Ferric/Ferrous/Iron sulphate - Ask Extension
I've got garden pH of 7.3. I think I'd like to reduce that with part elemental S which is slower acting and part Ferric sulphate which is faster acti...
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Ferric/Ferrous/Iron sulphate #924919
Asked February 22, 2026, 9:43 PM EST
I've got garden pH of 7.3. I think I'd like to reduce that with part elemental S which is slower acting and part Ferric sulphate which is faster acting. (N addition would be ammonium sulphate to help acidify.)
Ferric Sulphate, ferrous sulphate, iron sulphate, etc.. Are there equivalences between these? Would the equivalence be based on the mass of SO4 to the total weight? Can I pretty much ignore the hydrate components (being so much lighter)? is the sulphate availability the same for all?
Inquiring Minds Need to Know!
Thanks.
Charles County Maryland
Expert Response
Hi, elemental sulfur ("wetting", "dustable") is typically 90% sulfur. Some garden sulfur products are mostly calcium sulfate and contain 30-40% S (by weight).
Ferric and ferrous sulfate differ by oxidation number. We don't know if one is more soluble and available than the other. Iron sulfate for ag and gardening use ranges from 6-11% S.
To reduce the soil pH from 7.3 to 6.5, incorporate the following into the top 6 inches of soil per 100 sq. ft.:
1.5 lbs. elemental sulfur or 10 lbs. of iron sulfate (11% S)
Note: nutrient availability at 7.3 soil pH is not significantly reduced when organic matter is >12%. If your plants are growing and producing well, there is no urgent ned to lower soil pH as long as it is not increasing. Also, soil test labs measure soil pH and nutrient levels more accurately than diy kits.
Jon
Ferric and ferrous sulfate differ by oxidation number. We don't know if one is more soluble and available than the other. Iron sulfate for ag and gardening use ranges from 6-11% S.
To reduce the soil pH from 7.3 to 6.5, incorporate the following into the top 6 inches of soil per 100 sq. ft.:
1.5 lbs. elemental sulfur or 10 lbs. of iron sulfate (11% S)
Note: nutrient availability at 7.3 soil pH is not significantly reduced when organic matter is >12%. If your plants are growing and producing well, there is no urgent ned to lower soil pH as long as it is not increasing. Also, soil test labs measure soil pH and nutrient levels more accurately than diy kits.
Jon
Thank you, Jon.
My garden has several problems. To be honest, it's probably predominantly mid-summer neglect after the thrill of seed selecting and starting has passed, and things get boring and dry. But I've never done substantial N fertilizing and the pH supposedly affects N uptake. (I visited Poland last year and as a result proclaimed loudly that this year I WILL raise extraordinary beets and cabbages. So it's time to put my money where my mouth is.)
Anyhoo, the haven't much grown and produced well; I've done substantial organic matter addition each year but the soil is still pretty heavy clay. No idea how to measure organic matter content, and it varies, it's pretty heterogenous. pH is just another dart to throw at the target.
I would have gonge UMD for my soil testing but they/y'all waved me off to UDel. I so really like your response time!! I hope y'all do testing and keep up the good work. Thank you! ___________Marty
From: ask=<personal data hidden> on behalf of Ask Extension
Sent: Monday, February 23, 2026 9:54 AM
To: The Carts Family
Subject: Re: Ferric/Ferrous/Iron sulphate (#0194835)
Sent: Monday, February 23, 2026 9:54 AM
To: The Carts Family
Subject: Re: Ferric/Ferrous/Iron sulphate (#0194835)
Hi, unfortunately, UMD closed its soil testing lab many years ago. UDEL is a great option. They test lead (Pb) as part of the basic soil test, along with organic matter levels.
We suggest continuing to add compost to improve soil health and fertility. Complete fertilizers higher in nitrogen than in phosphorus or potassium would be a good choice for fertilizing vegetables.
https://extension.umd.edu/resource/fertilizing-vegetables/
Jon
We suggest continuing to add compost to improve soil health and fertility. Complete fertilizers higher in nitrogen than in phosphorus or potassium would be a good choice for fertilizing vegetables.
https://extension.umd.edu/resource/fertilizing-vegetables/
Jon
