The short answer: Polyhalite tends to become more compelling when the program needs sulphur plus potassium, magnesium or calcium, when seed-zone salt load matters, or when a more gradual multi-nutrient supply has value. Ammonium sulphate remains a strong choice when the primary objective is immediately available nitrogen and sulphate-S and placement risk is manageable.
Start with the nutrient job, not the product name
AMS is 21-0-0-24S and supplies immediately soluble ammonium-N and sulphate-S. Polyhalite is a naturally occurring mineral supplying sulphur, potassium, magnesium and calcium. Both can contribute sulphate-S, but the rest of the nutrient package is fundamentally different.
So the first question is simple: does the field need only N + S, or does the program also need K, Mg or Ca? If the additional nutrients have agronomic value, comparing only dollars per pound of sulphur understates what the polyhalite application is doing.
When AMS has the advantage
AMS is useful when a crop needs rapidly available sulphate-S and the ammonium-N is also valuable. Alberta Agriculture notes that ammonium sulphate is rapidly available to crops and can be effective when banded or used to correct sulphur deficiency where moisture can move sulphate into the root zone.
If the field is not deficient in K, Mg or Ca and fertilizer can be safely separated from sensitive seed, AMS may be the simplest and most economical way to deliver N + S.
When polyhalite can earn its place
Polyhalite supplies four nutrients from one mineral source. Its value proposition is strongest when a grower needs sulphate-S while also replacing K, Mg or Ca, wants a lower-salt multi-nutrient source near the seed, or prefers a more gradual dissolution pattern rather than delivering all sulphate immediately.
A 2026 Agronomy Journal meta-analysis evaluated polyhalite across a large body of crop and soil trials and compared it with conventional nutrient sources including ammonium sulphate, SOP, gypsum and other fertilizers. As always, response depended on nutrient balance and local conditions.
Seed placement changes the economics
Canola is particularly sensitive to seed-placed ammonium sulphate. The Canola Council notes that salt effects and ammonia toxicity can reduce emergence, and safe rates depend on moisture, opener width, row spacing and seedbed utilization.
That means the cheapest sulphur source at the blender is not necessarily the cheapest if its placement forces a second band, limits rate, or reduces stand. In a seed-row system, product selection should include the value of stand establishment and equipment logistics, not only $/lb S.
Make the comparison on a field-by-field basis
Use a soil test and crop removal targets to price the nutrients the field actually needs. Credit the N in AMS. Credit K, Mg and Ca in polyhalite only where those nutrients have value. Then account for placement, seed safety and application logistics.
The winner can change from field to field. That is the point of fertility management: choose the source that solves the nutrient problem at the lowest whole-program cost.
Related reading: Fertility Is Not the Same Thing as Fertilizer.
Related products
- Polysulphate: polyhalite supplying S, K, Mg and Ca
- SUL4R-PLUS: calcium sulphate with low salt index
- PKpluS: multi-nutrient P/K/S/Ca/Mg fertility
