Abstract: Determinations by Kumada method showed that with the improvement of black soil fertility,the free and combined humus contents in soil and its different size organo-mineral complexes increased,but the humification degree of free humus decreased,which was more obvious in silt and fine sand size complexes.The organic carbon content in complexes,humus extraction rate,free humus content,and humification degree of free humic acid decreased with the increasing particle size of complexes.All free humic acids in fertile soil were Rp type,while in unfertile soil,they were Rp and B type.With the increasing particle size of complexes,the type of free humic acids changed in the sequence A type(clay)→B type(silt)→Rp type(fine sand).Combined form humic acid mainly belonged to A type,no matter what particle size the complex was.The improvement of soil fertility could make the humification degree of free humus in soil and its complexes decrease,and furthermore, result in type change.In black soil,the type change of free humic acid mainly occurred in silt size complex,and that of combined form humic acid mainly occurred in fine sand size complex.

Key words: Gnotobiotic culture, Bacterivorous nematodes, Bacteria, Population