Cannabis nutrients Part Six: Magnesium and Cannabis.

by Alex Robles

Greetings my fellow cultivators, I hope everyone’s gardens are thriving and looking great.  The march down the list of nutrients continues and this time I’m going to talk about the wonders of Magnesium.

Magnesium (Mg) is a secondary nutrient that plays a primary role in a plants life cycle and will use it in large amounts.  Photosynthesis is the process of taking inorganic carbon (carbon dioxide) and changing it into organic compounds, this process can’t happen without Mg.   It’s also known that Mg is used to make the chlorophyll that gives plants their color and is the active part of light energy absorption.  Magnesium can’t be absorbed in its raw form by the roots from the soil, it has to be processed by beneficial micro-organisms first.  The uptake of phosphorous is also easier when a cannabis plant has enough magnesium. 


A magnesium deficiency is the most common secondary nutrient problem and can happen in any type of medium or system but it rarely appears in cannabis that is grown outdoors in the ground.  The reason for this is because most grow systems aren’t really designed to hold on to nutrients without some help from microbes.

Since it is a mobile element, the symptoms of a deficiency will first show up on older lower leaves which will show signs of chlorosis (yellowing of plant leaves caused by a nutrient deficiency) between green veins.  Dead spots will begin to appear and give the leaf a speckled look, while the edges of the leaf will sometime turn brown, curl and die back.  If the deficiency is left untreated the plant will start to lose a lot of the older leaves and really start to struggle.  The chlorosis will move up the plant to newer leaves and the plant will die.

Here are some of the reasons why a magnesium deficiency shows up and how to fix it in dirt.

  • Cannabis absorbs Mg best at a 6.0 – 7.0 pH range but just to be safe I wouldn’t go over 6.5 pH.
  • Too much calcium and potassium will make a marijuana plant lockout magnesium and cause a deficiency. Nutrient lockout happens when there are high levels of one kind of nutrient in the soil that blocks other nutrients from being absorbed.  One remedy for lockout is to flush the plant to break up the nutrient buildup that is on the roots and in the soil.  Remember, magnesium is easily washed away during a flushing also.
  • Make sure your feeding the plant a balanced nutrient mixture for its stage of growth. It could be as simple as double checking the math because sometimes the plant just needs magnesium.   Here are a few different ways to add it in.  Top feeding dolomite lime or worm castings is a great source of magnesium and both of them can also be mixed into the soil at about 5 teaspoons for each gallon of soil.  Epson salt is also good source of magnesium and can be mixed into water at about one teaspoon per gallon of water.
  • Low temperatures and a constantly wet soil/grow medium will cause the plant to absorb magnesium slower also. Making sure your soil is dry before watering again and adjusting your watering schedule will heip.

Note:  When you top feed a plant dry soil amendments like worm castings or dolomite lime, it will take about a week them to start to be broken down and absorbed by the plant.  For faster absorption, worm castings can be made into a compost tea.  The damaged leaves will more than likely not recover from the magnesium deficiency damage.


Too much magnesium could make the plant lock out calcium and cause other deficiencies.

And that my friends is all I can say about this.  Leave a comment or question.

Grow, Learn, Teach.

Click here for pH up down kit

Click here for Worm castings

Click here for Digital pH meter

Click here for Epson salt

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