# Sulfur in fertiliser

## Main Question or Discussion Point

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!

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.

chemisttree
Homework Helper
Gold Member
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.

chemisttree