Validating Sulphur Rates and Sources for Alfalfa in Ontario

Photo of alfalfa hay in a strip with a red stake at the front

Figure 1: Response of alfalfa to sulphur application/(100lb S/ac elemental to right of stake)

Alfalfa has the highest sulphur (S) requirements of any of the field crops. A 5 ton/ac crop of alfalfa removes about 25 lbs/ac of sulphur. By comparison, a 45 bu/ac spring canola crop, also a high user of sulphur, removes 15 lbs/ ac. A 150 bu/ac corn crop removes 10 lbs/ac of sulphur.

In Ontario, sulphur deposition from acid rain has decreased steadily. The amount of S deposited has decreased by over 50% since 1990. Instances of S deficiency have also increased due to reductions in the organic matter pool, higher crop yields and higher protein yields. S deficiencies in alfalfa are more likely to occur on soils that have not had a manure application within 2 years.

S Tissue Testing In Alfalfa

There currently is not a reliable soil test for sulphur in Ontario. However, tissue testing of alfalfa (at late-bud stage) is considered a suitable diagnostic approach for determining sulphur deficiencies. The critical level below which alfalfa is considered S deficient and may benefit from applying sulphur is 0.25%. A 2012 field survey of Ontario alfalfa stands (Figure 2) indicated that 21% of fields had S- tissue analysis below this level. Put another way, 79% of these fields would have been unlikely to have an economic response to applying sulphur. It is also noteworthy that 37% of these fields tested below the critical K value of 1.7%.

2012 Tissue Sample Survey of Nutrient Deficiencies  in Ontario Alfalfa Fields

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Note: to convert kg/ha to lbs/acre multiply by 0.9 (eg 8,000 kg/ha X 0.9 =7,200 lb/ac)

Figure 2: 2012 Tissue Sample Survey of Nutrient Deficiencies in Ontario Alfalfa Fields

What Rate of S?

A general thumb rule for S application on alfalfa is 5 lb/ac per ton of dry matter yield. The University of Wisconsin recommends 15 - 25 lbs/ac of S in the sulphate broadcast on established stands, or 25 - 50 lbs/ac of elemental S incorporated at seeding. Research is required to verify these numbers in Ontario.

What Form of S?

What is the most economical source of S to use with alfalfa? The sulphur must be in the sulphate form to be usable by the plant. Sulphate fertilizers include:

  • ammonium sulphate (21 - 0 - 0 - 24)
  • potassium sulphate (0 - 0 - 50 - 18)
  • sulphate of potash magnesia (aka Sul-Po-Mag, or K-Mag) (0 - 0 - 22 - 20)
  • calcium sulphate (0 - 0 - 0 - 17)

Elemental sulphur, rather than sulphate, could be a good fit for forage stands, supplying a potentially cheaper S source over a longer period of time. Elemental S must be converted to sulphate by soil bacteria before plants can use it.

Elemental S consists of finely ground sulphur that has been pelletized. The sulphur content is often 80-90%, but less than 50% is considered available in the year of application. The remainder becomes slowly available.

Limited Ontario Research

Ontario research on sulphur rates, source or application timing for alfalfa has been limited. Results from 2012 research trials applying sulphate to alfalfa were mixed. Some sites showed no response to applying sulphur. However, one site showed a dramatic yield increase in an alfalfa-grass mix of 1.55 ton/ac (including increasing first-cut yield from 2.2 to 3.1 ton/ac). Crude protein in- creased 4 percentage points, with the percentage of alfalfa in the harvested forage improved from 33 to 56%.

2013 Research Trials

A new field project was started in 2013 to look at the rates, timing and form of sulphur on alfalfa-grass yield and quality. Another goal was to refine tools for identifying S- responsive sites, using tissue and soil testing.

Three forage stands were selected where a positive yield response to S was anticipated based on low S-tissue analysis. Treatments included:

  • no S
  • 50 lbs/ac S fall-applied elemental S
  • 100 lbs/ac S fall-applied elemental S
  • 50 lbs/ac S spring-applied potassium sulphate (which also includes 142 lbs/ac K20)
  • no S + muriate of potash (which includes 142 lbs/ac K20)

In these trials where we anticipated a S response, sulphur application improved forage yield an average of 9% (Figure 3). The yield response was greater in second-cut than first cut. The third-cut yield response was greater than first- or second-cut in a 3-cut system. There were no significant differences between sulphur sources (i.e. elemental S vs. sulphate of potash).

Yield response to Sulphur in 2013 Trials on Responsive Sites

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Figure 3: Yield response (kg/ha) to Sulphur in 2013 Trials on Responsive Sites

These results verified the 0.25% S-tissue test threshold. Treatments without S remained below this level, while S application improved S-tissue concentrations. Figure 4 shows S-tissue concentration in first and second-cut. Sulphate rather than elemental-S had the highest S-tissue concentration in the first-cut, indicating more availability to the plant. Sulphur application did not improve crude protein. At 2 sites, % alfalfa in the alfalfa-grass mix was increased by 10%. Spring K application did not improve yield, even though soil tests indicated a requirement for potash.

Tissue S concentration at Site W

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Note: Tissue concentrations below line (0.25%) are considered low and responsive to S application

Figure 4: Tissue S concetration at Site W

In 2013, elemental sulphur provided a similar yield increase to sulphate. It has been generally regarded that because of slow availability of elemental S that the product should be applied 12 to 18 months ahead of crop. The mild winter in 2012-2013 may have improved the S availability. These trials will be repeated in 2014-2015.

Acknowledgement: Thanks to Greta Haupt, Shane McClure, Katie Walch, OMAF students, and John Lauzon, University of Guelph. Special thanks to the farmer co-operators. Financial support for the project: Heartland Regional Soil & Crop, Ontario Forage Council, Canadian Agricultural Adaptation Council. For more information on the research project, refer to "Crop Advances".

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