Supplement with sulfur for plant growth, health

Written by Scott Wilburn on .

Higher yields, earlier planting and successful clean air policies have increased the need for sulfur fertilizer and put a much-deserved spotlight on this often-overlooked nutrient. New research is being conducted at the university level and by MFA to determine the best management practices for sulfur on your farm.

Sulfur is a vital contributor to nitrogen efficiency, protein produc­tion and chlorophyll formation. In fact, sulfur is ranked only behind nitrogen (N), phosphorus (P) and potassium (K) in importance to crop production. Plants low in sulfur are often pale and yellow, symptoms that may be confused with nitrogen deficiency. Sulfur in the sulfate form does share sever­al characteristics with the nitrate form of nitrogen. It is mobile in the soil and subject to leaching, especially in soils with lower cation exchange capacity. It is also difficult to determine sulfur need through soil testing because of spatial and temporal variability.

However, unlike nitrogen, sulfur is generally immobile within plants. We recognize “firing” on the lower leaves of a corn plant as a sign of either N or K deficiency. This is because these nutrients are mobile in the plant and can be moved from old growth to new growth. Sulfur deficiency shows up on the young­est growth first and must be supple­mented from outside the plant.

Clean air policies have done an excellent job removing sulfur from the atmosphere, resulting in much less “acid rain” than in the past. As a grade-schooler in the 1980s, I was regularly taught about acid rain and its negative impact on the environment as well as the damage it inflicted on man-made objects such as machinery, buildings and other structures. Acid rain is rarely discussed today. There is still some sulfur in rainfall due to pollution and the natural activity of volca­noes, but it is much less compared to previous years. This is obvious­ly a positive, but the free 15-20 pounds of sulfur that Midwest fields received in the past have been reduced to 5 pounds or less and must be replaced with fertilizer.

Sulfur fertility is delivered in three main forms: elemental sulfur (90%, dry), ammonium sulfate (21-0-0-24S, dry) and ammonium thiosulfate (12-0-0-26S, liquid). Ammonium sulfate and ammonium thiosulfate are readily available to crops. Elemental sulfur must be ox­idized or “broken down” before it is available, so for a quick response in front of or over a crop, ammonium sulfate or ammonium thiosulfate should be used. That is why these products are our primary sources for wheat top-dress.

Another source of sulfur utilized by growers is Croplex, which has an analysis of 12-40-0-10S-1Zn- 0.3B. The sulfur in Croplex is a combination of elemental and ammonium sulfate. This “best of both worlds” approach helps assure there is plenty of sulfur throughout the growing season.

Sulfur is often associated with wheat top-dress and for good reason. Wheat needs sulfur when it breaks dormancy and begins to green up. Much of our sulfur comes from the soil’s organic matter through the mineralization process, and the cold soils of late winter/ early spring are not conducive to this activity. However, wheat is not the only crop that uses and needs sulfur. In fact, it is not the crop that needs it most. Producing 80-bushel wheat takes a total of 18 pounds of sulfur, but 180-bushel corn will use 29. Soybeans will use 27 pounds of sulfur to make 60-bushel yields, while 5-ton alfalfa uses 25 pounds.

Recognition of the need for increased sulfur has prompted recent research into the nutrient. Purdue University in Indiana has several years of plot data that shows response to sulfur, not only in corn but also in soybeans. Sulfur applied near planting (from planting to V2-V3) to early-planted soybeans resulted in significant gains. More surprisingly, this occurred on soils with higher-testing (greater than 3.5%) organic matter. The cool, wet soils that early-planted soybeans encounter seem to be the catalyst for these gains.

Magazine articles discussing this research last year generated interest and questions in MFA’s trade terri­tory, so MFA added soybean sulfur studies to our field trials in 2021. We did not have early-planted soy­beans in our trials last spring so we were unable to do that research, but we will look at it in the future.

If you are planting soybeans early this year, I would encourage you to consider an at-planting or early postemergence application of sulfur to evaluate this practice in your own fields. I would also encourage you to make sulfur a part of your overall fertility program, giving special attention to crops that are planted earlier into cool soils and to fields with lower organic matter values.