An evolving story

SULPHUR NUTRITION CONTINUES to be a hot topic for field crop production in Ontario. Between 2014 and 2016, University of Guelph and Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) researchers demonstrated that sulphur fertilizer applications provided significant yield increases at five of seven alfalfa and four of 10 canola sites in southern Ontario. While these species have high sulphur demands, these results still raised concerns for sulphur deficiency in less demanding crops such as corn, soybeans, and winter wheat.

SULPHUR CYCLING

Sulphur is essential for plant growth and development. It is a component of two amino acids, which are the building blocks for proteins. Photosynthesis, nitrogen fixation in legumes, and plant defence against pests and diseases all depend on sulphur-containing compounds. Because of this range of functions, sulphur deficiencies can lead to reduced plant growth and stunting. Afflicted crops may show general yellowing of aboveground growth with striping sometimes being noted in corn and small cereals. Yield and quality can ultimately be reduced, causing crop revenue losses.

In soils, and particularly in neutral to higher pH soils like those commonly found in southern Ontario, plant available sulphur (sulphate) is lost to leaching over the winter and early spring. This prevents the buildup of sulphate over time. Moreover, it means that over 90 per cent of the sulphur found in Ontario topsoils is in organic matter, which is unavailable to plants. Through biological processes this organic sulphur can be broken down to provide sulphate. Atmospheric inputs such as acid rain can deposit plant-available sulphur into our cropping systems as well.

However, these atmospheric sulphur inputs are declining in Ontario. Legislation introduced in the U.S. and Canada beginning in the 1970s gradually reduced sulphur emissions from industry, power generation, and other sources. This represented a win for human health and the environment but also limited sulphur for crop production. In 1990, we received upwards of eight to 13 pounds of sulphur per acre (S/ac) per year just as wet (which is roughly 60 per cent of total) sulphur deposition. Numbers from 2014 indicated wet accumulations of only two to 4.5 lbs S/ac per year. This decline, combined with steady rises in crop yields and nutrient requirements, means there is an increasing risk that sulphur supply for Ontario crops is insufficient.

The recent findings in canola and alfalfa support this notion. Yet, more research was necessary to gauge the effect of sulphur additions in Ontario corn, soybean, and winter wheat stands. A collaborative project with university researchers, OMAFRA staff, and grower cooperators was initiated in 2018 to determine the frequency of sulphur response in these prevalent grain and oilseed species.

Sulphur was spring-applied as an available sulphate or thiosulphate source at five corn, 15 soybean, and eight winter wheat trials across southern Ontario from 2018 to 2019. Grain yield measurements were collected at all locations. In addition, simple cost-benefit analyses were completed to determine if crop revenue gains would cover sulphur fertilizer costs.

WHAT THEY FOUND

Significant yield increases were noted at two of five corn locations. This is greater than in 2012 and 2013 experiments where no sulphur-driven yield advantage was measured across 21 OMAFRA-led corn starter fertilizer trials. Soybeans did not show response. None of the 15 site-years assessed displayed yield boosts upon sulphur application.

This mirrors recent Michigan results where preplant ammonium sulphate gave yield response at just one of eight experiments between 2017 and 2018. Winter wheat generated significant yield gains at three of eight sites, which is similar to the response seen in corn. This is surprising given winter wheat’s early season nutrient demands when soils are less able to supply sulphur through mineralization. Furthermore, the response we saw was less frequent than in OMAFRA efforts between 2010 and 2015, where 13 of 22 winter wheat sites tested offered significant yield gains from sulphur application.

Of the responsive trials, only one corn and one winter wheat location provided economic gains from sulphur application. Yield advantages at these sites were +16.5 bushels per acre (bu/ac) for corn and +7.5 bu/ac for wheat. Despite this, economic analyses averaged across all experiments failed to show significant profit from sulphur application for any of the three crops.

There are a few conditions to these results. All trials over the past two years have been on mid- to fine-textured soils, and we would expect there to be a greater chance of response on sandier sites which are more prone to leaching and generally lower in organic matter. The findings do indicate that sulphur application can make economic sense at certain sites in certain years. In fact, response could be more likely for years with cooler, wetter springs since these conditions slow sulphur mineralization. This was seen in Elora wheat trials that were unresponsive in 2018, but did respond in the damper, cooler spring conditions of 2019. If growers are interested in applying sulphur, field strips with and without sulphur can be compared to help determine whether a given site is likely to provide profitable response.

A third year of sulphur response trials is underway for the 2020 season to add to the collection presented here. An official Ontario sulphur soil test is also being developed through the project to aid in identifying responsive field sites.

This research project is funded by Grain Farmers of Ontario and the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), through the Ontario Agri-Food Innovation Alliance. Some of the field work was carried out at research stations owned by the Agricultural Research Institute of Ontario and managed by the University of Guelph. Support from OMAFRA staff and grower cooperators has been greatly appreciated.

Alex Sanders recently completed his M.Sc. at the University of Guelph School of Environmental Sciences under the direction of Dr. John Lauzon. Sanders was a student researcher for this project. •