Higher content of soil organic matter can greatly reduce the difficulties of a good crop production system. Content of organic matter in the majority of Bulgarian soils is very low which is evident from the results of soil survey and the data from mass chemical soil tests – nearly 60% of studied soils are with humus content lower than 2.5%. The organic fertilization has direct effect on supplying humic substances in soil. On the basis of some Bulgarian soil types, it is shown the inclusion of organic fertilizers into the fraction of humic acids. A pot experiment and a small field study with Haplic Chernozems show an increase into the fraction of free humic acids and those bound with monovalent cations and mobile R2O3 oxides.