In this paper,the basic substances of soil fertility and their variations in black soils with various exploiting periods and various levels of fertility are discussed.The results show that:1.According to the correlation analyses,soil organic matter has significant correlations with soil N status and CEC.It means that the status and amounts of organic matter have a marked influence on the physico-chemical and bio-chemical properties of soil,and the activities of enzymes characterize the variations of the status and amount of orgenie matter to a certain degree.In black soils,the organic matter is predominant.It is considered that the core of fertility in black soil is organic matter and the activity of enzymes can be used as an important auxiliary index to characterize the level of fertility in black soils.2.After exploited,the total amount of organic matter and the amounts of humic and fulvic acids are decreased obviously with the increase of exploiting periods.The amount of active humic acid has a little increase in thick black soil or few changes in meadow black soil in initial years of exploitation,but decreases gradually after that.Humin,a more stable component of humic substances,also decreases with the increase of exploiting periods.Correspondently,the activities of all sig enzymes tested show a tendency to decrease with the increase of exploiting periods.But the activity of polyphenol oaidase in cultivated soils is markedly lower than that in virgin land.As for enzymatic activities,the activities of all enzymes (expect that of polyphenol oaidase) are weakened with the depth of both virgin land and cultivated land.3.In thick black soil,there is a remarkable difference in levels of fertility between fertilized lands and usual lands.In the former,the total amounts of organic matter and various components of humic substanses are higher,the activities of catalase and invertase are higher,and those of polyphenol osidase and phosphatase are lower than the latter.