plant uptake under these conditions, whereas they have no effect on the dgt flux. moreover, labile complexes are not expected to contribute to the plant uptake if diffusion is not limited, but they are measured with dgt. therefore, if plant uptake is not limited by diffusion, interpretation of the observed correlation in terms of the labile species measured by dgt is inappropriate.