NC State Floriculture

 

Low Substrate Magnesium Corrective Procedures

Brian E. Whipker, James L. Gibson, Paul V. Nelson, Todd J. Cavins, and John M. Dole

Disclaimer: Growers should read and follow all label directions. Test the corrective procedure on a small number of plants prior to applying it to the entire crop.

Possible Causes
Corrective Procedures
The amount of fertilizer supplied is lower than the crop's nutrient demand (crops are categorized as light, medium or heavy feeders), or at a particular stage of development (high Mg demanding crops can deplete substrate Mg levels, especially near bloom). 1. Increase fertilization frequency (use more frequent fertilization irrigations instead of clear water irrigations).
2. Increase fertilizer rate.
3. Avoid leaching the substrate with clear water irrigations.
4. Recheck the EC values to make sure they are within the acceptable range.
Not supplying preplant Mg as dolomitic limestone. 1. Use dolomitic limestone for initial adjustment of substrate pH.
Not using a fertilizer containing magnesium (eg. 20-10-20 or 20-20-20). 1. Use a fertilizer which contains Mg.
Monthly applications of magnesium sulfate not made. 1. Apply monthly epsom salts (magnesium sulfate) drenches at the rate of 1 to 2 lbs per 100 gallons of water.
Little or no magnesium provided through the irrigation water. 1. Use a fertilizer which contains Mg.
2. Apply monthly epsom salts (magnesium sulfate) drenches at the rate of 1 to 2 lbs per 100 gallons of water.
Excessive leaching. 1. Avoid leaching the substrate with clear water irrigations.
2. Recheck the EC values to make sure they are within the acceptable range.
Injector malfunction. 1. Check the EC of the clear water and the fertilizer solution. Fertilizer solution EC minus clear water EC = EC contribution of the fertilizer. Compare this value with the tables on the fertilizer bag to determine the fertilization rate being applied.
Fertilizer mixing rate error and too little fertilizer used. 1. Check if the amount of fertilizer used, the size of the concentrate bucket, and the proportioner setting are correct.
Don't forget to check out the Plant Root Zone Management (PRZM) Manual for more information! Want to know more? Click here!

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