Unfenced Answers: Soil’s Biological Activity Key to Process Nutrients

What does the increase in the soil's biological activity do to the plant's ability to process available nutrients? — Shawn, South Dakota

The soil's biological activity, such as the oxidization of elemental sulfur, is a key aspect to ensure that the plant is getting the nutrients that it needs throughout the season. 

"Today's hybrids and varieties are creating higher-yielding crops that require more nutrients throughout the growing season," says Dr. Matt Clover, manager of research and alliances for The Mosaic Company. "A high-yielding crop year equals fewer nutrients that are available to the plant for the next year, and therefore need to be replenished. This is why it is important to make sure that your crops have the right nutrients available to them from the beginning to the end of the growing season." 

Sulfur (S) is one nutrient that can be readily available to the plant throughout the season, but that requires it to oxidize. There are two forms of S that the plant needs: elemental sulfur (S⁰) and sulfate sulfur (SO₄²⁻). Sulfate sulfur provides plants with the S they need in the beginning of the season, and elemental sulfur oxidizes in the soil over the growing season to become available to the plant later. S⁰ must oxidize into SO₄²⁻ to be available for plant absorption. Sulfate sulfur can be applied directly to the soil, but is prone to leaching. 

"To help reduce leaching, sulfur can be applied by top- or side-dressing later in the season," adds Clover. "Another option is to utilize an elemental sulfur fertilizer, because the oxidation process occurs over the growing season, which is beneficial to the crop when leaching is a concern."

The rate of oxidation is an important factor, because it can impact the availability of S for the plant. Elemental sulfur's oxidation rate affects the availability of sulfate sulfur, which ultimately determines the amount of S that the plant has available. Oxidation is a biological process, and the rate can be affected by five major factors:

  • Particle Size: The bigger the particles, the slower the oxidation process, because there is less surface area for the particles to colonize. Smaller particles are ideal for a faster oxidation process.

  • Temperature: As temperatures increase, so do oxidation rates. For every 18 degrees' Fahrenheit increase in the soil temperature, the oxidation rate increases an estimated 3.6 times.

  • Soil Water Content: For oxidization to occur, water and oxygen are needed. For saturated soil, more oxygen is needed because the oxygen supplies are reduced in this type of soil. Conversely, in dry soil, more water is needed.

  • Soil Chemical and Biological Properties: Both pH and organic carbon directly impact the rate of oxidization. The higher the pH and organic carbon content, the faster the oxidation.

  • Degree of Dispersion: When S⁰ is co-granulated into fertilizers, the oxidation process occurs more slowly. This allows S to be available to the plant for the duration of the growing season.

"These five factors play a huge role in whether sulfur will be readily available for the plant," Clover says. "The soil's biological activity of oxidizing elemental sulfur is a key aspect to ensure that the plant is getting the nutrients that it needs throughout the season after high-yielding crop years."

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