epsom salt for weed plants

Magnesium deficiency in cannabis plants – how to spot it, and what to do about it

After the three main nutrients (nitrogen, potassium and phosphorous), magnesium is the most crucial secondary nutrient for healthy growth and development of cannabis plants. Without it, plants wither, fail to thrive, and eventually die. What does a cannabis plant with magnesium deficiency look like, and what can be done to help it recover? Help is at hand!

Why do cannabis plants need magnesium?

The reason that plants – and not just cannabis plants – need magnesium is simple: it is essential for photosynthesis. Without it, leaves cannot absorb and process light into energy. Plants with magnesium deficiency will eventually starve to death, even if they are getting the correct amount of hours of light.

How can magnesium deficiency in cannabis plants be identified?

As the primary ‘building block’ of chlorophyll, magnesium is what gives it, and therefore the plant itself, a healthy bright green colour. The first and most easy to identify sign of magnesium deficiency in cannabis plants is the leaves showing a worrying fade of green to pale green to yellow.

This is known as chlorosis. Although there are other problems that also cause chlorosis, magnesium deficiency shows up first around the edges and in between the veins on the leaves, rather than all over them or from the stems outwards or tips inwards.

The first leaves to show these signs will be the oldest ones, and the ones closest to the bottom of the plant (often, these are the same ones). This is caused by the plant withdrawing magnesium from these leaves in order to send it to the newer ones, in an effort to keep itself alive (in much the same way as the human body pulls blood away from the extremities in cold weather to keep the vital organs warm).

The leaves might also start to feel crunchy and dry, although it is not advisable to wait until this happens before starting treatment!

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What are the dangers of magnesium deficiency in cannabis plants?

Not only is magnesium essential for photosynthesis, it is needed for the absorption of other vital nutrients. These especially include nitrogen and phosphorus, two of the three primary nutrients. As well as being unable to photosynthesise, plants will be unable to develop sufficiently large and healthy root systems and flowers. This is obviously bad news for the plants themselves, and for anyone hoping to harvest those flowers!

Indeed, cannabis plants with magnesium deficiency may not even make it to the flowering stage. As its supply of magnesium is depleted, more and more of the leaves will turn yellow, then brown, and eventually drop off, causing the plant to die.

What causes magnesium deficiency in cannabis plants?

Unsurprisingly, the first culprit is not enough magnesium in the soil. It is an unfortunate fact that magnesium can be quite easily washed away by flushing or overwatering. The good news is that depending on where you live, your tap water may actually contain useful levels of magnesium!

Even if there are sufficient levels of magnesium in the soil, the plant may not be able to make use of them. A substrate with a pH value that is too low, i.e. very acidic, prevents the roots from absorbing magnesium even if it is plentiful. It is also important to know that adding more magnesium to a substrate that already has enough can make the situation worse.

Other necessary nutrients can be ‘locked out’ by an excess of magnesium in the substrate, and suddenly the gardener has multiple nutrient deficiencies to deal with, not just one!

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How can magnesium deficiency in cannabis plants be corrected?

Testing levels of magnesium in a substrate is difficult without laboratory analysis, something that very few people have easy access to. The best solution would be to correct everything that could be causing problems rather than attempting to tweak one thing at a time.

The most common way to correct pH and remove excess nutrient build-up in a non-soil substrate such as coco coir, or in a hydroponic system, is a thorough flush with pH-balanced water followed by another flush with pH-balanced water supplemented with regular cannabis nutrients. In hydro systems, the pH should be 6.0 – 6.5 for the best magnesium uptake.

The exact type of nutrient ratio will depend on whether the plants are in the vegetation or flowering phase. Obviously, it’s important to make sure that the nutrient solution used contains magnesium. It is also recommended to check that it contains calcium, as calcium deficiency often occurs in tandem with magnesium deficiency.

Growing cannabis plants in soil, especially in open ground rather than pots, makes them far less likely to suffer from magnesium deficiency. However, it does not guarantee immunity! In soil, the pH should be 6.0 – 7.0. The same flushing technique can be applied as above, and then a specialised magnesium and calcium supplement (available at good garden centres) can be added.

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Quick fix for magnesium deficiency in cannabis plants

As a quick-fix emergency measure, mixing magnesium sulphate with water to make a foliar spray is often effective. Magnesium sulphate is sold under the name Epsom salts, and is easy to get online or from large pharmacies.

A magnesium foliar spray is made by mixing one tablespoon of Epsom salts with four litres of water. This mixture is then sprayed onto the plants every three days, with a clean water spray every ten days to ensure residue does not build up on the leaves.

We hope that this article has been informative about magnesium deficiency in cannabis plants! Do you have any tips for us? Please leave them in the comments below. Do you have questions about magnesium, or other types, of deficiencies? Please consult our forum, where a thriving community can help you find the answers.

Magnesium deficiency is a common problem for gardeners, and cannabis plants are not immune to it! Luckily it’s quite easy to detect and fix. This article explores what causes magnesium deficiency, what it looks like, and what can be done to reverse it.

Epsom Salts: a natural hack to grow healthy cannabis plants

Epsom salt is the best and most natural way of adding magnesium and sulfur to your cannabis plants. The benefits are astounding and there is little risk of overfeeding. Find out why you should use Epsom salt in your next grow.

Epsom salt is the nickname given to magnesium sulfate. The name comes from the spring water of Epsom, England. Farmers were quick to discover that Epsom water had “magical” healing properties, and its fame quickly travelled far and wide.


Epsom salt looks very much like coarse kitchen salt. But make no mistake, they are entirely different in molecular terms. Epsom salt is also used in health and beauty products in the form of bath salts, exfoliators, muscle pain-relievers, migraine-relievers, and those that treat cold and flu symptoms. Be cautious not to use these forms of Epsom salts to feed your plants, as they are often mixed together with all sorts of dyes, alcohols, aromas, and other ingredients that are likely to damage your cannabis plants. You will easily find the right product in any garden store, and even in pharmacies and health food stores.

Epsom salt is a fantastic and completely natural way of giving your cannabis plant an extra boost of both magnesium and sulphur, with the added benefit that you really can’t go very wrong with it. Excess feeding of these micronutrients will not have a negative effect on your plants. It is almost impossible to oversaturate the soil over time. Epsom salts also can be used in hydroponic systems; though here, you should be particularly mindful of potential pH swings.

It is very easy to use, easy to source, and cheap to boot. It is highly soluble in water, so it doubles as a great foliar spray. With the evermore urgent pursuit of completely organic ways of growing great weed, Epsom salts deserve an honourable spot in your arsenal of all-natural and environmentally conscious products.


There is no major difference in the magnesium found in Epsom salts as opposed to say, Cal-Mag supplements. The major difference is Epsom is an all-natural product while bottled supplements are usually petrochemical-derived.

High-performing crops have a tendency to exhibit some sort of deficiency if nutrition is not perfectly tailored. Cannabis certainly is one of these crops. In particular, plants grown hydroponically tend to exhibit a higher propensity for magnesium deficiency. Even is soil, certain strains seem to complain and demand a considerable boost of this micronutrient to maintain lush green foliage and vigorous growth.

Magnesium also facilitates the uptake of nitrogen (N), phosphorous (P), and potassium (K). Should a magnesium deficiency occur, it is easy to detect due to the characteristic yellowing of the leaves, except in the veins. This contrasts with the typical nitrogen deficiency, where the entire leaf turns yellow. Magnesium deficiency also tends to manifest first in the lower sections of the plant, with the yellowing leaves curling slightly upwards.

Magnesium is also highly implicated in photosynthesis, as it facilitates the creation of chlorophyll. It also strengthens cells walls, which are responsible for plant vigour and steady growth.

Magnesium is essential to achieve those superbly high yields. To form big buds, you need a healthy dose of this element. Furthermore, magnesium is essential for seed formation, too.


Sulphur is one of the building blocks of the plant’s immune system. It is also what gives garlic and onions their pungent tinge on the tongue. For cannabis, it works together with magnesium to increase the uptake of NPK, and is implicated in the production on enzymes, amino acids, and vitamins.

Chlorophyll also requires sulphur; but an area where this element is of particular interest is when keeping mother plants. Sulphur helps to promote longevity and reduce environmental stress. Mother plants are kept in veg for inordinate amounts of time. As cannabis is not a perennial species, this can pose a nutritional challenge, and Epsom salts certainly alleviate the issue of constant maintenance.


Epsom salts are very forgiving. As mentioned, it is difficult to overdo. But as with all things, little at a time goes a long way. These salts may be cheap, but costs do add up.

General rule of thumb would say 1 tablespoon per 5 litres of water can make for a great foliar application every other week. Difficult soils that have a pH above 7 will naturally lock out calcium and magnesium ions, so amending with some Epsoms salts is a great idea.

A more diluted version of Epsom salt water can also be used to help with seed germination, and even to help clones root faster. Harder-to-clone strains have been known to have a higher success rate by both misting and adding this mix to the actual cutting.

In the case of treating a deficiency, you should allow 3 to 4 days before evaluating the results by verifying new growth patterns and recovery from the less-affected leaves. More-affected leaves may well be scarred by magnesium deficiency and never fully recover.


As a final note, agricultural-grade Epsom salts are of great benefit to humans too. It is the base ingredient to fancy bath salts and many health and cosmetic products. Feel free to experiment with some of its known applications, should you need some relief from various common ailments. Now you and your plant can bond even further by using something in common.

Epsom salts get their name from the spring water of Epsom, England. But what exactly do they do for cannabis plants?