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.
Crop rotations are designed to maintain crop and soil health to ensure long-term sustainability. Crop sequences deal with the effects of previous crops on current crop choice. A successful crop rotation encompasses many farm management components, including economics, fertility, soil biology, insect, disease, weed and pesticide considerations. The following eight questions should be asked when planning crop rotations:
1. What crops should I include in my rotation?
Diversity is key. Growing cereals, oilseeds and pulses in a long-term rotation has been shown to provide the best benefit to soils and to crop yields. Wheat grown in a rotation with oilseeds and pulses was 16 percent higher yielding than continuous wheat grown on the same land at Scott, Saskatchewan, from 1993 to 1999. Wheat yields following flax, pea, and canola were 16 percent, 11 percent, and eight percent higher, respectively, than after wheat (Manitoba Crop Insurance data, Bourgeois and Entz, 1996).
2. What are the nutrient levels in each field?
Soil testing is important, particularly after dealing with excess moisture conditions. Nitrogen level variability will also be an issue in fields with flooding of depression areas. Knowing the soil nutrient levels through soil testing allows producers to balance nutrient levels in the field with crop nutrient requirements. Oilseeds such as canola have the highest nutrient requirements of the major Saskatchewan field crops, followed by cereals and then pulses.
Pulses or legumes can fix up to 80 percent of the nitrogen they need from the air, which reduces the amount of nitrogen removed from the soil. For annual crops, fababean has the highest nitrogen-fixing capability and can fix amounts similar to perennial legumes such as alfalfa (Table 1). Pulse or legume residue has a significant amount of nitrogen in it, which then will become available to future crops as it decomposes. Thus legumes increase the nitrogen supplying power of the soil and can reduce the requirement for nitrogen fertilizer for succeeding crops. A nitrogen credit is included in the soil test recommendation made by Saskatchewan soil-testing labs if you report your previous crop as a legume.
3. How much water is available and where are the nutrients?
Rooting patterns, crop maturity and growth stage can influence nutrient uptake and water use. The deeper the roots, the more accessible they are to water and nutrients found farther down in the soil profile. Varying crops in a sequence allows you to take advantage of the different root patterns and growth habits to access water and nutrients at different levels and at different times of the year.
Canola and mustard love water and nutrients, and are deeply rooted. Thus, canola and mustard are a good fit for wetter areas of the province as they can penetrate deep into the soil to reduce subsoil moisture and access nutrients that may have leached deeper into the soil profile. The deeper tap roots also help with improving soil aeration and drainage. Cereal crops are also deep rooted but tend to need the moisture earlier in the season and can withstand drier conditions better than canola. Pea and lentil are shallow rooted and have shorter maturities. This means that there will likely be more moisture left in pea or lentil stubble than cereal or canola stubble.
4. Are there soil biology considerations that may influence crop choice?
Soil biology is also important. For example, mycorrhizal fungi in the soil form mutually beneficial relationships with most plants. The fungi penetrate the roots and extend hyphae (threads) into the soil where they can access more nutrients and water for the plant. Thus, they serve as highly effective transport systems. Pulses form strong associations with these mycorrhizal fungi, while cereals are less dependent, and canola and other Brassicas do not generally form these associations. It is suggested that highly mycorrhizal crops may fit best after a crop that is at least somewhat mycorrhizal; for example, planting peas on cereal stubble. This may somewhat explain why crops such as peas and flax tend to do better on cereal stubble than on canola stubble.
5. What disease issues did I have in the past and when was the last time I grew this crop?
Crop rotations can be a significant management tool when it comes to residue and soilborne plant disease organisms. Leaving a rest period between certain crops can successfully reduce plant pathogen populations to a level where other disease control methods will work more effectively. Table 2 shows the risk associated with shortening the recommended crop rotation based on the disease of concern.
The general recommendation for cereals is to plant no more than two years in a row and incorporate different cereal species and varieties into the rotation. With canola, blackleg spore numbers and viability of sclerotia (sclerotinia stem rot’s resting bodies) are significantly reduced after four years. Therefore, canola should only be planted once in a four-year rotation. Sclerotinia stem rot can also affect crops such as lentil and pea so it is good practice to avoid growing pulses on canola stubble and vice versa. In fact, the recommendation is to grow peas or lentils no more than once every four years in rotation with canola.
Choose varieties that have disease resistance and consider fungicide use during the growing season. Fungicide products should be rotated (use different active ingredients in succession) to minimize development of resistant pathogen populations.
6. Are there weed issues to consider and is there potential for a high number of volunteers from the previous crop, or is the field fairly clean?
When selecting a crop it is important to consider its weed control needs or limitations. Crops such as lentils, which are uncompetitive and have limited weed control options, should be seeded into the cleanest fields. Matching weedier fields with crops that are more competitive and have better herbicide options is important.
It is not just the presence of weeds but potential volunteers from the previous crop that should be considered. Canola, for example, can be problematic as a volunteer, so having options in next year’s crops is key. Rotating cereals with broadleaf crops usually allows good control of volunteers.
7. Are there residual herbicide considerations?
It is important to know the residual properties of the herbicides you are applying in order to avoid any unwanted cropping restrictions in your crop rotation. The length of time it takes herbicides to break down can vary and is dependent upon a number of factors, including soil organic matter, soil pH, rainfall and temperature. In saturated soil conditions, herbicides that rely on aerobic microbes requiring oxygen may take longer to deactivate. On the other hand, herbicides that rely on a process called chemical hydrolysis will break down equally well in aerobic or anaerobic conditions. Some basic guidelines to follow include:
- Herbicides with re-cropping restrictions under dry conditions will most likely have limitations under saturated conditions.
- Fields that were seeded but that were saturated for a significant part of the season are unlikely to have seen much herbicide breakdown.
It is important for producers to check with herbicide manufacturers for recommended re-cropping options in fields that were recently saturated. Further information on soil residual herbicides and re-cropping restrictions can be found in the Weed Control Section of the current Guide to Crop Protection.
8. Does my crop selection allow me to rotate herbicides?
Herbicide resistance has been increasing in frequency, particularly with Group 1 and Group 2 herbicides. Weeds that have developed resistance to particular herbicide groups in Saskatchewan include: cleavers, chickweed, green foxtail, kochia, wild mustard, Persian darnel, Russian thistle, stinkweed, wild buckwheat and wild oat. It is estimated that over 90 percent of kochia populations are now resistant to Group 2 herbicides. Rotating or mixing herbicides from different groups on each field (on your farm) is critical to preventing the development of resistance. This is the case with all pesticides, including fungicides, insecticides and herbicides. More information on herbicide groups can be found in the current Guide to Crop Protection, at the beginning of the Weed Control section.
Planning crop rotations is complex. For more information, visit the governement of Saskatchewan's website or contact your Saskatchewan Ministry of Agriculture Regional Crop Specialist or the Agriculture Knowledge Centre at 1-866-457-2377.
While drones have a foothold in the game of precision agriculture, some researchers are toying with the idea of using them as pollinators as well.
Researchers ordered a small drone online and souped it up with a strip of fuzz made from a horsehair paintbrush covered in a sticky gel. The device is about the size of a hummingbird, and has four spinning blades to keep it soaring. With enough practice, the scientists were able to maneuver the remote-controlled bot so that only the bristles, and not the bulky body or blades, brushed gently against a flower’s stamen to collect pollen – in this case, a wild lily (Lilium japonicum). To ensure the hairs collect pollen efficiently, the researchers covered them with ionic liquid gel (ILG), a sticky substance with a long-lasting “lift-and-stick-again” adhesive quality – perfect for taking pollen from one flower to the next. What’s more, the ILG mixture has another quality: When light hits it, it blends in with the color of its surroundings, potentially camouflaging the bot from would-be predators. | READ MORE
But natural pools of nitrogen have plenty to offer, and McDonald believes if producers don’t wait to soil test or judge the impact of weather on the crop’s early development, they might be missing an opportunity for improved N management.
Back in 2001, McDonald and OMAFRA’s then-corn specialist Greg Stewart decided to evaluate levels of organic N across Ontario’s soil zones. “Most N went down as urea or urea ammonium nitrate (UAN) applied on bare ground before the corn was planted. We wanted to raise awareness of how much organic N was available from the natural pool across Ontario,” he says.
OMAFRA’s corn N soil survey was born, in which Stewart and colleagues annually sampled between 75 and 100 different sites, evaluating available organic N in natural pools by soil type, geography and cropping history.
“We did that on an annual basis in the hopes of better understanding how the natural pool was mineralized, and to give people other options than throwing all the N up front. Pre-plant N application works from a time perspective but doesn’t give producers much opportunity to use management thinking to customize the rates based on a sound knowledge of the year’s yield potential,” McDonald says.
In 2016, the survey morphed into the “N Sentinel Project,” a three-year Grain Farmers of Ontario and Growing Forward 2 sponsored effort designed to improve current tools for estimating N fertilizer requirements. Instead of visiting dozens of sites the team focused their analyses on 23 dedicated sites across clay-loam, loam and sandy soil zones.
“In the past, we were just haphazardly going out and sampling fields across the province and it wasn’t a very organized or targeted process,” McDonald says. “It was a one shot-in-the-dark per year analysis, and we felt that although it was giving us generalities, it wasn’t able to answer the important questions that needed to be answered on an annual basis.”
What are those questions? The researchers are mostly interested in discovering the impact of weather patterns on the mineralization of the natural N pool each year, as well as the effects of previous cropping practices, temperature, moisture and soil type on background N levels.
Three sites are located in eastern Ontario, two in central Ontario, and eight between London and Guelph; the rest are scattered to the west, with the furthest located at Dresden. Eight of the sites are maintained by the University of Guelph as part of a number of the Ontario Corn Performance Trials. The other sites are maintained by farmer co-operators.
Sites are established with zero N (max 30 pounds of N per acre) in a starter band and a full-rate non-yield limiting commercial N rate. Each site is sampled four times per year between May 1 and July 1, and has its own weather station installed by Weather Innovations Network, which processes local weather data and hosts results on a dedicated website.
All sites, McDonald says, will have two replicates of the zero and full N treatments harvested for yield at maturity. “This will allow a calculation of the delta yield for each location that measures yield response to N rate,” he says. “This will also provide a [maximum economic rate of nitrogen] MERN for each site and a calibration of the [pre-sidedress nitrogen test] PSNT taken at the mid-June sample timing.”
In the previous soil N survey, he notes, there was no correlation to crop yield and thus no way of determining whether the PSNT taken to generate the survey was in the ballpark of predicting what the crop needed for economic yield.
“We know more of the background on these sites — previous management, previous rotations, etcetera, that might influence soil nitrogen levels,” says Ben Rosser, OMAFRA’s corn specialist.
“We have more info being generated on each site than in the past,” McDonald agrees.
In 2015, the corn survey generated surprising results: soil N levels were “considerably higher” than previous years’ data, due to an unusually dry spring.
“The higher-than-usual average soil nitrate levels observed in this year’s survey suggest that fertilizer N requirements in 2015 may be less than the rates generally needed in most years,” the team’s field crop report suggested, while cautioning that producers should confirm fertilizer N requirements on a field-by-field basis.
“In 2016, the results were closer to normal, or maybe just above,” Rosser says. While the season began with cooler than normal temperatures, it warmed up by June.
“With an overall average of 11.2 [parts per million] ppm in 2016, soil nitrate levels tended to be average or slightly above average relative to the five previous survey years (2011-2015), while slightly lower than 2015 values, which were well above normal,” states the team’s 2016 field crop report, before recommending normal N application practices.
But recommendations should never be taken as holy writ, the authors again caution: “Soil nitrate values are highly influenced by the environment and agronomic practices. For instance, if you are in an area which has received significantly more rainfall than other parts of the province, you may have also experienced more loss than is reflected in these results.
“The only way to know soil nitrate concentrations on your own farm is to pull soil nitrates from your own fields.”
McDonald believes producers are beginning to realize the value of managing N application more tightly, thanks to their use of Internet and social media resources promoting the practice — and the genetics they’re employing.
“The biggest change that’s occurred is that the genetic potential of new hybrids has really increased, and with that, producers are understanding how important nitrogen management is to achieving that yield potential,” he says.
A previous version of this article originally appeared in the October 2016 edition of Top Crop Manager East.
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