Sulphate of Ammonia

(SOA) 21-0-0-24

Sulphate of ammonia is commonly used as a source of nitrogen and sulphur. Prior to the general acceptance of Urea in the 1970’s, Sulphate of ammonia was the main source of nitrogen.

It has good storage and handling qualities, and can be readily blended with other fertilisers. It has the desirable agronomic features of being a source of both nitrogen and sulphur.

Higher nitrogen cost can be one of the main factors limiting the use of SOA, however, if the value of the available sulphate sulphur is considered it can be good value.

3. PHYSICAL PROPERTIES

4. MANUFACTURE

SOA is often manufactured by the direct mixing of sulphuric acid and ammonia, or as a by product of refinery or caprolactum manufacture.

5. USES

SOA is a valuable fertiliser as a straight product or in blends.

Sulphur in SOA is sulphate sulphur which is the form taken up directly by plants. Sulphate sulphur is readily soluble in water and goes straight into solution when applied to the soil (providing there is moisture).

SOA releases nitrogen to the soil directly as ammonium NH4+, which is converted by bacteria to nitrate nitrogen. This process is favoured by conditions conducive to microbial activity, such as warm temperatures, moisture and organic matter. The rate of this reaction depends on those conditions and complete nitrification of applied ammonium can vary from a few days to several weeks.

Apart from leaching losses, the other source of loss is volatilisation from the soil surface. These losses vary with the prevailing conditions.

Losses will increase under the following conditions:

•          High temperatures, >25° C
•           Dry soil conditions and low humidity
•          Higher soil pH > pH 7.0
•          Soils with CaCO₃ at the surface or recently limed soils
•          Coarse textured soils with low cation exchange capacity e.g. sands

Losses are negligible if the SOA is incorporated with the soil or is washed into the soil by rain (2-3mm is sufficient) or is irrigated within 2-5 days of application.
Volatilisation losses of ammonium from SOA can be comparable to losses from Urea in equivalent conditions.

6. STORAGE AND HANDLING

As with all nitrogen fertilisers, SOA is hygroscopic and so susceptible to absorbing moisture from the atmosphere. SOA has a higher critical humidity (79.2%) (the humidity level at which it begins to absorb moisture) than Urea (75.2%).

Although it has lower moisture uptake qualities than Urea, care must be taken to minimise exposure to moisture. When stored in a bulk shed, ensure all adverse weather is excluded and preferably keep covered with plastic. When used in the field, all efforts must be made to avoid exposure to moist air. Keep trucks tarped, spreaders covered, empty drills, airseeders and spreaders overnight or between jobs.

Of the nitrogen fertilisers, SOA has the most acidifying effect on the soil.
Generally for broad acre crops and pastures with moderate rates (less than 50kg of SOA per hectare) and on loam or clay soils, the overall contribution to acidification is minimal.