THE ISOTOPE-EXCHANGE KINETIC TECHNIQUE - A METHOD TO DESCRIBE THE AVAILABILITY OF INORGANIC NUTRIENTS - APPLICATIONS TO K, P, S AND ZN

This paper presents the interests and the limits of the isotope exchan ge kinetic method to assess soil K, P, S and Zn availability for plant s. This method allows to quantify the amount of nutrient present on th e soil's solid phase which can arrive in the solution of a soil/soluti on system at a steady state. Three parameters characterize this exchan ge in addition to exchange time: R/r(1) which is the ratio between the total introduced radioactivity (R) and the radioactivity remaining in the soil solution after 1 minute of exchange (r1), n which describes the rate of disappearance of the radioactivity from the solution for e xchange times longer than 1 minute, and C which is the concentration o f the studied element in the soil solution. These three parameters can be used to calculate the amount of element isotopically exchangeable with time (E(t) values). This approach gives access to the three param eters characterizing the availability of inorganic nutrient: the inten sity factor (the concentration of nutrient in the solution), the quant ity factor (the quantity of nutrient which can be taken up by a plant E(t)), and the capacity factor which describes the relationship betwee n the intensity and the quantity factors (R/r(1) and n). Results obtai ned by this approach are relevant to areas such as fertilizer manageme nt and organic matter mineralization. This method bears also a large p otential for assessing the bioavailability of heavy metals, such as Zn , either in agricultural soils where those nutrients may be limiting o r in polluted soils where they may be present in excessive concentrati ons. However, results obtained by this approach may be properly interp reted if and only if the basic assumptions underlying its use are fulf illed, and if the various parameters (C, R/r(1), n) are measured with enough precision.