By STACY CAMPBELL
Cottonwood Extension District
Sulfur (S) deficiency in wheat has become more common in Kansas, particularly in no-till wheat. The reasons for this is a reduction in sulfur being emitted to the atmosphere and cooler soil temperatures because of no-till, which slows S mineralization in the soil. Some crops in the rotation, such as soybean, can take up significant amounts of S.
In wheat plants S-deficiency is yellow and stunted and is observed in patches in the field, especially in areas where there has been previous soil erosion or soil movement. The patchy S-deficient areas of the field are often found on hilltops or side slopes where erosion has occurred and soil organic matter is reduced, or where leaching is more pronounced. Wheat in areas where topsoil was removed or significant cuts were made (i.e. terraced or leveled fields).
Sulfur deficiency in growing crops is often mistaken for nitrogen (N) deficiency. However, unlike N deficiency, older leaves show firing and yellowing, with S deficiency, the pale yellow symptoms appear first on the younger/uppermost leaves. Plants with S deficiency eventually become uniformly chlorotic (yellow leaves).
Sulfur 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 any available S (sulfate) accumulations. Deficiencies of S are often difficult to identify because the chlorosis is not always obvious. Crops lacking S also may be stunted, thin-stemmed, and spindly. In the case of wheat, maturity is delayed. Winter annual weed competition is also enhanced due to the slower growth and lack of tillering.
Sulfate is relatively soluble, it tends to leach down into the subsoil. In many Kansas soils, it will accumulate in the B horizon (subsoil) in two forms. Clay surfaces and coatings will retain some sulfate, and sulfate will also be present in the subsoil as gypsum (calcium sulfate).
A soil test for available sulfate-S in the soil profile is available. For proper interpretation of this test, soil organic matter, soil texture, the crop to be grown, and the expected yield level all need to be considered. Accurate estimates of S needs cannot be made from a surface sample alone. Since sulfate is mobile, a 24-inch sampling depth is needed. However, due to the relatively high demand for S during the rapid vegetative growth phase of wheat, and relatively shallow rooting by the wheat crop at this time, the S measured in the deeper, subsoil levels by the test may not be available to wheat in the early spring, especially where soils are cold.
There are many S-containing fertilizer materials. Several dry materials are available that can be blended with other fertilizers for winter/spring topdressing. However, some of these products are best used in pre-plant applications because they are slow in becoming plant available in the spring for topdressing. For more information on choosing the right S fertilizer material, go to www.cottonwood.ksu.edu scroll down the home page to timely topics.
Stacy Campbell is an Agriculture and Natural Resources agent in the Cottonwood District (which includes Barton and Ellis counties) for K-State Research and Extension. You can contact him by e-mail at [email protected] or calling 785-628-9430, if after hours please leave a voice-mail for a return call.