Zinc Sulphate Monohydrate vs Heptahydrate: Which Grade Should You Buy?

The only real difference: water of crystallisation

Zinc sulphate monohydrate (ZnSO4·H2O) and heptahydrate (ZnSO4·7H2O) are chemically the same compound — zinc sulphate — carrying one water molecule versus seven in the crystal lattice. Because that extra water adds mass but no zinc, the heptahydrate is diluted on a weight basis. The monohydrate has a molecular weight of about 179.5 g/mol and the heptahydrate about 287.5 g/mol, which is why the same kilogram of product delivers very different amounts of actual zinc.

In practice the monohydrate runs around 33–35% Zn and the heptahydrate around 21–22% Zn (often quoted as a 21% guaranteed minimum; the theoretical maximum is 22.7%). These are typical figures and should be confirmed against the batch COA — do not buy or formulate against a single fixed number without it.

Cost per kilogram of zinc, not cost per tonne of product

This is the number that decides most procurement calls. Zinc sulphate is priced per tonne of product, but what you are really buying is the zinc inside it. A tonne of monohydrate carries roughly 330–350 kg of zinc; a tonne of heptahydrate carries roughly 210 kg. So one tonne of monohydrate delivers about 1.6 times the zinc of one tonne of heptahydrate.

To compare two offers fairly, divide the delivered price per tonne by the zinc fraction:

• Monohydrate at 34% Zn: price ÷ 0.34 = cost per kg Zn
• Heptahydrate at 21% Zn: price ÷ 0.21 = cost per kg Zn

Use the divisor that matches the batch COA assay, not the guaranteed minimum: heptahydrate is often specced at 21.5–22% Zn, so dividing by 0.215–0.22 can be the fairer comparison. The monohydrate almost always wins on cost per kg of zinc once freight is included, because you are not paying to ship and store six extra molecules of water. For long-haul export especially, the freight-per-kg-zinc gap is the single biggest argument for the monohydrate grade.

Solubility and where each grade is actually used

Both grades are highly water-soluble, so for tank mixing and fertigation either one will dissolve readily — the heptahydrate dissolves to roughly 55 g per 100 mL of water at 20°C, and the monohydrate is comparable in practice. Water solubility matters because plant roots take up only zinc that is dissolved, and on neutral-to-alkaline soils a highly soluble source is what corrects a deficiency.

Where they diverge is the application format:

• Monohydrate is the workhorse for dry handling: granular grades blend into NPK bulk blends and are broadcast or banded, and the powder is used where a concentrated, low-moisture zinc source is wanted. The higher zinc loading means fewer kilograms of product per hectare to hit the same zinc rate.
• Heptahydrate, in fine white crystals, is the traditional pick for foliar sprays and solution feeding, where rapid, clean dissolution and a familiar product form matter more than zinc concentration per bag. Foliar zinc sulphate is taken up quickly by the leaf, though it has limited movement within the leaf, so spray coverage matters.

Note that neither grade is automatically "better" agronomically — the right choice is driven by your application method, soil pH, and logistics, not by the hydrate alone.

Handling, storage and blending

Granular monohydrate is the friendlier grade for mechanised handling and bulk blending: a matched granule size flows well, broadcasts evenly, and resists segregation in an NPK blend. The watch-out with any concentrated zinc source is sparse distribution — because so little zinc is needed relative to the macronutrients, very high-analysis granules can spread too thinly across a field, so granule count and blend uniformity should be checked.

Both salts are hygroscopic and should be kept sealed and dry; powders in particular can cake if exposed to humidity. For containers crossing several climate zones, the lower water content of the monohydrate is one less variable to manage.

A simple decision rule

• Buying zinc to put into dry blends or to apply to soil, or shipping long distances where freight per kg of zinc dominates: choose monohydrate (granular for blends, powder for concentrated soluble use).
• Buying zinc for foliar programmes or fertigation where the market or spec calls for the classic crystalline heptahydrate: choose heptahydrate.
• Either way, compare suppliers on cost per kg of zinc, delivered — and always against the batch COA, not a catalogue number.