SUPPLY AND DEMAND
Increasingly, and for a number of reasons, sulphur deficiencies are an issue for farmers around the world. Here in Ontario, because we have better air quality standards, there is less sulphur from air pollution. Not only that, but we have purer, high analysis fertilizers that are reducing the input of sulphur as a contaminant in fertilizers. We also have higher crop yields with increased demands, especially corn and canola.
“If you’ve got higher crop yields, you have higher nutrient removal, and you need a higher amount of nutrient to support that yield, either naturally from the soil, coming in from the air, or being added as fertilizer,” says Cynthia Grant, a soil management and fertility scientist with Agriculture and Agri-Food Canada.
Sulphur deficiencies, she says, tend to be influenced by weather conditions. They also tend to be much more common after wet conditions. “That’s because sulfate sulphur is negatively charged, so it’s mobile in the soil and will leach,” says Grant. “If you have a lot of wet conditions, a lot of rainfall moving the sulfate through the soil, you may run into sulphur deficiencies more often in situations where leaching isn’t occurring.”
Soil degradation will further reduce your supply of sulphur in organic matter, she says. “We talk about mineralization of nitrogen – we also get mineralization of sulphur.”
SOURCES OF SULPHUR
There is a wide range of sulphur sources available to farmers. Sulphur comes in two forms: as sulphate or elemental sulphur. Ammonium sulphate, ammonium thiosulphate, gypsum, and microessentials S-15 are some of the sulphate sources. Elemental sulphur, bentonite blends (Tiger 90), and Microessentials S-15 are some of the elemental sources.
“But remember that they [elemental sources] have to convert to sulphate before the plant can use it because plants will only use sulphate sulphur,” warns Grant. For this reason, each source needs to be managed very differently. On top of that, each source’s response to the environment will be different, as well.
Sulphate sources are immediately available so the timing of application is flexible. “You can apply it ahead of seeding. It will not volatilize, but it may immobilize or leach,” says Grant. “But in wet conditions, you maybe don’t want to put it on too far ahead of seeding.”
On the other hand, if you apply it near seeding it’s readily available for the plant to use and reduces the risk of leaching below the rooting zone. If you notice a sulphur deficiency, sulphate sources can also be put on post-seeding, meaning you can go in and rescue the crop, if necessary. Even when they’re delayed, sulphate sources can be effective, as long as there is sufficient moisture to carry it into the rooting zone where the plant can use it.
Because sulphate sources are mobile in the soil, placement options are flexible as well. “You can band it, broadcast it, dribble-band it or seed place it, as long as you’re avoiding excesses that can cause toxicity,” says Grant.
The major difference between the sulphate form and the elemental form of sulphur is that the elemental form has to oxidize to sulphate before the plant can use it. “This is a microbial conversion, so any of the environmental conditions that encourage microbial activity also encourage the conversion of elemental to sulphate,” says Grant. “You get much more rapid conversion when soils are warm and moist. And the conversion is much slower under very wet or very dry conditions, and very slow under cold conditions.”
In Canadian soils, you usually can’t really rely on elemental sources to provide enough available sulphate in the year of application. “It may be delayed for a year,” says Grant. “It may be delayed for two years.” Elemental sources can be beneficial in long-term planning, though.
When evaluating your nutrient management practices, it is important to consider the effects of the preceding year on nutrient supply, especially for nitrogen and sulphur. Always be certain to match your management practices to the environment around you. For example, use site-specific management that is targeted to loss pathways, says Grant. You also want to minimize the amount of time between nutrient applications and crop uptake, especially under wet conditions. Finally, sulphate sulphur is required for use in the year of application. If your crop requires sulphur that year, don’t use an elemental form. Make sure that you use an available form, like a sulphate source, so that it’s there when your crop needs it. •