Antagonistic K/Mg ratios: is foliar application of MgSO4 a superior alternative to root resupply?

Backround and aims The main cause of magnesium (Mg) deficiency is its competition with potassium (K). Maize, as the world's most widely grown crop, has a high risk of interplay with poorly balanced soils. Since foliar fertilization is applied when availability and distribution of nutrients is limited, this study aimed to determine whether Mg foliar application (FA) is to be favored over root resupply (RS) under such conditions. Methods Plants of Zea mays L. were grown hydroponically with low and sufficient Mg supply under adequate to excess [K]. These ratios were combined with FA or RS of Mg, and plants were analyzed for Mg uptake, translocation and distribution. The primary physiological functions of Mg were quantified by chlorophyll content, photosynthetic rate and starch accumulation. Results Maize showed a restriction in the uptake antagonism of Mg by K, synergism in translocation, but no effect at distribution. Whilst tissue [Mg] remained unaffected by K, the increased K/Mg ratio impaired the physiological functions of Mg. The FA significantly reduced this K/Mg ratio, but neither the decline in chlorophyll levels and photosynthesis nor starch accumulation was reduced any faster or more markedly than by RS via nutrient solution (NS). Conclusions Foliar Mg application is an effective alternative under high K/Mg ratios, but due to the restricted antagonism and the unknown suppressive mechanism of K on the physiological functions of Mg it is not superior to a RS via NS. Under field conditions when compared to soil fertilization, however, it may offer a decisive advantage.