During harvest, nutrients that are necessary to the vitality of a crop are removed from the soil. We’re all familiar with potassium, nitrogen and phosphorous and how they’re essential ingredients for crops to flourish. However, they aren’t the only valuable nutrients that contribute to soil fertility. Let’s talk a bit about the unsung heroes of soil fertility — sulfur and zinc.
Sulfur
Sulfur is considered the fourth major nutrient in crop production and is necessary for protein synthesis in a plant. Protein synthesis, when supported by sulfur, promotes plant vitality and development, including pollen development. Most sulfur in the soil is located in the organic matter and cannot be taken up by the plant until mineralization, which is when sulfur is converted to sulfate, or SO4-2, by bacteria in the soil. Sulfate is mobile and, in many cases, acts like nitrogen, which is why it’s recommended to apply sulfate at the same time as nitrogen. Sulfur deficiency even mimics nitrogen deficiency, resulting in yellowing of leaves.
Sulfur deficiency is generally a result of a few events.
Harvest: It is estimated that harvest removes approximately .1 of 1 pound of sulfur per bushel of corn and .17 of 1 pound per bushel of soybeans.
Deposition: Reduced greenhouse gas emissions in the atmosphere means less sulfur dioxide is deposited back into the soil. This situation is largely in part because of the increased restrictions on these emissions.
Soil matter: Soil makeup may affect the availability of sulfur. Mineralization, the conversion of sulfur to sulfate, occurs only when organic material in the soil exists — organic material decomposes and encourages mineralization. If mineralization does not occur because of lack of organic material in the soil, then sulfur becomes immobile, resulting in sulfur deficiency.
Zinc
Zinc plays a critical role in plant growth. In fact, without zinc, the enzymes that are responsible for plant development would not be able to complete their metabolic reactions in the plant, which would stop the growing process. Zinc is responsible for transporting calcium through the plant and is necessary for the production of chlorophyll and carbohydrate metabolism. When there’s a lack of zinc in the soil, leaves begin to yellow and eventually turn a bronze color.
Zinc deficiency is usually caused by one of a few different factors.
Harvest: Whether it was simply used up by last year’s crop, removed from the top soil during combining or compacted in the soil by heavy equipment, harvest can play a factor in diminishing zinc from soil. We recommend soil sampling in fall to detect for nutrient deficiencies, including zinc, especially in areas that generated high crop yields in 2018.
Soil type: The type of soil can also affect zinc prevalence. Because zinc is a natural element that emits from rocks, areas where there are sodic or calcareous soils may already be equipped with the proper amounts of zinc. Sandy soils or soils containing low organic matter may benefit greatly from increased zinc applications. High pH levels in the soil may also inhibit the presence of zinc, and cold soils in the spring limit uptake of the nutrient. The more you know about the levels of zinc in your soil, the better off you’ll be come spring.
Previous nutrient applications: Other nutrient applications can affect the production of zinc. According to the University of Minnesota Extension, there is a direct relationship between phosphorus and zinc, where large levels of phosphate can antagonize the micronutrient, resulting in zinc deficiency and, eventually, yield loss.
Application Recommendations
Both of these critical nutrients can be deficient within a plant because of unavailability within the soil biosphere. We recommend .10 pounds of sulfur per bushel of corn and .17 for soybeans. For example, for 200 bushel per acre corn, this would require about 200 plus pounds of nitrogen and 20 pounds of sulfur per bushel. Zinc should be added with a pre-plant fertilizer according to soil tests and crop removal rates. The key to any proper fertility plan is balance.
If one nutrient level is too high, another nutrient in comparison will be too low. Make sure you have a well-rounded fertility program in place that understands the unique characteristics of your individual soil’s composition and how to best manage both its strengths and weaknesses.
Additional Resources
https://www.agry.purdue.edu/ext/corn/news/timeless/sulfurdeficiency.pdf
https://www.sulphurinstitute.org/fertilizer/
https://extension.umn.edu/micro-and-secondary-macronutrients/zinc-crop-production