The importance of magnesium in the production of CBD rich hemp

What is magnesium?

There are three groups of plant nutrients: primary, secondary and micro. The three primary nutrients are the ones represented by the numbers on the front of all fertilizer containers:  N-P-K, Nitrogen-Phosphorous-Potassium. The three secondary nutrients are Calcium, Magnesium and Sulfur. Their symbols are Ca, Mg, S. There are eleven micronutrients to be concerned with when growing, and we will not be discussing those  in this post.


Most farmers and gardeners focus only on the primary nutrients, NPK. However the secondary nutrients are just as important and either too much or too little of one of them can seriously decrease your yields.  In fact, there is a school of thought championed by Albrecht, Kinsey and others that the ratio of Ca:Mg:K is more important to pkant/soil health and yields than NPK. This known as the Basic Soil Cation Ratio.  The subject and focus of this blog post is the secondary nutrient Magnesium.


Plant leaves are nature’s solar cells. Through the process of photosynthesis, green plants use solar energy to convert water and carbon dioxide into carbohydrates and then release oxygen as a waste product. The green pigment in the leaves that  allows photosynthesis to take place is called chlorophyll. AT the very center of each chlorophyll molecules sits one magnesium atom encased in what is known as a chlorin ring.  Four nitrogen atoms in the chlorin ring surround and bind the magnesium atom. Nitrogen is well known for the nutrient that promotes deep green leafy growth, and now we understand why…because 4 nitrogen atoms are in the chlorophyll molecule binding to the sole magnesium atom. This arrangement also tells us something about magnesium. Magnesium is also essential to get than deep green leaf color. Without magnesium, chlorophyll does not exist. Without chlorophyll, most life on earth does not exist.


You would think magnesium’s presence being require for chlorophyll would be enough, but magnesium is a classic over-achiever. Magnesium also acts as a phosphorous (the “P” in NPK) carrier in plants. Without magnesium phosphorous uptake could not occur. This is very important information for cannabis growers as phosphorous demands in cannabis increases as it passes from the vegetative stage to the bloom stage. This is the reason bloom fertilizers have a higher amount of phosphorous. If your plants lack sufficient magnesium to make use of that expensive phosphorous then your yields are lower and you have wasted money.  But wait, there’s more! Magnesium is also essential  for plant respiration and the activation of several enzyme systems. What a workhorse for a secondary nutrient.

How do you identify a magnesium deficiency?

Magnesium is one of the nutrients that is mobile in a plant. That means that when a deficiency occurs the plant is able to move magnesium from older growth to support new growth. So a magnesium deficiency will first show in the older leaves. Visually it appears as interveinal chlorosis…the loss of color between the leaf veins, beginning at the leaf tips and margins and working its way to the center. So the edges and tips of the leaves will yellow and the veins and center of the leaves will be green. When you see this you want to take quick action to correct the deficiency, especially if your plants have already entered the bloom phase.

magnesium deficiency in cannabis
This cannabis plant has a severe magnesium deficiency caused by use of a high potassium bloom fertilizer
closeup of magnesium deficiency in cannabis
In this closeup you can clearly see the interveinal chlorosis. Note the yellow margins and green veins and center of the leaves.

The two photos to the left show a cannabis plant 8 days after the first symptoms of a rather serious magnesium deficiency showed itself. The classic yellow margins and tips with green veins and venters of the leaves. You can see the flower buds at the tips of the stems so this plant is at a  very critical stage in its development and this magnesium deficiency needs immediate attention.

How to correct a magnesium deficiency:

The quickest, easiest and most cost effective way to correct a magnesium deficiency is with magnesium sulfate…Epsom Salts. I live in a rural farming area so I can just go to an agronomy center and purchase a 50lb bag of solution grade Epsom Salts for under $25.  That 50lb bag will last me several years and will be used on CBD rich hemp as well as the solanum family: tomatoes, peppers, eggplant and tomatillos.

You can apply magnesium sulfate in several ways. 1) sidedress, scatter a small handful of Epsom Salts right on the soil surface around the outside of the effected plant.  2) add it to your nutrient solution at a rate of 1 pound/100 gallons of water. 3) as a foliar spray, which is the quickest acting method.  This plant is well into the bloom stage though so we don’t want to use a foliar spray that might leave a residue that will give a harsh or off flavor to the final product. So in this case we used the sidedress method and scattered a small handful of Epsom Salts on the soil surface and watered it in well…until now granules were visible on the surface.

Sidedressing and watering it in was still very quick acting. The plant was noticeably greener in 3 days. Within 6 days the plant was looking very healthy and growing again.

magnesium deficiency cured using epsom salts
This photo is of the same plant take 6 days after being treated with Epsom Salts. Green, healthy and growing again

This plant was one of a planting of 52 cannabis plants in a licensed, legal medical marijuana grow. About 12/52 plants developed symptoms of the magnesium deficiency when the grower changed over to his “bloom nutes”. We went ahead and gave every plant the same treatment. After seeing the results in 5 days, the grower went ahead and gave all of the plants a second treatment at 14 days.

When you make your fertility plan for a grow whether it is cannabis, or any other crop, be sure to include magnesium. I this case, it was the grower’s first outside grow. He had been growing for years indoors and had been taught that he only needed to concern himself with calcium and magnesium when using a coir based growing medium.

This case study illustrates the importance of having soil tests done well before you plant the first seed. Get the full soil test with NPK, secondaries and micronutrients. Also %organic matter, base cation ratios and cation exchange capacity. The cost isn’t that much more and the possible increase of yields by making your fertility decisions based on more complete data will pay for the tests many times over.


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