Ion composition and proportion of soluble soil salts and activities of Mg²⁺, Ca²⁺ and K⁺ ions were determined in soils different in greenhouse cultivation history and in degree of magnesium deficiency reflected in tomato growing therein, and their relationships with Mg deficiency of tomato were analyzed. Results show that among the soluble soil salts Ca²⁺, K⁺ and NO₃^-in the soil increased in concentration linearly with the history of greenhouse cultivation going on. Ca²⁺ ion was the dominate cation in the soluble salt and NO₃^-ion became the dominate anion after 5 years of greenhouse cultivation. significant relationships were found of Ca²⁺ and NO₃^-ions with total salts and NO₃^-ions in the soil, indicating that NO₃^-ion was the major factor contributing to soil salinization. Mg deficiency symptom became more obvious, when contents of Ca²⁺, K⁺ and NO₃^-ions, total salts, and mole ratios of Ca²⁺/ Mg²⁺ and K⁺/ Mg²⁺ rose in the soil. In soils where tomato displayed Mg deficiency, salt content reached the level of soil salinization. With the content of soil salt rising, Ca²⁺ and Mg²⁺ activities decreased exponentially. Activities of Ca²⁺ and Mg²⁺ were significantly lower in soils where tomato displayed Mg deficiency than in soils where tomato grew normally, while activity ratios of (K⁺)/( Mg²⁺) and (Ca²⁺)/( Mg²⁺) were the other way round, and the activity ratio of (K⁺)/( Mg²⁺) increased significantly with the extent of Mg deficiency, reaching higher than 1. The decrease in Mg²⁺ activity due to salt accumulation in soil and the antagonistic effect of K⁺ enrichment on plant Mg²⁺ absorption were the two major causes inducing appearance of the symptom of magnesium deficiency on tomato growing in calcareous soil.