Based on many-year researches on black soil and brown soil,different performances of various fractions of soil microaggregates in storing and supplying moisture and nutrients are discussed.In vlew of the significant differences of these perforrnances between fertile and infertile soils and between small and large sizes of microaggregates,the authors attempt to take 10μm as alimit,puling various fractions of microaggregates under 10μm "characteristic microaggregates" and using their specific value to draw the quantitative limits of compositions of microaggregates between soils with higher and lower fertility levels and judge the levels of soil fertility and the effects of its improvement and regulation.The results show that the specific value of compositions of "characteristic microaggregates" can better reflect the present situation of soil fertility and the actual effect of its improvement and regulation,being a new and useful index for the overall evaluation of soil fertity.