The effect of the carbon:nitrogen ratio (C/N) of organic amendments on soil structure modification was studied in laboratory incubation experiments. Variable C/N ratios were obtained through application of carbonaceous substrates (cellulose, starch and wheat straw) and different levels of urea. Optimal responses of soil density and soil aggregate stability were found for the C/N range of 20–40. The levels of sugars extracted with boiling water and those of uronic acids were highest for the same C/N range. The preference of this C/N range substrates is also justified theoretically, considering microbial carbon and nitrogen budgets.