Forms of sulfur
What are the pros and cons of elemental and sulfate sulfur as sowing fertiliser? Is either form more likely to lead to moisture being absorbed by the fertiliser, affecting it’s handling?
good to see comments appearing here!
Fossil-based sulfur deposits from salt domes were until recently the basis for commercial production in the United States, Russia, Turkmenistan, and Ukraine.
Sulfate is immediately available for crop uptake while elemental Sulfur (ES) requires oxidation to sulfate in the soil before it can be available to plants. This oxidation is carried out by a range of soil microbes whose activity is affected mainly by soil temperature and soil pH. Particle size of elemental sulfur also affects the rate of oxidation and release of sulfate – smaller particles oxidise faster and oxidation is faster in high pH soils, at higher temperatures and in higher organic matter soils.
The source of elemental sulfer is critical and typically supplied in cropping systems as sulfer bentonite pastilles (usually 90% elemental S) or in sulfur fortified ammonium phosphate compounds. The rate of supply of S from these products is predominantly related to S content in the granule, granule size, and size of ES particles in the product (the last factor is probably the least important of these three). For two of the co-granulated or compound products available on the Australian market S release is faster than for the pastilles but slower than finely powdered ES mixed through soil. There is at least one more compound that doesn’t have any published particle size data.
Some pastillated (granulated/prilled) forms of ES are very slow to oxidise (and slow to disperse) as they have inadequate added bentonite to cause the pastille to swell and disperse. Typically the pastille needs to be at least 10% bentonite for this to happen and even then it is no guarantee that the S will oxidise in time for that crop. The form of bentonite can also affect oxidation.
The compounded fertiliser products generally have ES dispersed throughout the granule in a very fine form – if the S is largely <70 um then it is likely that most of the S will be oxidised within a growing season.
Sulfur powder is not known to be hygroscopic, but any powder divided finely enough can behave as if it is hygroscopic to some degree. Commercial MAP products with added S have good handling and storage characteristics.
Sulfate S has the potential for leaching on sandy coarse textured soils, particularly in wet seasons, when all the Sulfur fertiliser is applied upfront (prior to or at sowing). Significant early rainfall can move the sulfate below the root zone before the crop is established.
Supplying sulfur in cropping systems
Co-granulated Elemental Sulfur/Sulfate Fertilizers
What sulfur source should I use?
I had one trial where the early growth of canola was significantly depressed when sulphate-S was applied at sowing. The seed and fertiliser bands were separated by about 4 cm of soil, so it wasn’t salting out. The depression occurred with both single superphosphate and sulphate of ammonia. The plants grew out of the problem, but in subsequent trials I’m applying the S as sulphate of ammonia with the first in-crop N application.