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Understanding Mechanisms of Sulfur Cycling in Minnesota Soils and Availability from Fertilizer

Start Date: 2019
Principal Investigator: Daniel Kaiser
Organization: University of Minnesota, Department of Soil, Water, and Climate
Status: Ongoing

Background Info

The response of corn grain yield to sulfur fertilization has been one of the major factors for increased productivity and profitability in some cropping rotations. Current projects on sulfur timing, rate, and placement have clearly demonstrated the need for sulfur. While a soil test is available for sulfur, differences in sulfate due to S application are difficult to detect with the soil test and soil test concentration of sulfate-S can be high even in soils where S responses occur. This highlights our limited understanding of how sulfur cycles among forms in the soil. Sulfate-S can be reduced in low oxygen situations but a complete reduction of sulfate to hydrogen sulfide which can be lost to the atmospheric via volatilization unlikely. Basic research on forms of sulfur in the soil is needed to better understand availability in soils across Minnesota.

Elemental sulfur is a low-cost option for supplying S to plants but must be oxidized to sulfate prior to plant uptake. Oxidation is mediated by bacteria, Thiobacillus thiobacteria. From previous work, we know that the activity of Thiobacillus tends to be low when soils remain cool. In fact, the optimum temperature for Thiobacillus activity is above 80oF and even at these temperatures the oxidation of elemental sulfur can take 30 days. Developing an accurate model of oxidation is important to understand how to effectively utilize elemental sulfur in cropping systems. In addition, long-term studies where elemental sulfur sources are compared to sulfate are needed to assess whether oxidation later in the growing season can lead to a buildup of sulfate which, over time, will supply enough available sulfate sulfur to a crop.

Objectives

  1. Evaluate the sulfur and nitrogen supply potential from soil organic matter in 26 Minnesota soil series at different incubation temperatures
  2. Determine the oxidation potential of elemental sulfur in 26 Minnesota soils
  3. Compare sulfur release and availability of a sulfate source of S versus two sources of elemental S in a continuous corn rotation
  4. Evaluate changes in sulfur redox state and changes in soil sulfur pools sorbed to soil solids over time
  5. Evaluate response among corn hybrids for single and split application of sulfur

Key Findings

All forms of sulfur produced equal yield potential at a sandy irrigated location.

Sulfate forms of sulfur generated the highest grain yield at one location while finely ground elemental S co-granulated with potash fertilizer (MST product) produced yield equal to sulfate.

Year 2 data points to slower availability of S oxidized from Tiger 90 as yield was increased by Tiger 90 but the increase in yield was less than sulfate or MST.

All forms of sulfur produced equal yield potential at a sandy irrigated location.

Ion probe data show that elemental S does take time to start oxidizing in Minnesota soils and may provide long-term S availability over the growing season. Finely ground elemental S was shown to be more effective in medium-fine textured soils than an elemental S- bentonite product such as Tiger 90.

Recovery of sulfate S following oxidation of elemental S at 5oC ranged from -5 to 25% across 26 Minnesota soils when incubated for 112 days.

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