14 AGRONOMY NEWS

Sulfur Fertilization Of Dryland Winter Wheat

Sulfur increased yield when soil pH was high and OM was low.

In the 1980's, CSU researchers Hunter Follett and Dwayne Westfall studied sulfur fertilization of winter wheat at 15 locations throughout eastern Colorado. Fertilizer treatments were injected about four inches deep at 12-inch spacings as liquid ammonium thiosulfate about two weeks before planting. The nitrogen and phosphorus applications were uniform across the plots. Three of the fifteen locations had significant yield responses. However, the average soil sulfate levels in the responsive sites was higher than the average level in the non-responsive sites.

Many wheat farmers apply sulfur with their pre-plant nitrogen and phosphorus applications. Often the stated purpose of the S is to reduce pH in the fertilizer band (thus increasing the availability of P, Zn, and Fe), not necessarily to supply S as a nutrient. A closer look at the Follett and Westfall dataset reveals that the yield response is related to the soil pH at the 15 study sites. One of the responsive sites had a low pH (6.6), but sulfur decreased yield significantly at this site. The other two responsive sites had yield increases due to S fertilization, and both had soil pH levels of 7.5 or greater.

However, there were two other sites with pH of 7.5 or greater which did not respond to S fertilization. Other research has shown that S fertilizer responses are more likely to occur in soils with low organic matter contents. This principle holds true in this case as well. The two sites with positive yield response of 3-4 bu/acre both had soil pH levels > 7.5 and soil organic matter levels < 1.5%. Therefore, S fertilization has the best chance of increasing yield when soil pH > 7.5 and soil OM < 1.5%. Be sure to consider the cost of the additional fertilizer when making your S fertilization decisions.

Table 3. Wheat yield response to sulfur fertilization.

soil pH	Yield Response	Details
< 7.0	1/5 responsive sites	The responsive site had a negative yield response.
7.0 - 7.4	0/6 responsive sites	--
> 7.5	2/4 responsive sites	The responsive sites had soil OM<1.5%, and the non-responsive sites had soil OM=2.0%.

Jessica Davis
Extension Soil Scientist
Colorado State University