Deficiency and excess of magnesium in cannabis plants

Functions of magnesium in cannabis plants

Magnesium is a very necessary secondary nutrient in all the stages of the plants life, and it’s needed in large quantities. It’s the central atom of chlorophyll and has a direct impact on the absorption of solar energy to be subsequently processed and used by the plant in the creation of sugars and carbohydrates.

The cannabis plant absorbs magnesium in ion Mg+2 form, being this the magnesium formulation normally found in most soils. Thus, the absorption of this nutrient will be determined by the available form of this element in the substrate for marijuana plants.

It’s important to find a balance between the available and the unavailable magnesium in the soil. Unavailable magnesium still hasn’t been transformed by the microbial life yet, so plants can't absorb it. It’s important to know the amount of magnesium available for the plant (which is very difficult to know without analyzing the substrate).

Then, how should we proceed when growing in soil/hydro to have the magnesium levels in the substrate under control? As it’s a mineral, we can supply our plants with it by using a mono-nutrient in Mg + 2 form or other products high in Mg, so we achieve a direct uptake by the roots without having to wait for the microbial life to transform it into assimilable elements for the plant.

Magnesium (Mg) deficiency in cannabis plants

If the Ph range of the substrate is lower than 7.0, then magnesium can be easily absorbed by cannabis plants. But if the soil is very acid - lower than 5.0 - magnesium won't be assimilable by the plants. In this case, we should increase the Ph level of the substrate by using limestone dolomite. There are other products on the market that contain magnesium in case of not having limestone dolomite to mix with the substrate.

As magnesium is a mobile element, any deficiency of this nutrient will be first visible in the oldest leaves and those of the lower part of the plant. As the deficiency advances, the central part will also be affected.

Magnesium - as nitrogen - is easily flushed through abundant watering. If we wash the roots to solve other other nutrient excesses (N, P, K) we must add magnesium and calcium to maintain a correct nutrient balance. Normally, growers use two parts of Ca for one of Mg (EC=0.4).

It’s important to note that there are other factors that may reduce or lock-out the magnesium uptake, such as a permanent humidity, low temperatures, or acid and cold substrates. If our substrate is costantly humid, we will proceed to put a fan in the growing space pointing directly to the pot's substrate, thus providing a more dry substrate and improving magnesium uptake. And, of course, we won't water our plants so ofently.

Once the substrate has the correct humidity level, we should adjust the amount of nutrient solution per irrigation to favour Mg absorption while avoiding a possible root rot, which is usually very harmful - even deadly - for cannabis plants. These cases are more usual in indoor crops, especially when temperatures aren’t well controlled during the night period.

To solve this problem, we should install a heat source to raise the temperature of the growing space to a minimum of 18 °C during the night period. In this way, the development of plants is not slowed down and we solve the problem in regard with magnesium uptake.

Remember that during the flowering stage, plants need a lot of nutrients such as phosphorus, potassium and of course magnesium to develop large flowers and the original smells and flavours naturally present in the plant.

Magnesium during the flowering stage of cannabis plants

Many of the deficiencies that we can observe during the flowering period are given due to an excess with the bloom fertiliser. Most fertilisers for this phase contain high amounts of phosphorus and potassium: it should be said that potassium and calcium are two nutrients that can cause magnesium lock-out when added in excess.

Usually, this lock-out occurs during the first weeks of flowering - when the plant is creating buds - approximately during the third-sixth week of flowering, depending on whether it’s an Indica or Sativa strain. During these weeks, and if we use too much fertilizer, we'll cause an excess of potassium (K) that will block the absorption of magnesium (Mg). To solve this nutrient imbalance, flush the roots to lower the nutrient excess in the substrate so that the plant can absorb Mg again.

It’s also necessary to emphasize that the deficiency often appears because the substrate doesn’t have enough Magnesium; in these cases, we should provide our plants with magnesium during the above-mentioned weeks, either directly in the soil - by adding limestone dolomite - or through irrigations, using a Mg supplement in he nutrient solution and via foliar with a Ph level of 7.1. You'll notice a quick recovery of marijuana plants.

Magnesium (Mg) deficiency

How to quickly detect a Mg deficiency in cannabis plants?

• It's difficult to detect in the early stages
• First signs are shown in the oldest or lower leaves of the plant
• The tips of the leaves turn brown and curl upwards
• The brown spots increase in number and size, advancing from the lower part to the top of the plant
• The youngest leaves, located in the top part of the plant, are also affected, showing brown spots and possible discoloration of the veins
• The deficiency may come preceded by an accumulation of other nutrients such as calcium, hydrogen, and potassium
• It’s possible to apply magnesium sulphate via foliar for better absorption (2 % of Mg with a PH level of 7.1)
• Temperatures shouldn’t be under 18ºC during the night period and 24°C during the day
• Adjust the Ph value of the substrate to 6.5 (soil) and 5.5 ( hydroponics)

Magnesium (Mg) excess

How to quickly detect a Mg excess in cannabis plants?

• Its detection is difficult since having magnesium excesses is rare as long as we use substrates suitable for growing marijuana.
• Magnesium ions come into conflict with calcium ions, causing Ca lock-out (look for symptoms of calciumdeficiencies)
• If there is excess of Mg you should flush the roots with triple the amount of water than the capacity of the pot. After that, water your plantswith a balanced fertiliser