Problem: A light green or yellow coloring will begin to show on the veins and edges of the lower & older leaves – this is one of the classic signs of cannabis magnesium deficiency. You may also see red stems.
Magnesium is a mobile nutrient, which means that the plant can move it from old leaves to new leaves.
If you don’t react to it promptly, a cannabis magnesium deficiency can spiral out of control and cause your plant to lose a lot of lower leaves quickly. The plant will pull magnesium out of older leaves and bring them to the newer leaves. That’s why a magnesium deficiency usually appears towards the bottom of the plant and on older, less important leaves.
The edges of the leaves may become yellow or bright green and may start feeling crispy to the touch. This crispiness around the edges is different from a nutrient burn, which does not lighten the margins inside the leaves.
You may see red stems with a magnesium deficiency, though not always.
Sometimes you will also get light brown spotting within the margins or along the edges of the problem that continues to get worse, though these may be partially other deficiencies, which often happen alongside a magnesium deficiency.
Magnesium deficiencies are easy to prevent and fix once you know what to do. Read on below to learn how.
Solution For Magnesium Deficiency in Cannabis
A magnesium deficiency sometimes happens after supplementing plants with something that contains calcium but not magnesium, such as agricultural lime (non-dolomite lime), egg shells, etc. This won’t happen with proper cannabis nutrients. If you’re using good nutrients, the cannabis plant may still show signs of a magnesium deficiency if the pH at the roots is too low, especially in hydro. That is because when the pH of your root zone is not in the correct range, your cannabis cannot properly absorb magnesium through the roots.
Often with this deficiency, magnesium is present, but the roots cannot absorb the magnesium properly due to an improper pH. Therefore it is very important to maintain the correct pH (and make sure the pH does not get too low/acidic) in order to avoid a magnesium deficiency.
Growers using Coco Coir or Reverse Osmosis (RO) water usually need to supplement their plants with extra Calcium & Magnesium in addition to regular nutrients. Treating coco coir with Cal-Mag and supplying extra throughout your grow is recommended for growers in coco coir, or those using RO water.
Adding more magnesium to a system when there is a pH lock-out will probably not help because the plant will not be able to absorb any magnesium until the pH has been corrected. If there’s already enough magnesium, adding more can cause other apparent deficiencies by locking out other nutrients from the plant.
Please note: Once a magnesium deficiency is cleared up, the problem (yellowing lower leaves) will stop spreading to other older leaves, usually within a few days. Please note that leaves that have been damaged by a magnesium deficiency will probably not recover or turn green, so you want to pay attention to other growth for signs of recovery.
In soil, magnesium is best absorbed by the roots in the 6.0 – 7.0 pH range (some growers say a 6.5 – 7.0 pH is best if you suspect a magnesium deficiency)
In hydro, magnesium is best absorbed by the roots in the 6.0 – 6.5 pH range (in hydro, it’s generally recommended to keep the pH between 5.5 – 6.5, but magnesium specifically tends to be best absorbed above 6.0)
If you suspect your growing cannabis plant has a magnesium deficiency, flush your system with clean, pH’d water that contains a regular dose of cannabis-friendly nutrients that includes magnesium. This will remove any nutrient salts that may be affected the uptake of magnesium and help restore pH to the proper levels.
To supplement with extra Magnesium…
Calcium and magnesium deficiencies often appear together in cannabis. Many growers decide to purchase some sort of Calcium-Magnesium (often called Cal-Mag) supplement for their grow room in case one of these common deficiencies appears.