Magnesium is a secondary macronutrient absorbed as Mg2+. It sits at the centre of the chlorophyll molecule, activates the enzymes that fix carbon and handle ATP, and helps load sugars into the phloem. It is mobile, so the plant strips it from older leaves when supply runs short.
In practice
Classic deficiency is interveinal chlorosis on lower and middle leaves: green veins with yellow tissue between, later turning brown. Hydro feeds usually run about 40-70 mg/L (ppm) Mg, most often from magnesium sulfate. High potassium, calcium or ammonium competes with Mg at the root, so a deficiency can appear even with Mg in the tank. Morad and Bernstein found cannabis responds to Mg supply across a range before plateauing.
Don't confuse with
- Iron deficiency: also interveinal, but on new growth.
- Potassium deficiency: starts at leaf margins rather than between the veins.
- Epsom salt: a common Mg source, not a synonym for magnesium.
Myth check
LEDs do not create a magnesium need (Evidence: house position, Myth). Mg demand is driven by cultivar, RO water and coco cation exchange. See Cal-Mag and Mythbusters.
The green pigment in chloroplasts that absorbs mainly blue and red light to drive photosynthesis; chlorophyll a and b are the main forms in plants.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.Magnesium sulfate heptahydrate, sold as Epsom salt, is the common hydro magnesium source, supplying about 9.9% magnesium and 13% sulfur.Iron (Fe) is an immobile micronutrient needed to make chlorophyll and run electron transport; deficiency yellows new leaves between green veins.Potassium (K) is a mobile macronutrient that controls water balance, stomata and enzyme activity; deficiency scorches the edges of older leaves.An LED (light-emitting diode) grow light uses solid-state diodes to produce light; modern fixtures are the most efficient grow lights, at roughly 2.5–3 µmol/J.RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.Cation exchange capacity (CEC) is a medium's ability to hold positively charged nutrients such as calcium, magnesium and potassium on its surfaces and release them later.Secondary nutrients are calcium, magnesium and sulfur: macronutrients needed in substantial amounts but not included in the N-P-K numbers on a label.A mobile nutrient is one the plant can move from older leaves to new growth, so its deficiency shows first on lower, older leaves: N, P, K, Mg, Cl, Mo.Cal-Mag is a supplement supplying calcium and magnesium, usually as nitrates; need depends on cultivar, water source (RO vs tap) and medium (coco), not on LED lighting.Nutrient antagonism is when an excess of one element reduces the uptake or use of another, such as high potassium suppressing magnesium and calcium.
