Magnesium Nitrate — Frequently Asked Questions

By where it appears. Magnesium is mobile within the plant, so it is withdrawn from older tissue to supply new growth, and the symptom, interveinal chlorosis with veins staying green, shows first on the lower and older leaves while the growing tip looks healthy. Iron and manganese deficiency produce similar chlorosis but on the youngest leaves, because those nutrients are immobile.

Magnesium sulphate is cheaper but the sulphate can precipitate with calcium in a fertigation tank, and in soils it adds sulphur that may not be wanted. The nitrate form avoids both, delivers nitrogen alongside the magnesium, and supports cation uptake at the root. Where those issues do not arise, magnesium sulphate is the more economic source.

A magnesium atom sits at the centre of every chlorophyll molecule, so a plant short of magnesium cannot build functional chlorophyll and photosynthesis is directly limited. It is also a cofactor for many enzymes involved in energy transfer. That central structural role is why deficiency shows so visibly and so quickly as loss of green colour.

As a dehydrating agent in nitric acid concentration, where its strong affinity for water is used to strip water from the acid, and as a catalyst carrier and in pyrotechnics. It is strongly hygroscopic, which is the same property that makes it useful for dehydration and a handling constraint in storage.