Zinc sulphate is one of the most widely used zinc sources in agriculture and animal nutrition. It is sold in two common forms, monohydrate and heptahydrate, and buyers often ask which one they should order. The answer depends on how you will use it, how you will store it, and how you compare prices.
The difference is water
Both products are zinc sulphate (ZnSO4). The difference is how much water of crystallisation is bound into the crystal:
- Zinc sulphate monohydrate (CAS 7446-19-7): ZnSO4·H2O, one molecule of water.
- Zinc sulphate heptahydrate (CAS 7446-20-0): ZnSO4·7H2O, seven molecules of water.
Because heptahydrate carries more water, each kilogram contains less zinc.
Zinc and sulphur content at a glance
| Monohydrate | Heptahydrate | |
|---|---|---|
| Formula | ZnSO4·H2O | ZnSO4·7H2O |
| Zinc (theoretical) | about 36% | about 23% |
| Typical commercial zinc grade | 33% Zn | 21% Zn |
| Sulphur (theoretical) | about 18% | about 11% |
| Usual form | Free-flowing powder or granules | White crystals |
In India, the Fertiliser Control Order (FCO) sets minimum zinc content for fertiliser-grade zinc sulphate: 33% for monohydrate and 21% for heptahydrate. Always check the grade on the Certificate of Analysis (COA).
When monohydrate is the better choice
- More zinc per bag. Roughly 1.5 kg of monohydrate supplies the same zinc as 2.4 kg of heptahydrate, so you move and store less material.
- Blending. The dry, free-flowing powder or granule mixes well into granular fertilisers, NPK blends and micronutrient mixtures.
- Animal feed premixes. Its low moisture and higher zinc content make it the common choice for feed-grade zinc.
- Humid storage. It is less prone to caking than the crystalline heptahydrate.
When heptahydrate is the better choice
- Solutions and sprays. The crystals dissolve readily in water, which suits foliar sprays, fertigation and liquid formulations.
- Direct soil application where farmers are used to the 21% grade and its application rates.
- Industrial uses where the process is designed around the heptahydrate form.
Keep heptahydrate in sealed bags in a cool, dry place. In very dry, warm conditions it can lose some of its water, and in humid conditions it can absorb moisture and cake.
Compare price per kilogram of zinc, not per kilogram of product
A lower price per kilogram does not always mean cheaper zinc. Divide the price by the zinc content to compare grades fairly:
Cost per kg of zinc = price per kg ÷ (zinc % ÷ 100)
For example, a product priced at 100 per kg with 33% zinc costs about 303 per kg of zinc, while a product priced at 70 per kg with 21% zinc costs about 333 per kg of zinc. In this example the monohydrate is the better buy, even though its price per kg is higher. Freight and handling usually favour monohydrate too, because you transport less water.
What to specify when you ask for a quote
- Form: monohydrate or heptahydrate.
- Zinc content (for example 33% or 21%) and, if relevant, sulphur content.
- Physical form: powder, granules or crystals.
- Intended use: fertiliser, feed or industrial, plus any limits on heavy metals such as lead, cadmium and arsenic.
- Quantity, packaging (for example 25 kg or 50 kg bags) and delivery location.
- Documents you need: TDS, SDS and COA.
Application rates depend on soil and crop. Follow a soil test and the advice of your agronomist or feed nutritionist.
Need zinc sulphate?
We supply zinc sulphate monohydrate and heptahydrate from Ankleshwar, Gujarat, along with other micronutrients such as ferrous sulphate, magnesium sulphate and copper sulphate. Request a quote with your grade, quantity and destination, and our team will confirm availability and documents.
