Ammonium Sulfate — Frequently Asked Questions

Because of what happens after application. Soil bacteria nitrify the ammonium to nitrate, and that process releases hydrogen ions. The sulfate anion remains behind rather than being taken up in equal measure, so the net effect over time is a measurable drop in soil pH. That is useful on calcareous ground and a problem on soils that are already acid.

Because atmospheric sulphur deposition has fallen sharply as industrial emissions have been controlled. Crops used to receive a significant part of their sulphur requirement free from the air, and that input has largely disappeared, so sulphur deficiency has become common. Ammonium sulfate supplies 24 percent sulphur alongside its nitrogen, which addresses both needs in one product.

Because the sulfate anion accounts for much of the molecular weight. At 21 percent nitrogen it is less concentrated than urea at 46 percent, so more tonnes must be spread and freighted per unit of nitrogen. That is the trade-off against the sulphur contribution and the acidifying effect, both of which may be the reason for choosing it.

It is precipitating a protein by raising the ionic strength of the solution until the protein can no longer stay hydrated. Ammonium sulfate is used for this because it is extremely soluble, cheap and does not denature proteins, and because different proteins precipitate at different salt concentrations, which allows selective fractionation. It is a standard first step in protein purification.