Vital UreaSulfur Coated Urea — N 32% min · S 13% min

The sulfur shell is not a wrapper. It is a barrier with a deliberate failure rate. Water finds its way in through pores and micro-cracks in the sulfur, dissolves the urea inside, and the solution diffuses back out through the same paths. Nothing releases until water gets in.

That is what produces the 7–18 day release profile on this page. Three things move it inside that range: soil temperature, because diffusion and the microbes both run faster when it is hot; moisture, because a dry seedbed releases nothing; and how the bag is handled, because a granule crushed at spreading has no barrier left and behaves like plain urea from the moment it hits the ground.

The mechanism is entirely this: nitrogen still inside a granule is not in the soil solution. Volatilisation, nitrification and leaching all act on nitrogen that is dissolved. What has not been released yet cannot be lost yet. That is the whole of the picture above — not less nitrogen, the same nitrogen, arriving while there is a root ready to take it.

A 25 kg bag carries at least 8.0 kg of nitrogen and 3.25 kg of sulfur — 32% N and 13% S are registered minimums, so that is the floor and not the content. Read as a sum, that is a nitrogen fertilizer with some sulfur in it. The next two sections are why it is not.

And 3.25 kg is the nutrient, not the material. The sulfur VAN charges to the coater is 80% pure, so putting 3.25 kg of elemental sulfur into the bag means putting in about 4.06 kg of sulfur material — 3.25 ÷ 0.80, and a sixth of a 25 kg bag by weight. A coating that is a sixth of the pack is not packaging.

Sulfur is not plant food until the soil oxidises it to sulfate — and that reaction happens only on the surface.

Same element, same mass, three geometries. Rapid oxidation calls for particles finer than about 20 µm, well dispersed (Bremer, 2022) — which is exactly what a shell micronised onto a granule is. The reaction is biological, so it roughly triples to quadruples for every 10 °C of soil warming (Janzen and Bettany, 1987): the heat that destroys surface urea is the heat that runs this.

And nitrogen that arrives without sulfur does not become yield.

Both are built into protein. Where sulfur is short, nitrogen the plant has already absorbed accumulates as nitrate and soluble amines, and nitrogen use efficiency falls with it (Grzebisz et al., 2022; Elbasyoni et al., 2026). Wheat builds protein at about fifteen parts nitrogen to one part sulfur, and above seventeen to one yield falls away — so each kilogram of sulfur the crop is short of is roughly 10–15 kg of nitrogen that cannot be finished into protein (Zapałowska et al., 2026; Wan et al., 2012). It is a cereal ratio: oilseed rape and the brassicas run far lower. The protons from that reaction also land exactly where the nitrogen is — dissolving phosphorus held as calcium phosphate and freeing zinc, iron and manganese held by alkalinity (Khoshru et al., 2023). That is the nutrient the ground already contains.

Everything above needs the nitrogen and the sulfur in the same place at the same moment. A mixture cannot promise that.

Two materials in one bag separate by size and by density, and the wider the size spread across the components, the more severe it gets (Antille et al., 2013). A coated granule cannot separate from itself. The shell is also what creates the surface in the first figure: the same sulfur as a lump would still be sitting there next season.