Abstract: 【Objective】It is generally considered that the contents of calcium and magnesium (Mg) in calcareous soil are rich. Therefore, Mg deficiency usually occurs in acidic soil in south China, where the weathering is very strong. However, Mg deficiency of tomato under solar greenhouses in calcareous soil in north China becomes common. It is urgent to understand the reason of Mg deficiency of tomato under solar greenhouses in the region, which will provide scientific knowledge to solve the Mg deficient problem in the solar greenhouse. 【Method】The different degrees of Mg deficiency of tomatoes with the same transplanting times and varieties in the greenhouses in Dazhai, Yangling, Shaanxi were chosen. The degrees of tomato Mg deficiency were divided into three types based on the severity of the symptoms. The three types of Mg deficiency included the leaves without Mg deficiency, the leaves with moderate (half of the upper leaves showed deficiency) and severe (all leaves showed deficiency) Mg deficiency. The soil samples (0-20 cm), lower leaves samples (the leaves between the second and third fruit sets) and upper leaves samples (the fully developed leaves on the top plant) were taken from different greenhouses. The basic soil properties were determined, including soil pH, electric conductivity (EC), organic matter, available nitrogen, phosphorus, and potassium, and soil exchangeable K⁺, Ca²⁺, and Mg²⁺, and their contents in tomato leaves. The ratios of exchangeable K⁺, Ca²⁺, and Mg²⁺ ions, and their saturation percentages in soil colloid were calculated. The fruit yields of different greenhouses were also surveyed. The relationships between Mg deficiency of tomato and the parameters determined were analyzed.【Result】There were significant differences in soil properties between the greenhouses with different degrees of Mg deficiency. The soil EC, contents of nitrate, readily available K in soil of tomato with severe Mg deficiency were significantly increased. The content of exchangeable Mg ions in soils was higher than the critical value of Mg deficiency both in tomatoes with and without Mg deficient symptoms. With the severity of Mg deficiency of tomato, the saturation percentage of K ion was increased and the ratios of Ca/Mg, and K/Mg also increased. On the contrary, the saturation percentages of Ca²⁺ and Mg²⁺ ions, and the ratio of Ca/K were decreased. Compared with tomatoes without Mg deficiency, the saturation percentage of K⁺ and the ratio of K/Mg in soil with moderate Mg deficiency were increased both by 22%, and soil with severe Mg deficiency increased by 56% and 88%, respectively. Compared to soil without tomato Mg deficiency, the ratios of Ca/Mg in soil with severe Mg deficiency was increased by 17%, and the ratios of Ca/K in soil with moderate and severe Mg deficiency were decreased by 19% and 37%, respectively. The soil Ca/Mg ratio of tomato with moderate Mg deficiency was slightly higher than optimum value of Ca/Mg ratio (7.0), and K/Mg ratios was higher than optimum value (0.6) by 47%. The ratios of soil Ca/Mg, and K/Mg were higher than optimum value by 28% and 125%, respectively. The Mg contents in both upper and lower leaves and fruit yield of tomato were significantly and negatively correlated with the readily available K, exchangeable K⁺, saturation percentages of K⁺, and K/Mg ratio, and positively correlated with soil organic matter content and Ca/K ratio; and their correlations with exchangeable Mg²⁺ and its saturation percentages, and the ratio of Ca/Mg were not significant. With the severity of Mg deficiency, the tomato fruit yields were decreased rapidly. The decreasing rates of fruit yields in moderate and severe Mg deficiency were 38% and 60%. 【Conclusion】 The contents of soil exchangeable Mg²⁺ was at optimum levels for tomatoes with or without Mg deficiency; however, the Mg²⁺ saturation percentages was lower. The ratios of Ca/Mg, and K/Mg in soil with different Mg deficiency were imbalance, especially for the K/Mg ratio. The high level of K resulted in imbalances of cation ions was the main reason of Mg deficiency in solar greenhouse of calcareous soil. Therefore, more attentions are needed to deal with this problem.

Key words: solar greenhouse, magnesium deficiency of tomato, soil properties, balances of cations, nutrient contents