While 2015 will rightly be remembered as the year when supplemental nitrogen applications paid huge dividends in much of the Corn Belt, post-planting applications of sulfur often returned strong results for growers as well. The question many are asking is why do we need to add sulfur when it hasn’t been a problem for years?
Sulfur deficiencies are more common in grower fields for a number of reasons:
- Reduced power plant emissions of sulfur that were deposited via rainfall. Many areas of the Midwest receive 10-20 lbs./acre less sulfur from this source.
- Higher yields remove more sulfur
- Fewer sulfur containing pesticides are used in today’s agriculture
- Reduced tillage reduces the mineralization of sulfur into our soil mainly due to cooler temperatures
The main source of sulfur for crops (other than inorganic fertilizers) comes via the mineralization of soil organic matter, however the soil releases a limited amount of sulfur via this avenue; roughly 2-5 pounds of sulfur are mineralized for every percent organic matter. Therefore, if you have a 3% organic matter soil and a good mineralization year you might expect to mineralize roughly 10-15 pounds of sulfur. However, a 200-bushel corn crop requires approximately 26 pounds of sulfur to produce that crop, so if we get 15 pounds via mineralization, where does the rest come from? In the past it generally came from rainfall deposition, but since that source has nearly been eliminated we now must supplement the crop with fertility.
One consideration for fertilizing a crop is that the sulfur gets to the plant in the sulfate form (SO4–) mostly via mass flow of soil water. Since the SO4 ion is negatively charged like your soil and moves with soil water it can move quickly through your soil profile especially in course textured soils. The other consideration is that the corn plant uses a little over 50% of sulfur after tasseling and because sulfur is immobile in the plant we must continue to supply sulfur for plant uptake.
This past year we witnessed a confluence of events that tended to exacerbate the issue.
- Cool spring temperatures and lack of soil oxygen slowed the mineralization of Organic sulfur
- Heavy rainfall moved sulfate molecules down through the soil profile while roots were more shallow
- Lower organic matter soils and/or no-till fields were of particular concern
As we increase our yield levels, the need to supply the plant with adequate fertility will require growers to adopt technology to meet the needs of the crop – often with elements like sulfur that we have not been concerned with before. For more on how your corn crop utilizes sulfur, watch our “Sulfur in Corn” video.