Twelve soils derived respectively from basalt, granite and seashore deposit were collected and cropped with grass (Coasteross-1 Pertrudagrass ) and corn in the greerhouse.The soil analysis data showed that all soils tut one bad relatively low content of total Mg. The soils derived from basalt were rich in exchangeable Mg( more than 100 ppm Mg in the top-soils ) .The soils derived from granite and seashore deposit were poor in exchangeable Mg. (ranging from 11.1 to 25.3 ppm in the topsoils) . There were significant correlations among different forms of magnesium.Exchangeable Mg in the topsoils correlated significantly with soil pH and CEC. Mg concentration in the grass and corn was well correlated with Initial total Mg, acid soluble Mg, exchangeable Mg and Mg saturation of CEC.Its correlations with exchangeable Mg and Mg saturation of CEC were much more significant. So was-the total Mg removed ty these two crops. Acid soluble Mg was available to the grass and corn,but its release was too slow to meet the requirement of plant growth.It waa concluded that exchangcable Mg and Mg saturation of CEC are better indices for evaluating the magnesium availability of the soils and the effects of magnesium fertilizer on plant growth .