3 Things to Consider Before Applying Sulphur in the Fall
With harvest having wrapped up in some areas and others nearing the finish line, it is time to start thinking about applying fall fertilizer. Whether you want to lighten the work load in the spring, free up some space in the tank to reduce the time spent filling during seeding, or take advantage of cheaper fertilizer prices but don’t have the infrastructure to store product over the winter, fall fertility has a variety of advantages but there are a few important factors to consider.
1. Fertilizer Forms
The two most common forms of fall applied sulphur would be sulphur fines/ammonium sulphate (21-0-0-24) or elemental sulphur (0-0-0-90). It is important to understand that there is a major difference between these two forms. The sulphur in ammonium sulphate (AMS) is already in the inorganic/plant available form and highly water soluble. Elemental sulphur is in the organic form which is unavailable to the plant and is insoluble in water. Therefore, applied elemental sulphur needs to be oxidized in the soil before it will become available to the plant.
2. Disadvantages of Drill Applied Sulphur
Both elemental sulphur and AMS have drawbacks when applied at the time of seeding. Because elemental sulphur requires time to oxidize before it is available to the plant, there is little to no benefit of placing elemental sulphur in the seed row. Also, placing the sulphur in a band reduces the surface area of the prill exposed to the soil, further inhibiting oxidization. There are two drawbacks associated with AMS. The first is when the fertilizer is exposed to moisture it can cause plugging of seeding equipment. The second is that the maximum safe rate for seed-placed AMS is very low, especially with canola.
3. Fertilizer Application
If you are choosing to apply sulphur fines, it is important to wait as long as possible before applying to insure it won’t absorb into the soil solution and leach into the subsoil or water collecting areas of the field. The best timing would be to apply just before snow fall. As for elemental sulphur, the window to broadcast is a little wider because sufficient soil contact is required to transform the fertilizer into the inorganic form. If the elemental sulphur is not incorporated, then oxidization to the inorganic form is relatively slow and you can expect about 20-30% of the applied sulphur to be available for next year’s crop. Therefore, we recommend applying enough actual sulphur for 3 to 4 crop years at one time. As for AMS, we will usually recommend enough actual S for one crop year only. If we were to recommend enough AMS fertilizer for more than 1 year, it is possible that the sulphur applied to the hills and upper slopes may leach.
In conclusion, there are benefits associated with applying either ammonium sulphate or elemental S in the fall to meet subsequent crop’s needs. Using a fertilizer like AMS can help take the guess work out of how much sulphur will be available the following spring, but using elemental allows you to reduce the amount of applications per field and can be easily monitored with a soil test to ensure the plant-available sulphur is on track.