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Managing Magnesium Deficiency in Spring Bedding Plants

Thu, May 9th, 2024, created by W. Garrett Owen

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 provides 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.

For a constant liquid feeding program, growers should consider using a fertilizer that provides Mg such as 13-2-13 or 15-5-15 Ca-Mg. Like mentioned before, 20-10-20 and 20-20-20 does not provide Mg or Ca so, monthly magnesium sulfate (MgSO4; Epsom salts) drenches should be applied at a rate of 1 lb Epsom salt per 100 gallons of water.

To correct Mg deficiency, monthly applications of supplemental Mg in the form of Epsom salts should be supplied. In areas with naturally occurring Mg in the irrigation water, provide a rate of 1 lb Epsom salt per 100 gallons of water. In areas lacking Mg in the irrigation water, provide Epsom salt per 100 gallons of water. It is recommended that growers should routinely sample and submit irrigation water for analysis to determine water quality. By doing so, this will enable growers to determine if supplemental Mg is needed.

Additionally, it is also recommended that growers monitor substrate pH and soluble salts [referred to as electrical conductivity (EC)] by easily performing in-house 1:2 Dilution, Saturated Media Extraction (SME), or PourThru procedures. Each method will determine substrate pH similarly; however, EC values vary with each method. For more information, refer to "Fert, Dirt, & Squirt: Monitoring pH & EC of Greenhouse Crops Book. Corrective procedures for low substrate pH should begin within the range of 5.4 to 5.5. Growers should be sure that your pH and EC meter is working correctly and calibrated regularly. For a "how to video, watch "How to Calibrate a pH and EC Meter. Also, growers should consider having plant tissue and substrate analyzed for nutrient content by submitting samples to analytical labs.

For low or high pH or EC corrective procedures refer to "Corrective procedures for high and low substrate pH and electrical conductivity.

About the Author:

W. Garrett Owen

Assistant Professor of Sustainable Greenhouse and Nursery Systems, The Ohio State University

W. Garrett Owen is an Assistant Professor of Sustainable Greenhouse and Nursery Systems in the Department of Horticulture and Crop Science at The Ohio State University. He has an appointment in research, teaching and Extension. His area of expertise is plant nutrition; plant growth regulation; and production problem diagnostics.

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