Magnesium (Mg) in Cannabis – Deficiency and Requirements

The tissue between the veins of the lower leaves turns yellow while the veins stay green. That is what typical magnesium deficiency looks like. Magnesium is one of the six macronutrients. It sits at the centre of chlorophyll, the green leaf pigment, and helps bind carbon dioxide from the air. The deficiency often only becomes visible once it is already severe. The window is narrow: in perlite 35 mg/L was optimal, and from 70 mg/L the leaves suffered. The unit mg/L means milligrams per litre of feed solution.

How do I recognise magnesium deficiency?

Magnesium deficiency shows as yellow tissue between green veins on the lower leaves. It often starts as a yellow net in the middle of the leaf that spreads outwards. Sometimes it starts as a yellow margin at the leaf tips instead. Later the contrast becomes sharp, and small light brown spots appear between the veins. The pattern climbs to the middle of the plant.

Magnesium deficiency often appears only after flowering has started. It then mainly hits the smaller, younger fan leaves and the sugar leaves on the upper branches. Fan leaves are the large fingered leaves, sugar leaves the small leaves inside the buds. Some older fan leaves stay green.

Every stage with its look-alikes is described in the article on nutrient deficiency and overdose.
Look at the bottom and middle of the plant and check for green veins.
Rule out potassium deficiency. It yellows more evenly from the margin and leaves larger brown spots at the edge.
In vegetative growth, rule out manganese deficiency. It shows a similar net but starts on the upper leaves.
If lower leaves turn purplish black, measure pH, the measure of how acidic the root zone is. In other crops this colour occurs with severe magnesium deficiency and with too much iron and manganese below about pH 5.

How much magnesium does cannabis need?

In perlite, 35 mg/L was optimal in both vegetative growth and flowering. Perlite is a heat-expanded volcanic rock that holds almost no nutrients itself. At 2 mg/L, flowering plants lacked 37.7 % of their bud mass. At 2 to 20 mg/L cannabinoid acids such as THCA were also at their lowest, as were most terpenes. Terpenes are the aroma compounds of the buds.

In peat with perlite, two CBD cultivars grew best at 50 to 75 mg/L. There, 100 mg/L gave fewer buds again. The two values come from different growing media and are therefore not averaged. Retailers often write ppm instead of mg/L. In irrigation water both units mean the same.

Perlite: give 35 mg/L in vegetative growth and flowering, and no less in flowering.
Peat with perlite: give 50 to 75 mg/L.
Count the magnesium in your tap water. It is listed on the water supplier’s analysis.
Garden soil, compost soil, coco and rockwool were not tested in these trials. There a lab analysis of substrate or solution together with a leaf sample shows whether enough arrives.

What does too much magnesium do?

Too much magnesium causes dead spots on old leaves and scorched leaf tips. In perlite, 70 and 140 mg/L caused dead spots on the old leaves low on the stem during vegetative growth. In flowering, the leaf tips burned first from 70 mg/L, and the leaves yellowed afterwards. At 140 mg/L bud mass was 8.6 % below the value at 35 mg/L, though sulphate also rose at that level.

Magnesium also pushes back calcium, potassium, zinc and manganese. At 2 mg/L magnesium the leaves held 61 % more calcium than at 140 mg/L. A calcium or potassium deficiency can therefore join the picture of an excess.

Check your tap water for magnesium.
Do not give Epsom salt repeatedly without measuring.
Lower the magnesium. New growth shows whether it works, because damaged leaves do not recover.
If new growth stays free of spots, keep the lower value.

How do I give magnesium correctly?

Usually as Epsom salt, and 1 g of it per litre supplies about 99 mg/L magnesium by calculation. Epsom salt is magnesium sulphate with water bound in its crystals and contains 9.9 % magnesium. Kieserite contains 17.6 %, because less water is bound in the salt. For the same amount of magnesium you need just over half as much kieserite.

As a one-off corrective drench the NC State group gives 2.4 g Epsom salt per litre. By calculation that is about 240 mg/L magnesium, more than the 140 mg/L that caused damage in perlite as a standing feed. The drench comes from advisory practice and has not been tested on cannabis as a standing value. Fertiliser labels usually list magnesium as MgO, and that value times 0.60 gives the magnesium.

The full calculation including tap water is in the article on calculating a nutrient solution.
Read the salt form from the data sheet first.
Multiply the MgO value on the label by 0.60.
Do not mix Epsom salt and calcium fertiliser in the same concentrate.
Epsom salt also supplies sulphur. That helps if sulphur is short as well.

What do I do about magnesium deficiency?

Look for antagonists first, then give magnesium. A lot of potassium, calcium or sodium in the root zone suppresses uptake. In flowering, when demand is high, soft water brings too little magnesium. In soil, magnesium is well available between pH 6.5 and 8.5.

Phosphorus deficiency can also acidify the root zone. The authors of a phosphorus trial see this as one route to magnesium and calcium deficiency.

Check fertiliser and tap water for high potassium, calcium and sodium.
Measure the pH in the runoff. That is the water draining out of the bottom of the pot.
Water once with 2.4 g Epsom salt per litre, letting 5 to 10 % run off.
For prevention the NC State group finds 1.2 g per litre once a month enough.
Watch the new growth. Strongly yellowed leaves usually do not turn green again, while new leaves stay green if it works.
Once new growth stays green, give no further shock doses.