A method for treating seeds, using a composition obtained extemporaneously by mixing at least one powdered substance and an aqueous medium, comprising the following steps: - automatically and cyclically mixing in at least one first tank (10; 10') by first introducing at least part of the aqueous medium and then the powdered substance, the latter being sucked into the aqueous medium within the tank due to a vacuum created in the tank, the injection of the powdered substance into the tank being carried out through a tapping point located below the level of the aqueous medium in the tank, if necessary taking into account the vortex of agitation of the aqueous medium, and connected to an inlet for the powdered substance, the tapping point having an internal diameter less than or equal to 17 mm, the powdered substance being a hydratable compound, the hydratable compound being chosen from the following compounds: a cellulose or one derivatives thereof, for example hydroxycellulose, alkylcellulose, such as methylcellulose, hydroxyalkylcellulose such as hydroxyethylcellulose, or hydroxypropylcellulose, hydroxypropylmethylcellulose, carboxyalkylcellulose, such as carboxymethylhydroxyethylcellulose, sodium carboxymethylcellulose, synthetic polymers such as polyvinylpyridine, polyvinylpyrrolidone, polyethylene glycol, polyethylene oxide, polyamide, polyacrylamide, polymethacrylic acid, polyvinyl alcohol, polyglycerol, polytetrahydrofuran, polyacrylic acid, polyacrylate, alginic acid and derivatives thereof, such as alginate, pectin, galactomannans and derivatives thereof, such as guar gum, hydroxypropylated guar gum, carboxymethyl-guar, carboxymethylhydroxypropyl-guar, cationic guars, gelatin, starch and derivatives thereof, such as cationic starch, corn starch, wheat starch, rice starch, potato starch, tapioca, waxy corn, sorghum, waxy sorghum, dextrin, chitin, chitosan, xanthan gum, carrageenan gum, karaya gum, a derivative of any of the aforementioned compounds, or a combination of these compounds, the hydratable compound being present in the mixture used to treat the seeds at a concentration between 4.5 and 70% by weight, - at least partially draining, using at least one flushing with compressed gas, its content towards a mixing bowl of a seed treatment device where the mixture is brought into contact with the seeds and in which the dispersion is mixed with the seeds before the viscosity of the mixture reaches a maximum value. - automatically and cyclically mixing in at least one first tank (10; 10') by first introducing at least part of the aqueous medium and then the powdered substance, the latter being sucked into the aqueous medium within the tank due to a vacuum created in the tank, the injection of the powdered substance into the tank being carried out through a tapping point located below the level of the aqueous medium in the tank, if necessary taking into account the vortex of agitation of the aqueous medium, and connected to an inlet for the powdered substance, the tapping point having an internal diameter less than or equal to 17 mm, the powdered substance being a hydratable compound, the hydratable compound being chosen from the following compounds: a cellulose or one derivatives thereof, for example hydroxycellulose, alkylcellulose, such as methylcellulose, hydroxyalkylcellulose such as hydroxyethylcellulose, or hydroxypropylcellulose, hydroxypropylmethylcellulose, carboxyalkylcellulose, such as carboxymethylhydroxyethylcellulose, sodium carboxymethylcellulose, synthetic polymers such as polyvinylpyridine, polyvinylpyrrolidone, polyethylene glycol, polyethylene oxide, polyamide, polyacrylamide, polymethacrylic acid, polyvinyl alcohol, polyglycerol, polytetrahydrofuran, polyacrylic acid, polyacrylate, alginic acid and derivatives thereof, such as alginate, pectin, galactomannans and derivatives thereof, such as guar gum, hydroxypropylated guar gum, carboxymethyl-guar, carboxymethylhydroxypropyl-guar, cationic guars, gelatin, starch and derivatives thereof, such as cationic starch, corn starch, wheat starch, rice starch, potato starch, tapioca, waxy corn, sorghum, waxy sorghum, dextrin, chitin, chitosan, xanthan gum, carrageenan gum, karaya gum, a derivative of any of the aforementioned compounds, or a combination of these compounds, the hydratable compound being present in the mixture used to treat the seeds at a concentration between 4.5 and 70% by weight, - at least partially draining, using at least one flushing with compressed gas, its content towards a mixing bowl of a seed treatment device where the mixture is brought into contact with the seeds and in which the dispersion is mixed with the seeds before the viscosity of the mixture reaches a maximum value.