Will AMS Be There in the Spring If I Apply It Now?
Nov 28, 2011
Question: Will sulfur applied in the fall as AMS be there in the spring, or will it leach out?
Answer: This is a lengthy answer, but I hope it helps. As with nitrogen, organic sulfur, contained in the soil and in crop residue, is mineralized by microbes into the inorganic form that plants can use. But other microbes are also at work, converting inorganic sulfur into the organic form. This process is called immobilization.
Mineralization and immobilization go on all the time. Sometimes we have net mineralization, and other times we have net immobilization.
Often, when soil warms up in the spring and soil microbes increase, there is a period of net immobilization. That creates a shortage of sulfur available to young corn plants. I call this situation the "carbon penalty" because the supply of carbon in old-crop residue causes increased microbial activity. Sulfur levels will drop like a rock.
If nitrogen is temporarily immobilized, plants can take it up later, when it becomes available. But, because sulfur is not mobile inside plants, they must take up a large portion of their supply in the spring.
So failing to apply sulfur to help pay the carbon penalty can have season-long effects in continuous corn.
If a field is flooded and the oxygen is removed, anaerobic microbes convert sulfate, the available form of sulfur, into the unavailable form. If the field stays waterlogged for a week or more, the microbes can convert sulfur to a gas, and it can be lost.
You can tell when sulfur is volatilizing out of a field by the odor. If a field has been wet for a long time, you sometimes detect a septic smell. That smell is sulfur, leaving the field as a gas.
When soil is flooded, the first microbes to activate are denitrifying bacteria. The microbes that convert sulfur to a gas become active later. If you smell that septic odor, your nitrate nitrogen is already gone--although nitrogen in the ammonium form may still be present.
Sulfur also can be lost through soil erosion, and it can be leached away by water. But it never travels alone.
Sulfate ions cling to base cations (calcium, magnesium and potassium) like a cocklebur. So if you put on too much sulfur and then get a lot of rain, you can lose all three.
The first cation it typically takes is potassium. If you overapply sulfate to light sand or a silt loam soil and use ammonium sulfate as your total nitrogen source, you risk losing potassium. That is especially likely to occur on a sandy soil with irrigation.
Sulfur can become unavailable by fixing to clay particles in certain types of soil. A kaolinite clay, which is often found in the South, has negative charges on only one side of the clay lattice, and positive charges on the other side. Negative-charged sulfur ions fix to those positive sites. Such soils require more applied sulfur to get the same response. A montmorillonite clay, on the other hand, has negative charges on both sides, so sulfur ions do not attach.
Acid soils will also tie up sulfur. Because sulfur availability is controlled by microbial activity, balanced pH is essential. Acid soils reduce microbial activity, and that reduces sulfur availability.
When pH gets above 7.5, calcium and sulfur ions form gypsum, so you may have a lot of sulfur, but it can’t get into the plant because it is bonded with calcium.
Sulfates are not acidic (although elemental sulfur is). But if you apply ammonium sulfate, it takes twice as much limestone to neutralize the soil as it would if you applied the same amount of nitrogen as anhydrous ammonia, UAN (urea-ammonium nitrate) or urea. That’s because ammonium is twice as acidic as ammonia.
For every pound of ammonium nitrogen applied, it takes 8 lb. of limestone to neutralize the acidity. Remember to account for acidity in your sulfur sources.
In highly leachable soils, apply elemental sulfur in the fall, rather than sulfate, to prevent loss. In spring, use the sulfate form. Elemental sulfur may not become available quickly enough when applied in the spring.
High phosphate levels cause poor sulfur uptake. Corn roots load up with phosphate, which precipitates to the root lining. Negatively charged phosphate ions repel sulfur, reducing sulfur absorption, as well as zinc and manganese.
In high-phosphorus soils, carry a high sulfur load by applying some of your nitrogen as ammonium sulfate. Your goal is to pull the excess phosphorus out of the soil with your crop, while sticking sulfur back in.
Maintain the proper ratio between nitrogen and sulfur. Tissue testing gives an indication of the ratio of sulfur to nitrogen in the plant. Corn needs about 1 lb. of available sulfur per 14 lb. of available nitrogen.
Pay special attention to sulfur in soils that have less than 3% organic matter. Organic matter is your main supplier of sulfur. With more than 3% organic matter, sulfur probably will take care of itself.
There are different sulfur extraction processes and reporting methods. Don’t compare results from one lab to another unless you know the same process was used. One lab might consider a reading of 8 ppm to 12 ppm to be medium, while another lab’s idea of medium might be 25 ppm.