Schoenau: Transformations and Fate of Seed-Placed Sulfur Fertilizers in Saskatchewan Soils
Date: February 2016
Term: 2 years
Status: Completed
Researcher(s): Jeff Schoenau and Derek Peak, University of Saskatchewan, Saskatoon SK, SS. Mahli, Agriculture and Agri-Food Canada, Melfort SK
SaskCanola Investment: n/a
Total Project Cost: n/a
Funding Partners: GF2, ADF, SPG, WGRF, NSERC
Project Summary
Sulfur (S) fertilizers have become an important part of the fertilizer mix for Saskatchewan growers, not only for canola, but also for pulse and cereal crops. The overall objective of this study was to evaluate the effectiveness of different fertilizer forms as sources of plant available S to crops. Field studies in three different soil zones showed that canola was the most responsive to S, and that most of the S uptake occurred over the period from one to four weeks after seeding and fertilizing. In addition, the Canadian Light Source synchrotron was used for the first time to track the fate of fertilizer S and P together in soils and shed light on the kinds of fertilizer reaction products formed and their availability to plants.
Sulfur (S) fertilizers have become an important part of the fertilizer mix for Saskatchewan growers, not only for canola and other Brassicae crops, but also for pulse and cereal crops as well. The form of sulfur (S) fertilizer can influence its behavior and crop response. Understanding the fate of the S fertilizer applied when attempting to predict the relative performance of different sulfur fertilizers for different crops under varying application conditions is important.
The overall objective of this study was to evaluate the effectiveness of different fertilizer forms as sources of plant available S to crops. Three soils were chosen for the study to provide a contrast in soil properties, and to represent the different agricultural regions of Saskatchewan. The specific objectives of the study were to investigate in these soils, under controlled environment conditions and in the field, the effects of five different S fertilizer forms applied alone and in combination with monammonium fertilizer (MAP) on supply rates and concentrations of plant available sulfate and phosphate in the seedrow following fertilizer application, crop (canola, pea, wheat) uptake of S and P, and yield.
Field studies were conducted in 2013 and 2014 at three Saskatchewan sites: Central Butte (Brown), Star City (Gray) and Melfort (Black), all marginally deficient in sulfur, along with a growth chamber study. In the studies, researchers evaluated five sulfur fertilizer forms (ammonium-sulfate, ammonium-thiosulfate, gypsum, potassium-sulfate and elemental-sulfur) applied in the seed-row with wheat, canola and yellow peas at 20 kg/ha S alone and in combination with MAP at 20 kg/ha P2O5. The fate of the fertilizer was evaluated by measuring soil available sulfate and phosphate in the seed-row, crop S and P uptake, and grain yields. Another goal of the study was to develop and employ new techniques for using the Canadian Light Source (CLS) synchrotron to study the fate of fertilizer in soil.
The results showed that canola was the most responsive to the sulfur fertilizers, with limited response from pea and wheat in the majority of cases. Most of the S and P uptake in canola, wheat and pea occurred over the period from one to four weeks after seeding and fertilizing. All soils generally had higher sulfate and phosphate concentrations in the seed-row seven days after seeding and fertilizer application, followed by a large decrease in concentrations from day seven to day 28, especially for sulfate sulfur, reflecting high plant uptake and sulfate depletion over this time period. The combination of sulfur fertilizer with MAP may provide some enhancement of phosphate availability, but typically the effects were small. Gypsum and gypsum plus MAP treatments had higher content of sulfate for all days in all soils, similar to the observation for supply rate of sulfate.
The fertilizer forms of sulfate and thiosulfate were effective in enhancing short-term soil available sulfate supplies in the seed-row, along with crop sulfur uptake and yield compared to the elemental-sulfur fertilizer form, which releases sulfate slowly by oxidation. However, for sensitive crops like canola and yellow pea, ammonium thiosulfate and ammonium sulfate can cause injury when placed in close proximity to the seed at high rates and are best placed separate from the seed. Gypsum maintained the highest seed-row sulfate concentrations over time, a consequence of its slightly soluble nature that reduced the sulfate leaching. Elemental sulfur was the least effective in increasing seed-row sulfate supply and providing plant available sulfur to plants in the initial weeks following application. Wheat, canola and especially peas were not highly responsive to phosphorus fertilizer addition on soils with a history of phosphorus fertilization. This suggests that residual P from previous years fertilization is effective as a supply of available phosphorus.
In the Brown soil zone, yield responses to the addition of S fertilizers were limited when there were adequate supplies of sulfate sulfur in the sub-soil. However, under conditions of unusually high moisture, responses to S fertilization may be observed even in soils with subsoil sulfates. Soils of high organic matter and with good mineralization capacity for S such as Black Chernozems also show reduced response. Under conditions of intense weed competition, peas may respond favourably to S fertilization as a result of their shallow rooted nature.
Researchers were also successful in developing a methodology for using the SXRMB XANES (X-ray absorption near edge structure) beamline at the CLS for the assessment of soil sulfur and phosphorus forms in samples collected from seed-row fertilizer bands in the field, and to follow their transformations in the soil in the growing season following application. Thiols and ester sulfates were identified as short-term products formed from seed-row placed sulfur fertilizers soils that likely originate from microbial immobilization of fertilizer sulfur in these prairie soils. The synchrotron also proved to have the ability to track the oxidation of reduced sulfur fertilizers like elemental S into more oxidized forms and eventually into plant available sulfate over time, which is of particular interest, as new fertilizer products become available to growers in Western Canada. Phosphorus XANES spectroscopy was also successfully used to identify P species formed from P fertilizer in the soil in the presence and absence of sulfate fertilizer. The successful application of the CLS methodology in this project breaks ground for further studies using the synchrotron in soil-plant nutrient studies.
Scientific publications.
H. Ahmed, J.J. Schoenau, T. King and G. Kar, 2017. Effects of Seed-Placed Sulfur Fertilizers on Canola, Wheat and Pea Yield, Sulfur Uptake and Soil Sulfate Concentrations Over Time in Three Prairie Soils. J. Plant Nutr. 40: 543-547.
G. Kar, J., J. Schoenau and D. Peak, 2017. Transformation and Fate of Seed-Row Placed Sulfur Fertilizers in Prairie soils. Manuscript submitted for review.
Full Report PDF: Transformations and Fate of Seed-Placed Sulfur Fertilizers in Saskatchewan Soils