Calcium Products - Raising Sulfur Levels on a Soils Tests - Update from Iowa State
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Raising Sulfur Levels on a Soils Tests - Update from Iowa State

On Monday one of our customers asked the question, “How much increase of sulfur will I see on my soils report for each pound of sulfur applied?”

This is an interesting question, and unfortunately does not have a direct answer. It depends on many things, soil texture, soil organic matter, soil temperature, soil moisture, drainage, and soil microbiology.

It also depends on the type of sulfur you are measuring. By that I mean are you testing for sulfate, sulfide, or oxides. Most reports only test for the plant available form, the sulfate. This problematic since the sulfate form of S is an anion (negative charge), and therefore is leachable. As a rough rule-of-thumb, it can be considered to leach through the soil profile at about 50% as fast as nitrates (NO3-).
 
Most farmers don’t worry about “building” N on a soils report. Due to its leachablity it is applied every year. As with Nitrogen, Sulfur can be converted into unavailable forms, lost as a gas, or leached in irrigation or rain water. Excess sulfur does not normally present an agronomic problem for crops, but it is need to build soil organic matter.
 
One percent soil organic matter can have 140# of sulfur locked up in it. Microbial action will break this down overtime and make the nutrients in the organic matter available to crops. It takes warm moist soils for microbial conversions, so a winter test of sulfur may not show true growing condition levels.
 
Another consideration is that when drying soils at the lab, this process increase the sulfur number on a soils report. I have a number of questions out to agronomist and soils labs to weigh in on this and will be posting the answers here.
 
Finally, while it is important to build soil levels, it really only means something if it gets into the crop, this study done by FC in Rake Iowa shows a 20% increase in sulfur and a 3x increase in calcium!
 
Update: 
From John Menghini, Midwest Labs In response to sulfur levels increasing during drying of samples. "I am not sure on sulfur specifically and I think it would depend on the method that they are using for the sulfur analysis.  I know that a wet soil when extracted for cations which includes the sulfur for us will be low when the sample is wet.  I have also evaluated soils that were dried at very high temperatures even for a a short time (2 hours at 120 C), we see the sulfur value increase by approximately 20%. It is extremely important when drying soils that the percent of moisture remaining in the sample is minimal (less than 5%) and that the temperature in the drying chamber is not too high to impact the S or any other cations in a negative way."
 
From Joe Thelen, Midwes Labs in response to sulfur increasing on tests.
"There is no good rule of thumb for anions. They are very leachable and will move with moisture in the soil profile.  The timeframe from application to sampling is also an important factor. If sulfur is applied today and we sample next week, there will be a higher value.  If we sample next spring or summer, there is no guarantee.Elemental sulfur will be around longer, but is not as readily available for plant uptake."
 
From Dr. John Sawyer, Extension Soil Fertility Specialist and Professor in Agronomy at Iowa State. "The theory would be that each lb of S/acre would increase soil test by 0.5 ppm, if the applies S is within the 6 2/3 acre furrow slice. That is assuming a sulfate form (assuming the test measures sulfate-S) and no uptak

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  • Article Reference:: Calcium Products, Inc.
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