Both are ferrous sulphate. One carries a single molecule of water, the other seven, and that alone changes the iron in a bag by roughly half.
The short answer. Ferrous sulphate monohydrate is published at 30% Fe. Ferrous sulphate heptahydrate is published at 19-20% Fe. A 50 kg bag of the monohydrate therefore carries about 15 kg of iron against about 9.75 kg for the heptahydrate. Compare them on cost per kilogram of iron, not per kilogram of bag.
Ferrous sulphate crystallises with water locked into the crystal. The heptahydrate holds seven molecules of it, the monohydrate one. That water is real weight in the bag and it is paid for by the kilogram, but it is not iron.
The heptahydrate is the familiar green crystal; the monohydrate is the drier, more concentrated material. Because the two look and behave differently as well as assaying differently, a quotation for ‘ferrous sulphate’ with no grade against it cannot be compared with anything, and should be sent back for a number.
| Product | Formula | Grade published | Standard packing |
|---|---|---|---|
| Ferrous Sulphate Monohydrate | FeSO₄·H₂O | 30% Fe | 50 kg net, HDPE bag |
| Ferrous Sulphate Heptahydrate | FeSO₄·7H₂O | 19-20% Fe | 50 kg net, HDPE bag |
Every grade in this table is the figure published on that product’s own page on this site. Nothing is estimated. The guaranteed analysis on the product page and the batch certificate of analysis govern any supply.
At the published grades, a 50 kg bag of the monohydrate carries about 15 kg of iron and a 50 kg bag of the heptahydrate about 9.75 kg. To land one tonne of iron you would need roughly 67 bags of monohydrate or roughly 103 bags of heptahydrate. Those are arithmetic on the published grades and the standard packing, and they are indicative.
Where two products carry the same element at different strengths, the price per kilogram of bag is not a comparison. Divide the delivered price per kilogram by the element fraction and you get the cost per kilogram of the thing you are actually buying. That is the only basis on which grades of different strength can be set against each other.
Our nutrient cost comparison calculator does that arithmetic on any two products in our range and prints its working, and the element content calculator converts between kilograms of product and kilograms of the element. Freight follows the same logic: moving a weaker grade means paying to move more material that is not the element.
That is decided by your process, and by things we cannot see from here: how the product is applied, what it is blended with, what has to dissolve and how fast, the specification you or your customer must meet, and how far the material travels. Any supplier who answers that question without asking about your process is guessing.
We manufacture the alternatives set out above, so we have no reason to steer you toward the dearer one. Describe the application and we will tell you what we would send and why, including when the cheaper grade is the right answer.
Four things let us price these products without a round of questions: the grade, the quantity, the delivery location, and any specification you have to meet. Send those and we come back with a price, the packing, and the documents that travel with the consignment. There is more detail on what to send us for a quote.
Know what you need already? Send the grade, the quantity and the delivery location and we will quote against them.
Send the grade, the quantity and the delivery location and we will quote against them.