Abstract: The invention relates to a new method for increasing the biological value of manures, dung-waters, organic wastes and/or soils rich in lysine. According to the invention manure, dung-water, an organic waste and/or soil rich in lysine, either of natural composition or supplemented with lysine, a lysine source and/or one or more agricultural chemical(s), is treated with at least 0.01 mole of formaldehyde or an equivalent amount of a formaldehyde source, calculated for one mole of free or bound lysine present.The treatment according to the invention results in a significant increase of plant growth and crop yield.