LE PRÉLÈVEMENT DU K PAR LA LUZERNE (Medicago sativa L.) ET SA DYNAMIQUE DANS 30 SOLS DU QUÉBEC

The understanding of the kinetics of K uptake by plants is necessary for the development of predictive models aimed at optimum fertilizer use and crop yields. The objective of this work was to study the kinetics of K uptake by alfalfa, in a long-term growth chamber experiment (366 d), on 30 soils from Quebec. The soils varied in mineralogical composition and by the amounts of available K in soluble, rapidly and slowly exchangeable forms. In 28 of the 30 soils, a minimal, steady-state level of NH 4 OAc-K was reached after 208 d and six cuts of alfalfa. This minimal level and the K uptakes were qualitatively related to soil texture. Only soils with clay content greater than 35% were still able to supply enough K to maintain alfalfa yields. The kinetics of K uptake was described by two parabolic diffusion equations. In the first 208 d, the rate and amounts of K uptake were better predicted by extracting procedures using a monovalent cation for the exchange. The amount of HNO 3 -K in the soils was very significantly related to the rate (r = 0.90**) and amount of slowly exchangeable K uptake (r = 0.89**) in the last 158 d of growth. The availability of K was greater in soils rich in feldspar and vermiculite than in soils where illite is a major component. The results of this study indicate that the amounts of HNO 3 -K and/or the clay content should be integrated into alfalfa fertilizer requirement tables along with amounts of K in soluble and rapidly exchangeable forms for maintenance of alfalfa fields. Key words: Slowly exchangeable K, alfalfa, uptake kinetics, HNO 3 -K, diffusion, K uptake