Magnesium: what it is and what it does in cannabis plants
Magnesium is one of the most important nutrients for the proper development of cannabis plants. Although nitrogen, phosphorus and potassium are often discussed more frequently, a magnesium deficiency in cannabis can directly affect leaf colour, growth rate, photosynthesis and the overall performance of the crop.
In this article from the Hydroponics Blanes blog, we explain in a simple and practical way what function magnesium for cannabis plants has, how to recognise the first symptoms of a deficiency, which factors can block absorption and which products can help correct the problem. This content is for informational purposes only. At Hydroponics Blanes, we do not sell marijuana with THC; we sell legal products for self-cultivation, such as fertilisers, nutrients, indoor cultivation lighting systems, substrates, meters, extractors, grow tents and cultivation accessories.
What is magnesium and why is it so important?
Magnesium is an essential secondary nutrient for many plants, including cannabis. It forms part of the structure of chlorophyll, the green pigment that allows the plant to capture light and transform it into energy through photosynthesis. For this reason, when magnesium is lacking, the plant loses the ability to produce energy, generate sugars and maintain vigorous growth.
In indoor cultivation, where development depends on the balance between light, nutrients, temperature, humidity, irrigation and pH, magnesium becomes even more important. A plant exposed to powerful LED lights, intense flowering or rapid growth may need a stable supply of calcium and magnesium to avoid nutritional imbalances.
When we talk about fertilisers with magnesium for indoor cultivation, we usually refer to Cal Mag products, magnesium supplements, Epsom salts or complete nutrients that already include this element in a balanced formula. The key is not to add magnesium without control, but to detect the problem correctly and adjust nutrition.
Functions of magnesium in cannabis plants
Magnesium participates in several fundamental processes. The best-known function is related to chlorophyll, but it is not the only one. It also plays a role in enzyme activation, energy transport within the plant and the use of other nutrients.
A plant with adequate magnesium levels usually shows green leaves, balanced growth and a better ability to respond to intense lighting. On the other hand, when a magnesium deficiency in plants appears, photosynthesis is reduced, the leaves lose vitality and yellowing, brown spots or dry areas may appear.
Main functions of magnesium
- Participates in the formation of chlorophyll.
- Promotes efficient photosynthesis.
- Helps transform light into usable energy.
- Contributes to the production of sugars and carbohydrates.
- Plays a role in nutrient absorption and transport.
- Helps maintain more stable growth during the vegetative and flowering stages.
Magnesium deficiency in cannabis: how the problem begins
A magnesium deficiency in cannabis usually appears progressively. At first, it can go unnoticed because the initial changes are subtle and can easily be confused with watering stress, incorrect pH, excess fertiliser or a deficiency of other nutrients.
Magnesium is a mobile nutrient. This means that, when the plant does not receive enough of it, it can move magnesium from the older leaves to the new growth areas. For this reason, the first symptoms usually appear on lower or middle leaves, especially in plants that already have some size.
One of the most common signs is yellow leaves due to lack of magnesium, but with a very specific characteristic: the yellowing usually appears between the veins of the leaf, while the veins remain greener during the initial stage. As the deficiency progresses, brown spots, oxidised areas and dry edges may also appear.
Symptoms of magnesium deficiency in cannabis
Recognising the symptoms of magnesium deficiency in cannabis in time makes it possible to act before the problem progresses too far. A plant with a mild deficiency can still recover well if the pH is corrected, irrigation is adjusted and the right nutrient is provided.
Most common symptoms
- Yellowing between the veins of the leaves.
- Green veins while the tissue between them becomes lighter.
- Appearance of brown spots, rust or dry points.
- Lower and older leaves affected before new leaves.
- Leaf edges with a dry or burnt appearance.
- Tips curling upwards in advanced cases.
- Dull overall appearance, with less vegetative vigour.
- Less stable flowering when the problem appears during key weeks.
It is important not to confuse a real deficiency with a lockout. Sometimes the substrate or nutrient solution contains magnesium, but the plant cannot absorb it due to incorrect pH, excess salts, low temperatures or saturation of other nutrients.
Cannabis magnesium nutritional problems: common causes
Cannabis magnesium nutritional problems can have several causes. They are not always due to a lack of magnesium in the fertiliser. In many cases, the problem appears because the root cannot absorb it correctly.
Incorrect pH
pH is one of the most important factors. In soil, a range close to 6.2 – 6.7 usually helps magnesium assimilation. In hydroponics or systems with inert substrates, an approximate range of 5.7 – 6.2 may be more suitable, although it is always advisable to follow the instructions of the nutrient manufacturer used.
When the pH drops too low, magnesium availability can be reduced. For this reason, before adding more fertiliser, it is advisable to measure the pH of the irrigation water and, if possible, also check the runoff.
Excess potassium or calcium
During flowering, products rich in phosphorus and potassium are used. An excess of potassium can make magnesium absorption more difficult. An imbalance can also occur when there is too much calcium in relation to magnesium. That is why Cal Mag for cannabis products should be used carefully, respecting dosages and observing the plant’s response.
Substrate that is too wet or too cold
A waterlogged substrate reduces the oxygen available in the root zone. When the root does not work properly, the plant absorbs nutrients less efficiently. Low temperatures also slow root activity, especially during the night period in indoor cultivation.
To avoid this problem, it is advisable to improve ventilation, control the minimum temperature, avoid excessive watering and use pots with good drainage. In crops sensitive to cold, a heat mat or better climate management can help keep the root active.
Reverse osmosis water without calcium or magnesium
Reverse osmosis water has a very low EC and contains practically no calcium or magnesium. This is useful for controlling nutrition from scratch, but it also requires remineralising the water before adding base fertilisers. In these cases, a Cal Mag supplement is highly recommended.
Magnesium and flowering in cannabis plants
During flowering, the plant increases nutrient demand. Phosphorus and potassium become more prominent, but magnesium is still necessary to maintain photosynthesis, energy and the overall functioning of the plant.
Many deficiencies appear between the third and sixth week of flowering, especially when stimulators or PK products are applied at high doses. At that point, excess potassium can block magnesium absorption and cause symptoms even if the nutrient is present in the substrate.
For this reason, before increasing doses, it is advisable to review EC, pH, watering frequency and the general condition of the plant. Excess fertiliser can create the opposite of the desired result: flowers with irregular development, damaged leaves and nutrient lockout.
How to correct magnesium deficiency
To know how to correct magnesium deficiency, it is first necessary to differentiate between a real lack of magnesium and a lockout caused by pH or excess salts. Adding more product without checking these points can make the crop worse.
First step: check pH and EC
Before applying any corrective product, measure the pH of the irrigation water. In crops with liquid fertilisation, it is also advisable to measure EC. If EC is too high, there may be salt build-up. In that case, adding more nutrients will not be the best solution.
Second step: adjust watering
If the substrate remains wet for too many days, reduce watering frequency and improve ventilation. A root with oxygen absorbs magnesium and the rest of the nutrients better.
Third step: apply a suitable supplement
When the deficiency is mild or moderate, a Cal Mag for cannabis product or a specific magnesium supplement can be used. In crops with reverse osmosis water, Cal Mag is often a basic tool to raise the initial EC and provide calcium and magnesium in a balanced proportion.
In some cases, Epsom salts, rich in magnesium sulphate, are also used. They can be applied through irrigation or foliar spraying with great care, always respecting low dosages, good ventilation and avoiding spraying with the lights on or at high temperatures.
Fourth step: observe the evolution
Damaged leaves do not always recover their original colour. The important thing is to check that the deficiency does not progress and that new leaves grow healthy. If the problem continues after several days, pH, EC, root temperature and possible excess of other nutrients should be reviewed again.
Recommended products to provide magnesium
At Hydroponics Blanes, you can find different solutions to prevent or correct a magnesium deficiency in plants. The choice depends on the type of cultivation, the water used and the fertilisation programme.
Cal Mag
Cal Mag is one of the most widely used options in indoor cultivation, especially with reverse osmosis water, coco, hydroponics or high-intensity lighting systems. It provides calcium and magnesium in a balanced formula, helping to prevent lockouts and deficiencies.
Complete base fertilisers
Many growth and flowering fertilisers already include magnesium in the formula. When working with a complete nutrient range, it is advisable to respect the cultivation chart and not mix products without checking compatibility.
Epsom salts
Epsom salts can provide magnesium quickly. They can be useful in specific cases, but they do not replace balanced nutrition. Excessive dosages must be avoided to prevent new imbalances.
pH and EC meters
A good pH and EC meter is just as important as the fertiliser. Many magnesium problems do not appear because of a lack of product, but because of incorrect values in the irrigation solution. Measuring prevents applying products blindly.
Excess and deficiency of magnesium in cultivation
Excess and deficiency of magnesium in cultivation can cause different problems, but both affect nutritional balance. Deficiency usually appears as yellowing between veins, brown spots and loss of vigour. Excess, on the other hand, is less common, but it can block the absorption of calcium and other nutrients.
How to recognise excess magnesium
- Slower growth than normal.
- Dark green leaves.
- Possible spots or damaged edges.
- Symptoms similar to calcium deficiency caused by lockout.
- General imbalance in nutrient absorption.
If excess is suspected, the safest approach is to check the runoff EC, verify the pH and temporarily reduce the fertiliser load. In more serious cases, it may be necessary to perform a root flush with adjusted water and then return to balanced nutrition.
Tips to prevent lack of magnesium
Prevention is always better than correction. To reduce the risk of yellow leaves due to lack of magnesium, it is advisable to maintain a stable cultivation routine and avoid sudden changes.
- Control pH at each watering.
- Use an EC meter if applying liquid fertilisers.
- Remineralise reverse osmosis water with Cal Mag.
- Do not overuse PK during flowering.
- Avoid cold or constantly waterlogged substrates.
- Maintain good ventilation in the growing area.
- Respect the manufacturer’s dosages.
- Observe lower and middle leaves to detect early symptoms.
Magnesium is an essential nutrient for photosynthesis, chlorophyll formation and the overall performance of the crop. A magnesium deficiency in cannabis can appear as yellowing between veins, brown spots, loss of vigour and problems during growth or flowering.
To correct the problem, adding more fertiliser is not enough. It is necessary to review pH, EC, temperature, watering, the water used and possible lockouts caused by excess potassium or calcium. With balanced nutrition, suitable products and good measurement, it is possible to prevent most cannabis magnesium nutritional problems.
At Hydroponics Blanes, you will find fertilisers, Cal Mag, pH and EC meters, substrates, indoor cultivation lighting systems and everything needed to maintain more controlled, efficient and healthy self-cultivation.