A study was made on the physico-chemical properties of micro-aggregates in meadow drab soils, yellowish drab soils and red earths of different levels of fertility. The results show that with the use of organic fertilizers the soil fertility is raised gradually, the organic and mineral colloids become combined more tightly, the ratio of humic and fulvic acids appears to be greater; all these making the micro-aggregates of the soils with high fertility to possess the following properties: a greater external-surface area, a lower electrophoretic velocity, a lower phosphorus fixing power, a larger amount of easily releasable ammonium, a greater available water holding capacity, and a lower hydroscopic water and swelling coefficient. From the results mentioned above, it is evident that the otgano-mineral colloidal complexes are of great significance in soil fertility.