With wheat topdressing season soon approaching, producers may be wondering if it would pay to add chloride or sulfur to their topdressing blend this year. Both are highly mobile nutrients in soils so split or topdress application may be beneficial, especially in regions of sufficient precipitation or with coarse texture soils that may cause leaching.
Chloride
One of the main benefits from good chloride (Cl) nutrition is the improvement in overall disease resistance in wheat. Wheat response to chloride is usually expressed in improved color, suppression of fungal diseases, and increased yield. It is difficult to predict whether chloride would significantly increase wheat yields unless there has been a recent soil test analysis for this nutrient. Chloride fertilization based on soil testing is slowly becoming more common in Kansas.
As with nitrate and sulfate, chloride soil testing is recommended using a 0-24" profile sample. More field testing is needed, particularly in western Kansas, to determine the extent of the chloride-deficient areas, and to improve soil test correlations and calibrations. But based on current data, the probability of a response to Cl in dryland wheat production in northeast and central Kansas seems higher than in western Kansas.
The interpretation of the Cl test and corresponding fertilizer recommendations for wheat are given in the table below. Chloride fertilizer is recommended when the soil test is below 6 ppm, or 45 pounds soil chloride in the 24-inch sample depth. Potassium chloride (potash) and ammonium chloride are the most commonly available and widely used fertilizer products, though other products such as calcium, magnesium and sodium chloride can also be used.
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Soil Test Chloride Interpretations for Wheat in Kansas |
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Soil Chloride in a 0-24 inch sample |
Chloride Recommended |
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Category |
lbs/acre |
ppm |
lbs Cl/acre |
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Low |
<30 |
<4 |
20 |
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Medium |
30-45 |
4-6 |
10 |
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High |
>45 |
>6 |
0 |
Deficiency symptoms appear as leaf spotting and are referred to as physiological leaf spot.
K-State has done considerable research on Cl applications to wheat since the early 1980’s, mostly in the eastern half of the state. Results have varied, but there have been economic yield responses in almost all cases where soil test Cl levels have been less than 30 lbs/acre. Deficiencies were most likely to be found on fields with no history of potash (KCl) applications. Early studies showed that there are variety differences in response to Cl, but no recent studies have been done on this.
Reports on the majority of these studies can be found in the Kansas Fertilizer Research Reports, published annually and available online.
For more information, see “Chloride in Kansas: Plant, Soil, and Fertilizer Considerations,” MF2570: www.ksre.ksu.edu/bookstore/pubs/MF2570.pdf
Sulfur
Sulfur (S) deficiency on growing crops is often mistaken for nitrogen (N) deficiency. However, unlike N where the older leaves show firing and yellowing, with S deficiency, wheat often becomes uniformly chlorotic. Like N, deficiencies normally occur in patches in the field -- especially areas where erosion has occurred, soil organic matter is reduced, or leaching is more pronounced. In recent years deficiencies have been showing up early in the spring, shortly after green-up, before organic S is mineralized from soil organic matter, and before wheat roots can grow into the subsoil to utilize sulfate accumulated there.
The pale yellow symptom of S deficiency often appears first on the younger or uppermost leaves, while N deficiency initially appears on the older, lower leaves. Deficiencies of S are often difficult to identify because the paling in-crop color is not always obvious. Crops lacking S also may be stunted, thin-stemmed, and spindly. In the case of wheat and other cereal grains, maturity is delayed.
Figure 1. Sulfur-deficient wheat. Photo by Dave Mengel, K-State Research and Extension.
Figure 2. Examples of patterns of sulfur deficiency in wheat fields. Photos by Dorivar Ruiz Diaz, K-State Research and Extension.
Sulfur is usually present in relatively small amounts in soils, and a majority is “stored” in organic forms. Sulfur-deficient soils are often low in organic matter, coarse-textured, well-drained, and subject to leaching. In recent years, finer-textured soils have shown S deficiency.
A soil test for available sulfate-S in the soil profile is useful and should be done prior to planting. For proper interpretation of this test, soil organic matter, soil texture, the crop to be grown, and the expected yield level also need to be factored in to accurately assess S needs. Since sulfate-S is mobile, sampling to a 24-inch depth is important. Accurate estimates of S needs cannot be made from a surface sample alone.
Significant amounts of plant-available sulfate-S can be added to the soil through irrigation water. In Kansas, the S content of irrigation water varies, but in some cases enough S could be added through irrigation to meet crop needs. The S content of irrigation water should be determined by testing and factored into S applications. The timing of irrigation may not coincide with plant S needs, however. If it is well into the growing season before the first irrigation is made, the plant may be S deficient early, even though more than enough S will eventually be applied during the growing season.
There are many S-containing fertilizer materials available. Several dry materials are available which can be blended with dry phosphorus or nitrogen fertilizers for preplant or winter/spring topdressing.
- Elemental S (typically 90-95 percent S) is a dry material marketed by several manufacturers. Before it becomes available for plant uptake, elemental S must first be oxidized by soil microorganisms to sulfate-S and this can be a slow process when surface-applied. So it is best suited for preplant applications. One benefit of elemental S is the potential residual effect that can provide S for multiple years.
- Ammonium sulfate, AMS (21-0-0-24S) is a dry material that is a good source of both N and S, but has high acid-forming potential, and soil pH should be monitored. Ammonium sulfate is a good source to consider for topdressing to correct existing sulfur deficiencies.
- Gypsum (analysis varies) is calcium sulfate, and is commonly available in a hydrated form containing 18.6 percent S. This material is commonly available in a granulated form that can be blended with other materials. Since it is a sulfate source, it would be immediately available, and is another good source for spring topdressing.
There are also liquid sources of sulfur fertilizers available.
- Ammonium thiosulfate (12-0-0-26S) is the most popular S-containing product used in the fluid fertilizer industry, as it is compatible with N solutions and other complete liquid products.
- Potassium thiosulfate (0-0-17-17S) is a clear liquid. Potassium thiosulfate (KTS) can be mixed with other liquid fertilizers.
Application guidelines supplied by the manufacturers caution that these products should not be applied in a foliar application or as foliar sprays to growing plants. Topdressing with thiosulfate and UAN can be done early, before Feekes 5, and at temperatures below 70 degrees. But some burn can be expected, especially with KTS.
For more information see “Sulfur in Kansas” www.ksre.ksu.edu/bookstore/pubs/MF2264.pdf
For estimations of required application rates of S see “Soil Test Interpretation and Fertilizer Recommendations” http://www.ksre.ksu.edu/bookstore/pubs/mf2586.pdf
Dorivar Ruiz Diaz, Nutrient Management Specialist
ruizdiaz@ksu.edu
Dave Mengel, Soil Fertility Specialist
dmengel@ksu.edu
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