Magnesium nitrate is a soluble salt of magnesium and nitrate, sold as the hexahydrate Mg(NO3)2·6H2O. It contains about 9.5% Mg and 10.9% N, all as nitrate. It carries no sulfate, so it does not precipitate with calcium.
In practice
It is used to add magnesium where sulfate is already high, or to build a one-part or Cal-Mag concentrate where calcium and magnesium must share a bottle. It also adds nitrogen, so it is less suitable than magnesium sulfate when tapering N in late flower. Liquid Cal-Mag supplements are often calcium nitrate plus magnesium nitrate.
Units and formula
1 g/L gives about 95 mg/L (ppm) Mg and 109 mg/L N.
Don't confuse with
- Magnesium sulfate (Epsom salt): Mg with sulfur, no nitrogen, must stay apart from concentrated calcium.
- Calcium nitrate: the calcium counterpart.
- Cal-Mag: a finished product that may contain magnesium nitrate.
Nitrate (NO3-) is the main form of nitrogen in hydroponic and soilless feeds; its uptake tends to push root-zone pH upward.Sulfate (SO4 2-) is the form in which plants take up sulfur, delivered mainly by magnesium sulfate and potassium sulfate in mineral feeds.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.Magnesium sulfate heptahydrate, sold as Epsom salt, is the common hydro magnesium source, supplying about 9.9% magnesium and 13% sulfur.Calcium nitrate is the standard hydroponic calcium source, supplying about 19% calcium and 15.5% nitrogen, mostly as nitrate.Sulfur (S) is a secondary nutrient taken up as sulfate, used in the amino acids cysteine and methionine; deficiency yellows new growth fairly evenly.Magnesium (Mg) is a mobile secondary nutrient at the centre of every chlorophyll molecule; deficiency causes interveinal yellowing on older leaves.An A/B nutrient is a mineral feed split into two concentrates so that calcium (part A) is kept apart from sulfate and phosphate (part B), which would precipitate together.
