Sulfur in Fertiliser: Why the % Mass of Sulfate?

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Discussion Overview

The discussion revolves around the reasoning behind fertilizer producers specifying "the percentage by mass of sulfur present as sulfate" in their products. It touches on the forms of sulfur in fertilizers and the implications for plant uptake.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant notes that sulfur in fertilizers is typically present as sulfates, suggesting that this is due to the insolubility of elemental sulfur in water and the instability of sulfites.
  • Another participant proposes that while sulfur may be present as sulfate, it could also be in the form of zero valent sulfur or sulfur-containing polymers, especially in time-released fertilizers, and that the assay simplifies the representation by assuming all sulfur is converted to sulfate.
  • A participant compares the representation of sulfur to that of phosphorus, which is listed as P2O5, indicating that this may be a convenient metric despite complicating matters for chemists.
  • Another participant adds that, similar to phosphorus, all P is converted to orthophosphate and expressed as P2O5, suggesting a complexity in how nutrient forms are represented.

Areas of Agreement / Disagreement

Participants express differing views on the forms of sulfur present in fertilizers and the rationale behind the percentage by mass representation. No consensus is reached regarding the implications of these representations.

Contextual Notes

The discussion does not resolve the assumptions regarding the forms of sulfur in fertilizers or the implications for plant uptake. There is also a lack of clarity on how these representations affect practical applications.

Who May Find This Useful

Individuals interested in agricultural science, fertilizer chemistry, and plant nutrition may find this discussion relevant.

Stroodle
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Hi,

I'd just like to know why fertiliser producers refer to "the percentage by mass of sulfur present as sulfate" in their fertilisers?

Thanks!
 
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Because sulfur in fertilizers is typically present in the form of sulfates ... I'll guess that's because elemental sulfur is not soluble in water, and sulfites are too unstable.
 
It's possible that it is indeed present as sulfate but if it is time-released, it is usually present as zero valent sulfur or as a sulfur-containing polymer and perhaps both. Rather than speciate all the forms of sulfur present, the assay is given as if all were converted to sulfate.
 
Phosphorous is listed as P2O5 though and clearly this is not the active form taken up by plants. I guess it's just a convenient metric, although for the chemist it makes things more complicated.
 
In the case as phosphorous, all of the P is converted to orthophosphate and the result is multiplied by a factor to express it as P2O5. A little more complicated but not by much.
 

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