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Does Canola Need Additional Sulfur?
By AJ Foster
Southwest Area Agronomist
 
Sulfur (S) deficiency was once limited to high-yielding crops grown on irrigated, sandy soils that were low in organic matter and subject to leaching. However, S deficiency has become a widespread problem, domestically and internationally, due to several factors including more concentrated fertilizers containing little to no S, a reduction of sulfur dioxide emissions, higher S uptake and removal by high-yielding crops, and decreased levels of soil organic matter (SOM).  
 
Studies in spring canola have found that S application can increase both yield and oil content. But winter canola response to sulfur application has not been studied in western Kansas thus a study was established in Hugoton, KS consisting of 2 acre plots with four replications and four S treatments to examine canola response to S.
 
Results from this study found winter canola yields increased 10 to 13% in response to an application of 30 lb S per acre [10 lb S as 10-18-0-10S (ammonium polyphosphate (APP) and 20 lb S as thio-sulfate (TS)] and [10 lb S as APP and 20 lb S as potassium thio-sulfate (KTS)]. The fertilizer was side-dressed in early spring.
 
These results led to the questions:
  1. How reliable is soil test S?
  2. Should canola growers be applying S? 
How reliable is soil test S?
 
Soil tests for inorganic S (sulfate–S) often have the reputation of being unreliable compared to other soil nutrients. While this may be true, it is important to note that interpretation of a sulfate-S test must take into consideration SOM levels, soil texture, crop to be grown, amount of S in irrigation water, crop residue input and manure applications, and the expected yield level. This is because a yield response to S application is most likely for crops with a high demand for S (i.e. corn, canola and alfalfa), sandy and/or eroded soils, soils low in organic matter, cropping systems with high residue removal, and soils with low sulfate-S within the profile.  Accurate estimates of sulfate-S cannot be made from a surface sample alone because sulfate-S is mobile. Therefore, sampling to a 24-inch depth is required. 
 
The study in Hugoton was conducted on a sandy soil with SOM less than 1%. Soil samples were collected to a depth of 36” and analyzed for inorganic N and S (Table 1). Based on the Great Plains Canola Production Handbook, https://www.bookstore.ksre.ksu.edu/pubs/MF2734.pdf, recommendations for canola, only soils having < 20 lb/acre sulfate-S should receive supplemental S. This would suggest there is adequate S available and no additional S is required.
 
In some areas, nitrogen-to-sulfur (N:S) ratio has been used with some level of success as an indicator of the plant sulfur status. The drawback to this approach is that deficiencies identified late in the season may not have time to be corrected until the following year. Table 2 shows plant analysis of canola (whole plant) taken at flowering for the different S treatments.
 
Table 1. Sulfur and inorganic N soil test results from Hugoton site.
 

Sample Depth (in)

NO-3 (lb/acre)

NH+4 (lb/acre)

SO2-4 (lb/acre)

0-12

4.0

5.0

52.0

12-24

4.0

4.0

48.0

24-36

4.0

4.0

80.0

 
 
Table 2. Plant analysis of canola taken at flowering with S treatments
 
 

Treatment

Total N

Total S

Total K

N:S

lb per acre

%

 

0 (control)

4.5

0.6

4.0

7.0

10 lb S (APP)

4.3

0.6

3.8

7.3

20 lb S (APP +TS)

4.2

0.6

3.7

7.2

30 lb S (APP + TS)

4.3

0.6

3.9

7.1

30 lb S + 29 lb K (APP + KTS)

4.6

0.6

4.1

7.7

 
Should canola growers be applying S?
 
According to both soil and plant analyses, there is no need for additional S fertilization.  However, the study found that application of 30 lb S per acre significantly increased yield by approximately 2.5 bu per acre (13%) over the control (Table 3). It should be noted that canola yields in southwest Kansas were significantly impacted by a snow storm on April 30, 2017 (Figure 1). Regardless, the yield response seen in this study was similar to results obtained in Saskatchewan that showed increased spring canola yield with fertilizer applications of approximately 30 lb S per acre.  In another study in Saskatchewan, S fertilization was critical to achieving any response to N fertilizer and at higher rates of N fertilizer, more S was required to increase yield. Yield was maximized with approximately 30 lb per acre S and 90 lb per acre N. According to the Great Plains Canola Production Handbook, a good rule to follow is to keep the N:S ratio about 7:1. 
 
Table 3.  Effect of S rate on canola yield in southwest Kansas.
 

Treatment
(lb per acre)

Yield
(bu per acre)

0 (control)

23.5

10 lb S

23.4

20 lb S

23.4

30 lb S

26.1*

30 lb S + 29 lb K

25.4*

 
*Indicates a significant difference compared to the control at LSD=1.5
 
 
Figure 1. Canola in Hugoton study before the snowstorm on April 30 (top photo); Canola in Hugoton study nine days after the snowstorm (bottom photo). 
 
More studies are needed in the Great Plains to better understand canola response to S fertilization. Until then, it is a good idea to apply S to soils with test levels less than 20 lb S per acre. Nitrogen-to-sulfur ratios should be kept to 7:1 or one may simply apply 10-30 lb S per acre, depending on the yield level. Canola is a heavy user of S and deficiencies may result in reductions in pod set and seed quality.