Magnesium (Mg) is an essential macronutrient and is integral to photosynthesis and the chlorophyll molecule. Despite its importance, many greenhouse growers use water-soluble fertilizers that provide little to no Mg, such as 20-10-20 or 20-20-20. When bedding plants are provided with little to no Mg, plants become Mg deficient and symptoms develop in the lower or older leaves as interveinal chlorosis (yellowing between veins), and in severe instances, marginal leaf necrosis (browning and death). In some instances, substrate amended with inadequate limestone or constant use of acidic fertilizers will lower substrate pH below 5.5. Low substrate pH will inhibit Mg uptake thus possibly causing plants to exhibit Mg deficiency symptomology. Therefore, growers should not solely depend on the lime charge of the substrate to provide Mg and are encouraged to monitor substrate pH.
To prevent Mg deficiency, growers should generally maintain a calcium (Ca)-to-Mg ratio of 3:1 to 5:1 in irrigation water and substrate. When this ratio is unbalanced, an antagonist effect can occur, i.e. excessive Ca supplied can cause Mg deficiency. Meanwhile, excess Mg can cause potassium (K) deficiency, so supplying plants with a K:Ca:Mg ratio (in ppm) of 4:2:1 will limit any antagonistic effects.
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