Abstract: Magnesium deficiency is one of the common phenomena occuring in the mountainous citrus orchards in South Fujian.It causes the short-life of foliage and lower chlorophyll content; it also retards the process of photosynthesis, arrests the growth of the roots and makes the plant to deteriorate and thus leads to alternate bearing of fruits.The amount of exchangeable magnesium in the soil of magnesium deficient orchard is usually below 50 ppm, and that of exchangeable potassium is usually above 150-200 ppm.The exchangeable potassium-magenesium ratio value is higher than 0.5.The soil shows highly or very highly acid.The potassium-magnesium content in the foliage correlated with the exchangeable potassium-magnesium content in the soil positivelly.The magnesium content of the magnesium deficient plant’s foliage is below 0.25% with the lowest is 0.03% only.The magnesium content in the old leaves is lower than that in the young leaves and visual symptom, could be observed.As far as potassium is concerned, it usually exceeds the intermediate range of 1.2-1.6%.The conditions that cause the magnesium deficiency in the mountainous land of South Fujian are two fold, l.the content of assimulab-Je exchangeable magnesium in the soil is low; 2.the ratio value between the exchangeable poatssium and the exchangeable magnesium is out appropriate proportion.The visual symptoms appear in the plant are mostly of A and B types.A spray of 1% magnesium nitrate may quickly increase the magnesium content of the plant foilage and cause its potassium content to drop to a reasonable range,while the calcium content of the foilage would correspondently be heightened and effect an increase of its chlorophyll. This, in comparison with the control plot shows a significant or highly significant differences.A basic measure for the remedy of magnesium deficiency is to apple magnesium fertilizer and use lime or dolomitic lime to adjust the pH value to 6-6.5, and also to apply potassium fertilizer,according to the existing potassium content of the soil.