Magnesium is a secondary macronutrient essential for cannabis plants. This element is the central atom within the chlorophyll molecule, the green pigment responsible for capturing light energy during photosynthesis. Without sufficient magnesium, the plant cannot properly convert light into the sugars it needs for growth, leading to an energy deficit. Addressing a magnesium deficiency quickly is essential for maintaining vigorous growth and preventing a reduction in final harvest quality.
Identifying the Symptoms
Magnesium is a mobile nutrient, meaning the plant can relocate it from older tissues to support newer growth. This characteristic dictates that the first visual signs of a deficiency will appear on the older, lower fan leaves.
The most distinct indicator is interveinal chlorosis, a yellowing of the leaf tissue between the veins. The veins themselves typically remain dark green, creating a noticeable marbled or striped pattern. As the deficiency progresses, the yellowed areas may develop into rusty or bronze-colored spots that become necrotic and cause the leaf edges to become brittle.
If the problem is not corrected, these symptoms will systematically climb up the plant, affecting progressively newer growth. The affected leaves may also start to curl upward and eventually wither and drop prematurely. While red or purple stems can sometimes signal a magnesium issue, this symptom should only be considered alongside the characteristic leaf yellowing, as it can also be caused by genetics or other nutrient imbalances.
Diagnosing pH Imbalances
Even when magnesium fertilizer is used, the plant may be unable to absorb the nutrient if the root zone environment is not correctly balanced. This situation is known as nutrient lockout, where the element is present but unavailable. Magnesium absorption is highly sensitive to the acidity or alkalinity of the growing medium.
To diagnose this, growers must test the pH of the runoff water in soil or the reservoir solution in soilless and hydroponic setups. For cannabis grown in soil, the optimal range for magnesium uptake is 6.0 to 7.0. When the pH drops below 6.0, magnesium absorption is severely restricted.
For soilless mediums, such as coco coir, or hydroponic systems, the ideal pH range is slightly lower, maintained between 5.5 and 6.5. If the measured pH is outside this range, the first step is to adjust the pH back into the proper range. This often resolves the deficiency by freeing up the already-present magnesium, though low pH is a more common cause of lockout than high pH.
Immediate Correction Methods
Once a deficiency is confirmed, the fastest way to deliver the nutrient directly to the plant is through a foliar spray. Epsom salts (magnesium sulfate) provide a readily available source of magnesium.
For a foliar spray, the common mixing ratio is one to two teaspoons of Epsom salts dissolved into one gallon of water. Apply this solution as a fine mist to the entire plant, focusing on the undersides of the leaves where the stomata are located for maximum absorption. To prevent leaf burn, perform this application right before the lights turn off or during the dark cycle.
A second, slightly slower method is a root drench, which provides a longer-lasting supply of magnesium to the growing medium. This involves watering the plant with a solution of one tablespoon of Epsom salt mixed per gallon of water. If a pH imbalance was diagnosed, this drench is often performed as part of a flush, using pH-adjusted water containing the magnesium solution to reset the root zone environment.
Long-Term Maintenance Strategies
After the immediate deficiency is addressed, growers must implement long-term strategies to prevent recurrence, focusing on consistent nutrient and pH management. Regular monitoring of the pH of the nutrient solution or runoff is the most effective preventative measure, ensuring the root zone remains within the optimal 5.5 to 7.0 range depending on the medium.
Many growers, particularly those using reverse osmosis (RO) water or soilless substrates like coco coir, benefit from the prophylactic use of a dedicated calcium and magnesium supplement, often called Cal-Mag. These supplements provide a balanced ratio of both minerals, which are frequently depleted in soft water or prone to competition for uptake. The typical maintenance application for Cal-Mag is about one teaspoon per gallon of water with every feeding.
For soil growers, incorporating dolomite lime into the medium offers a slow-release source of magnesium and calcium. Dolomite lime has a neutral pH of approximately 7.0 and helps to buffer the soil, stabilizing the pH and gradually releasing the minerals over several months. Six to seven teaspoons of fine-grade dolomite lime can be mixed into each gallon of soil before planting to establish a long-term magnesium reserve.