Ammonium Sulfate as a Nitrogen and Sulfur Fertilizer

Most fertilizers are sold on one nutrient. Ammonium sulfate is sold on two, and they do not behave the same way in the soil: the nitrogen sits on the exchange complex and converts slowly, while the sulfur is already in the form roots absorb but travels with water. Getting that difference straight is what makes it possible to position the product properly, and to explain to a customer why it is worth buying rather than simply adding more urea.

Nitrogen in the ammonium form

The nitrogen in ammonium sulfate is ammonium nitrogen (NH4+), not nitrate and not urea, and most of what follows from that is about how long the nitrogen stays where it was put. Urea applied to the soil surface and left unincorporated can hydrolyse and lose nitrogen to the atmosphere as ammonia, whereas ammonium sulfate generally carries a lower ammonia volatilisation risk than surface-applied urea under many soil conditions. On pastures, no-till ground and top-dressed situations where incorporation is impractical, the advantage shows up as more nitrogen staying in the system. The product is not exempt from volatilisation, particularly when surface-applied to alkaline or calcareous soils, so incorporation, rainfall or irrigation still help move the fertilizer into the soil and reduce exposure to surface conditions.

The ammonium ion carries a positive charge and soil clay and organic matter carry negative charges, which is why ammonium is held on the exchange complex rather than immediately leached, unlike nitrate, which is mobile and moves with water. Soil bacteria nitrify ammonium to nitrate over time, a normal process and the reason ammonium nitrogen still eventually becomes leachable. The release is gradual rather than immediate, so the supply spreads over a longer period than a straight nitrate application would give, and the acidification that comes with nitrification is dealt with below.

Sulfur in the sulfate form

The sulfur in ammonium sulfate is sulfate (SO4²⁻), the form plant roots absorb. That is the substantive difference from elemental sulfur products, which must be oxidised by soil microorganisms before the sulfur becomes plant-available. Elemental sulfur is therefore slower-acting, with effectiveness that depends on soil temperature, moisture and microbial activity, while sulfate sulfur requires no transformation step at all.

Sulfate is mobile, though. Like nitrate it moves with soil water and can be leached below the root zone by heavy rain or over-irrigation, so applying it long before the crop needs it raises the chance of loss, which is why spring applications are usual in many systems. Where the crop has a long demand period and the risk of leaching is high, supplying sulfur in more than one application can be the better choice.

Soil acidification

Repeated ammonium sulfate applications can contribute to soil acidification, because hydrogen ions are released during nitrification and pH can fall over time. The actual effect depends on soil properties, application rate and management practices, which is one reason the product is not the default nitrogen source on every soil type. On acid soils, continued use requires lime management to prevent pH dropping to the point where it constrains yield.

On alkaline, calcareous soils the same reaction works in the other direction. As pH rises, iron, manganese, zinc and phosphorus become less available, and the acidifying effect can improve micronutrient availability in the immediate root zone. Growers on high-pH soils sometimes choose ammonium sulfate partly for that reason rather than purely for the nutrition.

Sulfur response

Sulfur deficiency has become more widespread in many regions, for reasons that have accumulated over decades. Cleaner industrial emissions mean less sulfur arrives incidentally in rainfall, the shift to higher-analysis fertilizers removed much of the sulfur that was once supplied as a by-product, and higher yields remove more sulfur from the field each season. Crops with a high sulfur requirement, and where deficiency most often shows up, include:

• Oilseeds, particularly canola and rapeseed, which have a high sulfur requirement for protein and oil formation
• Cereals, where sulfur affects protein quality as well as yield
• Forages and pasture, where sulfur influences both yield and nitrogen-use efficiency
• Brassica vegetables and other high-demand horticultural crops

Where a grower is applying nitrogen heavily and not seeing the expected response, sulfur limitation is one of the possibilities worth checking. Nitrogen-use efficiency falls when sulfur is short, because the crop cannot build protein without it. The symptom is easy to read as a nitrogen problem instead, which is how sulfur deficiency gets missed.

Application positioning

Because the two nutrients behave differently, application practice is a compromise between them. Nitrogen benefits from being applied when the crop’s demand is high and the risk of loss is managed, which for ammonium nitrogen is more forgiving than for nitrate or surface-applied urea, while sulfur as sulfate benefits from going on close to the demand period because it is mobile and leachable. Where the two align, in a spring application to a sulfur-responsive crop, ammonium sulfate fits naturally; where they do not, growers often manage the sulfur separately.

Buying considerations

Nothing here changes the specification. It changes which customers the product suits, which is useful when you are selling it. Distributors with oilseed, cereal and forage growers on alkaline soils are the natural audience, blenders need the granular form so the sulfur arrives in the field at the intended rate, and organic-matter-rich or high-rainfall regions need attention to sulfate leaching, which argues for timing rather than for a different product.

Next steps

• Ammonium Sulfate in Agriculture
• Ammonium Sulfate 21-0-0-24S — specifications and supply
• What Is Ammonium Sulfate Fertilizer?

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