The present invention relates to a humic substance based Si-fertilizer product having storage-stable, bio-available Si, which is produced by forming a mixture of, 45-95% by weight of a humic acid-containing raw material, 5-50% by weight of amorphous silica, and 0.5-10 % by weight of an alkali, all amounts based on the total weight of the dry components, incubated in an aqueous medium, and optionally dried and granulated, wherein the bio-available silicon is in the form of water-soluble chelates of monosilicic acid-humate compounds. The invention also relates to a method of producing humic substance based Si-fertilizer, a method for increasing the uptake of silicon in plants and use of the humic substance based Si-fertilizer.