Canna - MgO 7% Magnesium

A single-element magnesium fertiliser with 7% magnesium oxide and 14% sulphur trioxide, based on magnesium sulphate. Fed through the water or applied to the leaf, at 1 to 2 ml per litre.

It is a sulphate, and that explains the warning

Beside the 7% MgO the label also states 14% SO3, and notes that it contains magnesium sulphate. That second figure is not incidental: there is twice as much sulphate in it as magnesium.

That is where this bottle's mixing warning comes from. Bring it together undiluted with a calcium-bearing fertiliser and the sulphate meets the calcium to form calcium sulphate. That is gypsum, and gypsum barely dissolves in water. It drops out as a white sludge you will not get back into solution.

Which is exactly why this and the calcium bottle in the range are the pair that must never share a measuring jug. Anyone buying the two separately to build their own Cal-Mag is also buying the obligation to keep them apart until both are dissolved in plenty of water.

The sulphur is not ballast, incidentally. Sulphur is itself a nutrient the plant needs for building proteins, so you are adding something useful with it.

A deficiency shows from the bottom up, and that is useful

Magnesium is the central atom in chlorophyll. No magnesium, no chlorophyll; no chlorophyll, no photosynthesis.

What makes it diagnostically useful is that magnesium is relocated within the plant. Run short and the plant pulls it out of old leaf and sends it to the young growing points where it pays back more. The result is a deficiency that starts at the bottom: lower leaves yellowing between the veins while the veins themselves stay green and the top looks fine.

That is the mirror image of calcium, which is not relocated and whose symptoms appear at the top. Yellowing low down with green veins points to magnesium. Trouble in the new growth points to calcium.

As a foliar spray it works faster

This is the only product in the range for which CANNA mention foliar application. Mix 1 ml per litre and spray the leaf.

Going through the leaf bypasses the root and the substrate chemistry, which helps when a shortage is not about quantity but about uptake, for instance at too high a pH or where an excess of calcium or potassium is crowding magnesium out.

One warning CANNA add themselves and worth taking seriously: do not spray in strong sun or under bright light. Droplets on the leaf act as lenses, and combined with a salt solution that causes scorch. Spray at the end of the day, just before the lights go off.

Technical Specifications:

• Type: single-element mineral fertiliser, liquid
• Manufacturer: CANNA
• Content: 7.0% magnesium oxide (MgO), water soluble
• Sulphur trioxide (SO3): 14%
• Contains: magnesium sulphate
• Dose: 1 to 2 ml per litre of water
• Foliar: 1 ml per litre, not in strong sun or under bright light
• Mixing: never mix in concentrated form with calcium-bearing fertilisers
• Volume: 1 litre
• Note: an excess of magnesium can lead to calcium deficiency

Frequently asked questions

Why can this not go in with calcium?
Because it is a sulphate. Undiluted, sulphate and calcium form calcium sulphate, gypsum, which barely dissolves and drops out.

Why does it state 14% SO3?
Because magnesium sulphate supplies sulphur as well as magnesium. Sulphur is itself a nutrient, used by the plant for building proteins.

Where do I see a magnesium deficiency?
Low down. The plant pulls magnesium out of old leaf and sends it upward, so lower leaves yellow between the veins while the veins stay green.

When should I spray it on the leaf?
When uptake is the problem rather than quantity. Do it at the end of the day and never in strong sun or under bright light.

Can I give too much?
Yes. An excess of magnesium crowds calcium out at the root and can cause a calcium deficiency.