The study looked at how different types of fertilizers affect the composition of humus in soil over a long period. They found that fertilization increased humus content, improved the complexity of humic acid molecules, and decreased a specific ratio (E4/E6). Different types of fertilizers increased the levels of humic acid and humin in the soil. The ratio of humic acid to fulvic acid also increased with fertilization. The carbon content in humin varied, with the highest amount found in undissolved humin.