Sulphur fertilizer’s form, such as elemental sulphur, gypsum or ammonium sulphate, affects its behaviour in the soil and its availability to the plant. The best form depends on the situation. Factors such as soil and crop type, weather conditions and timing all come into play. A Saskatchewan study has evaluated the effectiveness of different sulphur fertilizer forms under various conditions, providing useful information for crop growers.
Jeff Schoenau, a soil scientist at the University of Saskatchewan, led the research. He has conducted various studies on sulphur fertilizers over the years, but this latest study delved into the transformations the different forms of sulphur fertilizer undergo in the soil and how those transformations affect crop uptake and yield.
“We need to consider the behaviour of different forms of sulphur following application if we’re going to do a good job of predicting when that sulphur is going to become available to the plant and how it relates to such factors as leaching,” he explains.
The study involved growth chamber and field trials in 2013 and 2014, as well as some additional work in 2015. Both types of trials compared five sulphur fertilizer forms applied in the seed row with canola, wheat and yellow pea, in Brown Chernozem, Black Chernozem and Gray Luvisol soils.
The five different sulphur forms
The five sulphur forms were: ammonium sulphate (a soluble form of sulphur); potassium sulphate (soluble); gypsum (calcium sulphate, slightly soluble); ammonium thiosulphate (liquid); and elemental sulphur (insoluble). These fertilizers were applied at a rate of 20 kilograms of sulphur per hectare, alone and in combination with monoammonium phosphate fertilizer (MAP) at 20 kilograms of phosphorus pentoxide (P2O5) per hectare. The researchers evaluated the effects of these fertilizer treatments on the amount of plant-available sulphate and phosphate found in the seed row, on crop uptake of these nutrients, and on crop yield.
The three field sites were located in Star City (Gray Luvisol), Melfort (Black Chernozem), and Central Butte (Brown Chernozem), Sask. The soils tended to be marginally deficient in sulphur. “We wanted the soils in the study to be typical Saskatchewan field soils, so the sites did have a history of sulphur fertilization in the rotation and therefore were not highly sulphur-deficient,” Schoenau says. “It’s difficult these days to find a field with soil that is highly sulphur-deficient because most growers now apply sulphur fertilizers regularly in their crop rotations, especially for canola.” Soils from these three sites were also used for the growth chamber experiments.
The research team used several methods to track the changes in sulphur forms from the time of fertilizer application to crop uptake, focusing mainly on sulphate because it is the plant-available form. They collected soil samples from the seed row at one, four and eight weeks after seeding, and determined the amount of sulphate in the samples through chemical tests. They also evaluated the sulphate supply rates using probes in the soil. As well, they used advanced spectroscopy technology to determine which sulphur forms were present in selected soil samples. At harvest, they determined the amount of sulphur and phosphorus in the grain and straw, and measured grain yield and crop biomass.
Using spectroscopies to determine absorption
For the spectroscopy work, the researchers used x-ray absorption near-edge spectroscopy, or XANES, at the Canadian Light Source synchrotron in Saskatoon. Schoenau explains XANES provided further insight into what was happening to the different sulphur fertilizer forms; those details would have been very difficult to determine using conventional chemical methods. For example, the researchers used XANES to document the oxidation of elemental sulphur – its conversion into plant-available forms by microbes – and some other microbial transformations.
“The ability to track the oxidation of sulphur fertilizers like elemental sulphur into more oxidized forms and eventually into plant-available sulphate over time is of particular interest, as new fertilizer products become available to growers in Western Canada,” Schoenau adds.
The uptake data showed the availability of sulphur from elemental sulphur in the season of application was significantly lower than from the sulphate sources. “You need to have microbial activity and give the microorganisms the time to oxidize elemental sulphur into sulphate for it to be usable by the plant,” he explains.
As a result, elemental sulphur can’t be relied on as a short-term source of available sulphur. He adds, “The role of the elemental sulphur product is to supply sulphur slowly over a number of years because the oxidation is incomplete in the season of application.”
The soluble sulphates (ammonium sulphate and potassium sulphate) and thiosulphate proved to be very effective in supplying available sulphur to the crop early in the growing season. “That early supply of sulphate appears to be important for plant uptake of sulphur and crop yield,” Schoenau says.
“The slightly soluble sulphur form, gypsum, is also an effective source of plant-available sulphur, producing a good crop response,” he adds. The study showed gypsum performs especially well in rainy conditions when there is a high risk for sulphate loss through leaching; gypsum tends to remain in the seed row while the soluble forms are leached away.
“Sulphur fertilizers that supply sulphate and/or acidify the soil may slightly enhance the supply of plant-available phosphorus from phosphorus fertilizer placed in the seed row with the sulphur,” Schoenau says. However, the effects tend to be small.
Wheat, canola and pea took up most of the sulphur fertilizer from the seed row in the first month after seeding and fertilizer application.
“Canola is more responsive to sulphur fertilizer than wheat or peas, reflecting the lower demand of cereals and pulse crops for sulphur and also perhaps a better ability of those crops to scavenge sulphur from the soil,” Schoenau explains.
“For sensitive crops like canola and yellow pea, ammonium thiosulfate and ammonium sulphate can cause injury when placed close to the seed. They are best placed separate from the seed.”
Soil zone effects
Soil type plays a role in sulphur fertilizer needs. “Growers have built up a capacity in many soils to supply available sulphur through mineralization. This was especially apparent in the Black Chernozem soil where a high mineralization potential, or ability to release available sulphur from the soil organic matter, was evident,” Shoenau says. Crop response to sulphur fertilizer was less in the soils with high mineralization potential.
“Sometimes in the drier Brown soils, we have a reserve of subsoil sulphate deeper in the profile, maybe at a 12- to 24-inch depth. That can come into play as a supply of available sulphur later in the season. So soils with that subsoil sulphate reserve sometimes aren’t highly responsive to sulphur fertilization, and only need starter sulphur to supply the crop until the roots access the deeper sulphate,” he says.
“However, under very wet conditions, as we had in 2014 at our Brown soil study site, the crops were responsive to sulphur fertilizer, despite the subsoil sulphates. The unusually high amount of growing season precipitation pushed the sulphate down and really restricted the ability of the crops to access the subsoil sulphate.”
Sulphur deficiency can be more common in Gray Luvisol soils than in Black or Brown soils because Gray soils tend to have a lower mineralization capacity and they don’t usually have subsoil sulphates.
A soil test will give a good indication of the availability of sulphur in a field. “But keep in mind that there is typically a high degree of variability in sulphur availability across a field,” Schoenau says. “So you really have to pay attention to careful soil sampling, taking lots of cores and staying out of the atypical areas like slough edges where sulphate salts may accumulate, in order to best represent the field in your sample and avoid skewing.”
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Schoenau collaborated on this research with Derek Peak from the University of Saskatchewan and S.S. Malhi from Agriculture and Agri-Food Canada. The Saskatchewan Canola Development Commission, Saskatchewan Pulse Growers, Saskatchewan’s Agriculture Development Fund, and Western Grains Research Foundation funded the study.
WEB EXCLUSIVE: Effectiveness of different sulphur fertilizer forms
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