Compared to MgSO 4 , the Use of Magnesium Nanofertilizer Alleviates Potassium-Magnesium Antagonism in Tomato Roots

Magnesium (Mg) deficiency is increasingly recognized as a critical factor limiting crop production, especially in soils with high potassium (K) application. This study investigated the effects of different Mg fertilizers (MgSO 4 and Mg(OH) 2 -based nanofertilizer) on K–Mg interactions in hydroponically grown tomato seedlings, with varying K (0.7, 7, and 21 mM) and Mg (0.1 and 1 mM) supply concentrations. The results observed with MgSO 4 application showed that high K levels (21 mM) significantly antagonized Mg uptake, reducing total Mg content and negatively affecting plant growth and root-to-shoot ratios at both Mg supply levels. Specifically, the K3 (21 mM) treatment reduced total biomass by 45.35% compared to the K2 (7 mM) treatment. Moreover, a high K supply combined with low Mg intake exacerbated Mg deficiency. The optimal K:Mg ratio for growth was found to be 7:1. K–Mg antagonism occurred primarily during root uptake, with excessive K leading to increased Mg 2+ efflux in the root elongation zone. Notably, the application of Mg(OH) 2 nanoparticles alleviated K-induced Mg deficiency, as indicated by the lack of a significant correlation between K supply and relative Mg concentrations in plants treated with nano-Mg across K:Mg ratios ranging from 7:0.1 to 21:0.1. However, Mg concentration decreased by 17.54% and 35.63% in shoots and by 27.72% and 37.08% in roots for K2 and K3, respectively, in plants treated with MgSO 4 . It is concluded that optimizing K:Mg ratios and using Mg-based nanofertilizers can improve K and Mg utilization in high-K soils.