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Important Differences in Fertilizer Materials Containing Sulfur and Zinc

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by J Stevens, Associate Professor and Extension Soils Specialist

As of recent, I have become aware that there are some fertilizer dealers who might be unaware of some of the differences in the fertilizers they are handling and selling to producers. Let’s take Sulfur and look at it first.

The use of sulfur in soil fertility programs has become more routine. The most common chemical forms of sulfur used in fertilizers are sulfate-sulfur and elemental sulfur. However, these two forms of sulfur react quite differently in the soil. It’s very important to understand the differences between sulfate-sulfur and elemental sulfur in order to use these two forms in the most effective manner possible.

Plants can only absorb sulfur through their root system in the sulfate form. Thus all soil sulfur must be converted to a sulfate in order to be utilized by plants. For the most part, sulfates move freely with soil moisture, especially in the upper part of the soil profile. This is very much like nitrate-nitrogen in soils. As a result, sulfate levels frequently increase with increasing depth in the soil profile. Like nitrates, sulfates can leach in sandy-textured and silt loam  soils.

Elemental sulfur is totally unavailable to plants. Plant roots cannot absorb elemental sulfur. Elemental sulfur is inert and is water insoluble. When elemental sulfur is added to a soil, it has to be converted to the plant-available sulfate form through the activity of soil bacteria. The rate at which this conversion takes place is the determining factor regarding the effectiveness of elemental sulfur as a fertilizer source of sulfur. This transformation of elemental sulfur to the plant-available sulfate form is a slow process often taking months to be accomplished. Thus, for most crops in the initial sulfur fertilization, a sulfate fertilizer like Ammonium sulfate is recommended and elemental sulfur is not.

Now, let’s look at Zinc, specifically zinc sulfate and zinc oxysulfate.  Most of the formulations of these two products contain 35.5 – 36% zinc. Among the inorganic zinc sources on the market, the most common sources are sulfates, oxides, and oxysulfates. Zinc sulfate is essentially 100% water soluble, while the Zinc oxides are essentially insoluble in a single crop year, thus unavailable to the crop to be planted. Many agronomists consider the oxides to be ineffective as a fertilizer source. Oxysulfates are a mixture of sulfates and oxides, with varying proportions of sulfates and oxides. The solubilities of the oxysulfates vary considerably, from 0.7 to 98.3%. The effectiveness of these can be highly variable. Low solubility materials may have some value in a long-term build up program, however, when immediate results are the goal, highly soluble fertilizers are the best choice. It is suggested that in order to be effective, a Zinc fertilizer should be at least 50% water soluble.

I’ll leave you with a few questions to ponder; Are you using sulfur and/or zinc in your soil fertility program? Are you soil testing to determine if your crops could benefit  from adding one or both of these nutrients? If you are applying sulfur and/or zinc, have you ever considered which form is being field-applied?  The answers to these questions could lead you toward a better soil fertility program and enable your crops to improve their yields as they come closer to reaching their genetic potential.

If you have any questions on this article or would like to discuss your soil fertility program, please feel free to contact me by email JStevens@agcenter.lsu.edu ,  telephone, 318-308-0754 cell, or text.

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