Soil Heating Significantly Increases Leaf Magnesium Concentration and Fruit Yield in Tomatoes Produced in a Plastic Greenhouse During Winter

Magnesium (Mg) deficiency frequently occurs in tomato leaves grown in plastic greenhouses during winter in northern China. In this study, field experiments were conducted to test the effects of soil temperature and the potassium (K) fertilizer application rate on tomato K and Mg absorption and fruit yield. The treatments were soil non-heating (control) and soil heating with electric hotlines, with three K2O application rates (180 kg ha−1, 580 kg ha−1, and 980 kg ha−1). The soil heating treatments increased the average soil temperature by 2.1 °C during the day, significantly increasing leaf Mg and chlorophyll concentrations by 21.3% (from 6.86 to 8.32 g·kg−1) and 12% (from 1.25 to 1.40 mg·g−1), respectively, and fruit yield by 5.5% (from 150 to 158.2 t·ha−1) and significantly decreasing the leaf K concentration by 10.5% (from 29.4 to 26.3 g·kg−1). However, the K fertilizer application rate had no significant effect on fruit yield and leaf K and chlorophyll concentrations. Moreover, the soil non-heating treatments showed a significant negative correlation between leaf K and Mg concentrations. Low soil temperature exacerbates K–Mg ion antagonism, which is the main driving factor for Mg deficiency in winter greenhouse tomatoes. Soil heating can significantly promote Mg absorption and improve fruit yield in tomatoes produced in plastic greenhouses during winter. The results of this study provide theoretical and technical support for regulating Mg nutrition in tomatoes grown in plastic greenhouses in northern China.