Abstract: The present invention relates to a composition intended for culture substrates and/or culture solutions comprising the combination of—(a) at least one clay, siliceous or clay-siliceous mineral compound and—(b) at least one carbon compound comprising at least 5% by mass of total carbon and/or at least 1% by mass of organic carbon relative to the total mass of the carbon compound, said combination being obtainable by heat treating, in the presence of water, the mixture of compounds (a) and (b). It also relates to the method of manufacturing and using same as well as a fertilizer or soil conditioner comprising same and the use thereof.