Fertilising and/or soil conditioning composition intended for culture substrates and/or culture solutions

A composition intended for culture substrates and/or culture solutions consisting of 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 obtained by heat treating, in the presence of water, the mixture of compounds (a) and (b), wherein the heat treatment consists of heating to a temperature between 30° C. and 95° C., wherein the water content of the combination is between 1% and 5% by mass relative to the total mass of the combination, and wherein the at least one carbon compound (b) is selected from the group consisting of saccharides, chars, biochars, hydrochars, lignosulfonates, and mixtures thereof.