Objective The effects of K and Mg and their interactions on nutrient uptake of tomato were studied.

Methods A hydroponic trial was conducted with cherry tomato ‘Bijiao’ as the test crop. On the base of Yamazaki nutrient solution for tomato, Mg concentrations of 0 and 1.5 mmol/L (Mg₀, Mg_1.5), and K concentrations of 4, 40 and 200 mmol/L (K₄, K₄₀, K₂₀₀) were tested as individual treatments and as treatment combinations. At seedling, flowering, and fruiting setting stages, the root, stem and leaf samples were collected to analyze biomass, K, Ca and Mg contents.

Results The average biomass of tomato root, stem, and leaves in all three stages were in the order of K₄>K₄₀>K₂₀₀, and those of root, stem and leaf biomass at flowering and fruit setting stages were Mg_1.5>Mg₀. The K content in all the plant parts across the three stages was K₄₀>K₄ under both Mg levels, while tomato plants were depleted and decayed at fruit setting stage under K₂₀₀. The Ca and Mg contents in the whole growth period showed K₄>K₄₀>K₂₀₀. The highest Ca content in all organs was recorded in Mg_1.5K₄ at fruiting stage, while the Mg content in all the plant parts across the three stages in Mg_1.5 was higher than in Mg₀. Highest K uptake occurred under K₄₀ at flowering and fruit setting stages. K-Mg interactions significantly affected the stem K content, leaf Ca content at fruit setting stage, and whole plant Mg content at flowering and fruit setting stages (P<0.01). At different levels of K concentration, K accumulation at seedling and flowering stages was in order of K₄₀>K₄>K₂₀₀, and Ca and Mg accumulation was in order of K₄>K₄₀>K₂₀₀. K accumulation in all organs at fruiting stage were higher in Mg_1.5 than in Mg₀. Ca and Mg accumulation in all organs were higher in Mg_1.5 than in Mg₀. The K and Mg interactive treatments had significant reciprocal effects on K, Ca and Mg accumulation at flowering and fruiting stages (P<0.01). Compared to seedling stage, the K allocation to leaf tent decreased at flowering and fruiting stages, and Mg allocation to roots under K₂₀₀ tent decreased at flowering stage. The main component scores of Mg_1.5K₄ at flowering and fruiting were higher than that of other treatments.

Conclusions Excessive K concentration inhibites the absorption, accumulation and rational distribution of K, Ca and Mg in various organs of tomato, and causes plant death at fruit setting stage. Suitable Mg supply concentration promotes the accumulation of biomass, and K, Ca and Mg nutrients. Mg deficiency aggravates the adverse effects of excess K on tomato plants, especially at fruiting period, while excess K inhibits the absorption of Mg, which in turn affects the absorption of Ca. Therefore, adequate mineral nutrition, especially the balanced supply of K and Mg, should be used for tomato cultivation.