Experiments including two in laboratory and one in greenhouse were carried out to study non-exchangeable magnesium release from fluvo-aquic soils sampled from Daxing and Changping counties located in the suburbs of Beijing and Mg relative availability of the two soils to plants. In a batch experiment in laboratory the soils were incubated under wet conditions and alternation of dry and wet conditions and determined for amount of Mg released at the 4th, 8th and 12th week, respectively, after extraction of exchangeable Mg with 1 mol L-1 NH4Ac. The amount of Mg released from the soil of Daxing was higher than from the soil of Changping, which was in accordance with the fact that the soil of Daxing had higher contents of all forms of Mg than that of Changping. There was little difference in Mg release from soils between wet conditions and alternation of dry and wet conditions. About 1%～2% of the total non-exchangeable Mg might be released within 12 weeks of incubation, restoring about 30%～35% of the original soil exchangeable Mg.Results of the experiment on kinetics of Mg release from the soils through continuous extractions with 0.5mol L-1 NH4Ac (pH 7.0) on a continuous flow apparatus in laboratory showed that Mg released rapidly in the beginning, decreased sharply with time and kept stable at 60 and 240 rain for the soils of Changping and Daxing, respectively. Among the five mathematical models used to describe the kinetics of Mg release,the parabolic diffusion equation best fitted the cumulative Mg release, indicating that diffusion of Mg out of the soils might be the controlling process. The experiment of exhaustive cropping with 1 crop of tomato (Lycopersicon esculentum Mill.) followed by six crops of corn (Zea mays L.) in greenhouse showed that soil exchangeable Mg decreased remarkably with cropping. After three crops, the percentage of the total plant Mg uptake that came from soil non-exchangeable Mg was 29.5% for the soil from Changping and 35% for the soil from Daxing. About 50% of the total Mg uptake by plants in the six crops was believed to come from the soil non-exchangeable Mg form.