Nitrogen sources are often treated as interchangeable units of a single nutrient, differing only in price per unit of N. Agronomically they are not interchangeable, and the differences are large enough to reverse a ranking based on price alone.
The candidates
The realistic alternatives to ammonium sulphate are urea (46 % N), UAN solution (typically around 28–32 % N, a mix of urea and ammonium nitrate in water), ammonium nitrate (about 34 % N), and calcium ammonium nitrate, which is ammonium nitrate blended with a carbonate carrier. Each brings a different combination of nitrogen form, secondary nutrients, soil reaction and loss pathway.
The four axes that actually separate them
1. Volatilisation risk. Urea hydrolyses in soil to ammonium carbonate, raising pH sharply at the granule surface and creating conditions for ammonia loss to the atmosphere. Losses from surface-applied urea can be substantial in warm, moist, windy conditions on high-pH or high-residue surfaces, and are the main reason urease inhibitors exist. Ammonium sulphate carries no equivalent mechanism on neutral and acid soils. The exception is important: on free-lime calcareous soils, surface-applied ammonium sulphate can also volatilise, through reaction with calcium carbonate. Incorporation resolves the issue for both.
2. Soil acidification. Per unit of nitrogen applied, ammonium sulphate is the most acidifying common source, urea and ammonium nitrate are intermediate, and calcium ammonium nitrate is the least because its carbonate carrier partially offsets the effect. On acid soils this argues against ammonium sulphate unless the lime programme accounts for it; on calcareous soils it argues for it.
3. Sulphur. Ammonium sulphate supplies about 24 % S. Urea, UAN and ammonium nitrate supply none. Where the crop needs sulphur, the comparison must be against a source plus a separate sulphur product, and often plus an extra pass.
4. Nitrogen form and timing. Nitrate-containing sources deliver immediately available nitrate that plants can take up at once but which leaches readily. Ammonium sources hold nitrogen on exchange sites until nitrification occurs, which is slower in cold soil. In cold early-season conditions, a nitrate-bearing source acts faster; in warm conditions the difference largely disappears.
Situations that favour ammonium sulphate
- Calcareous or alkaline soils, where mild acidification helps phosphorus and micronutrient availability
- Sulphur-responsive crops: oilseed rape and other brassicas, onions and garlic, forage grasses, tea, sugarcane
- Surface application on non-calcareous soils where volatilisation from urea is a live concern
- Blends where a low-analysis, sulphur-bearing component is wanted
- Markets close to a source of by-product supply, where delivered cost is favourable
- Regulatory environments that restrict ammonium nitrate handling
Situations that favour alternatives
- Acid soils where further acidification is unwelcome
- High-rate nitrogen programmes over long freight distances, where nutrient density dominates
- Situations with no sulphur requirement
- Cold-soil early applications where nitrate availability matters
- Fluid fertiliser systems, where UAN’s handling advantages are decisive
How substitution actually behaves
Because the differences above are real, substitution between nitrogen sources is partial and lagged rather than instantaneous. A price signal alone rarely moves a grower who is buying ammonium sulphate for its sulphur or for its behaviour on a calcareous soil. Conversely, growers buying it purely as cheap nitrogen switch readily when the delivered cost per unit of N moves against it. The market therefore behaves as if it has two layers: a structural core of agronomically committed demand that responds little to relative pricing, and a swing layer of price-sensitive nitrogen demand that responds quickly.
Distinguishing those two layers is more useful than any single substitution ratio. The honest way to make the comparison is a delivered cost per unit of nutrient at the field, counting both nutrients, the number of passes, expected losses under local conditions, and the multi-year lime implication.