Not since the spring of 1998 has ammonium sulfate been in such short supply, and the problem has left canola producers with hard choices.
If they haven’t planted their canola yet, should they do so without the usual seed row or sidebanded sulfur application?
If they have planted already and shifted to elemental product, what should they do about nutrient deficiencies?
“It can be a tough decision, but luckily sulfur is something you can apply by top dressing later in the season,” said Rigas Karamanos of Westco Fertilizers in Calgary.
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He said an early season application of ammonium sulfate can help avoid sulfur deficiencies or correct problems if they show up.
“As the (ammonium sulfate) becomes more available again, you can top dress those sulfur sensitive crops and deal with the deficiency.”
Elemental sulfur is limited in its short-term availability to plants, so producers will generally see only a minor benefit in the season that it is applied. Ammonium sulfate is the plant-available form of the mineral.
Canola demands the greatest amount of sulfur in the sixth week after seeding, at the five to six leaf stage.
Westco’s research indicates that no matter when the crop was seeded or where, canola’s demand for the nutrient rises rapidly between the fourth and sixth weeks.
Uptake of sulfur doubles in that period, from half to three quarters of a pound per acre per day to 1.1 to 2.2 lb. per day.
Maximum sulfur uptake in canola averages 1.6 lb. per acre per day in the sixth week and then drops off nearly as quickly as it increased.
“This would then mean that in order for a top dressed application of sulfate to be effective, it has to take place prior to or at that time,” Karamanos said. “Of course, a very important ingredient in attaining effectiveness of top dressed sulfate applications is sufficient precipitation in order to dissolve and wash the fertilizer into the root zone.”
He said the sooner the product can be applied, the greater the chance that rain will fall and move the sulfur down to the roots.
Agriculture Canada research has shown that sulfur applications just before the bolting stage can still correct for most deficiency problems.
“However, timelines of rains become more critical if applications are delayed beyond the bolting stage.”
Karamanos said research has shown broadcasting the fertilizer as much as a month after seeding produces significant yield advantages compared to not applying it at all or adding it later.
Top dressing of plant-available sulfur as late as early flowering, provided there is enough moisture to transport it to a crop’s root zone, will increase yields but likely will be too late to maximize yield potential in a nutrient stressed crop.
To be plant-available, the fertilizer must be soluble. There are several versions that perform equally well, the most common being 21-0-0-24, 20-0-0-24 and 20.5-0-0-24.
The first, 21-0-0-24, is typically not of a uniform size and is a crystalline product, which means it will work in a fertilizer spreader but is not well suited for blending with other dry fertilizer products such as urea nitrogen.
The other two are a uniform, granular product and blend well with other fertilizers.
Ammonium thiosulfate is a liquid form of the fertilizer – either 12-0-0-26 or 15-0-0-20 – and about half of the sulfur is plant-available at the time of application. It can be dribble banded into the crop but should not be foliar applied because of the risk of plant leaf damage.
“A number of blends of ammonium sulfate (20-0-0-24) and bentonitic sulfur (0-0-0-90) are also available,” Karamanos said.
“Although these products contain ammonium sulfate, the actual quantities of readily available sulfur are surprisingly small and may not be adequate to satisfy crop needs.”
Sulfur is not prone to gassing off the way nitrogen does. If left at the soil’s surface for an extended length of time, some nitrogen in ammonium fertilizer will be lost but not the sulfur.
Karamanos said producers who anticipate a light to moderate sulfur deficiency should plan to apply 10 to 15 lb. per acre of ammonium sulfate.
For severe shortages the rate should be 25 lb.
“Economically, a yield increase of only a bushel or two will pay for the application,” Karamanos said.
“If the field is deficient the benefit will likely far outweigh any costs.”
Sulfur deficiency in canola appears as a yellowing and cupping of the newest leaves at the top of the plant. Shortages are sporadic within fields because sulfur is mobile.
“There are such large benefits to yield (in the case of severe deficiencies), treating a whole field for those areas (that are deficient) often makes sense,” Karamanos said.
