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Wednesday, October 1, 2014

Assessing your Need for Sulfur Application in Corn

By Daniel Kaiser, Extension Soil Fertility Specialist

It is important to understand where sulfur that is utilized for crops comes from in order to determine where to best target fertilizer application. In Minnesota, sulfur was not recommended for many crops grown on medium and fine textured soils. Numerous studies were conducted during the 1970's, 80's, and 90's with little to know positive benefits shown except for a limited number of studies where corn was grown on eroded soils. Over the past 10-15 years reports increased as to sulfur deficiencies and research has found that sulfur may be needed for crops most sensitive to sulfur deficiency.

Soil tests for sulfate-sulfur only account for a small fraction of the total amount of sulfur in the soil.  Soil organic matter is a large storehouse of sulfur with as much as 95% of the total sulfur contained in organic matter.  Sulfur in organic matter must be mineralized to sulfate before it can be taken up and assimilated by plants.  It is within this process of mineralization which can explain, in many instances, why responses to sulfur fertilizer have increased.

The total amount organic matter in a soil has a significant impact on whether sulfur fertilizer is required.  Soils with low soil organic matter have a smaller pool of sulfur that can be mineralized.   A general rule of thumb is that somewhere between 3-5 lbs of S, as sulfate, can be mineralized per acre per year for each percent organic matter in the top six inches of the soil.  Soils with 3.0% or less organic matter have demonstrated a more consistent need for some sulfur to be applied for corn or alfalfa.  Targeting fields that fit within this range is justified as yield can be greatly increased with as little as 10-15 lbs of S broadcast on an annual basis.

Some of our initial concern with corn on corn systems was sulfate tie up during the decomposition of corn stalks.  However, out data has not shown there to be a high potential for tie up as most of the corn stalks analyzed should have enough sulfur for microbes to break down residue without having to utilize sulfate within the soil.  Mineralization is a process that is affected by soil temperature.  As soils cool the amount of sulfate mineralized slows.  While average air temperature plays a big role in this, soils can also remain cooler under dense mats of crop residue.  This effect seems to be greater in soils with poor internal drainage that will remain wet for longer periods of time.  In high residue continuous corn systems low rates of broadcast sulfur (10-15 lbs S per acre) or a low rate of ammonium thiosulfate (1-2 gallons per acre) banded on the soils surface to the side of the seed row have been shown to aid in stand establishment and potential increase yield.
Red Wing soil sulfate data
Figure 1. Soil sulfate data from Red Wing.

Sulfate carried over in soil from one year to the next is a major pool of sulfur that needs to be considered.  While sulfate is mobile in the soil, the rate of leaching is not as fast as nitrate.  If enough sulfur is applied there may be benefits for crops grown one or more years after application.  In a study funded by the Minnesota Agricultural Fertilizer Research and Education Council (AFREC) we have studied residual soil sulfate levels each fall to a depth of two feet in a corn soybean rotation.  Sulfur was applied before each corn crop in the spring at a rate of 25 lbs of S per acre.  The figure to the right summarizes data from a field site on a silt loam soil near Red Wing which is representative of a medium soil with good internal drainage.  Fall soil samples from 2011 show that was a higher concentration of sulfate sulfur in the top six inches indicating very little movement of sulfur applied the previous Spring.  After the 2012 soybean crop elevated concentration of sulfate could still be detected at one to two feet below the soil surface indicating some movement of sulfate but there still was some of the sulfur that was applied in 2011 that was detectable.  The soil sulfate data only represents concentrations so we do not know the total amount of sulfate still available.  The data only gives an indication that some of the sulfate was still within two feet of the soil surface.
Sulfur deficiency in corn
Figure 2. Sulfur deficiency in corn.

The answer may be no.  Some early season striping is expected in many fields.  However, if soils warm and the visual symptoms disappear at the V5 growth stage it is doubtful that any yield potential was lost.  If symptoms persist sulfur can be applied as a side dress up to the V5 growth stage with no expectation of lost yield.

Low corn prices may cause some to question certain practices such as applying sulfur fertilizer.  Corn producers in Minnesota have increased their usage of sulfur although benefits may not be likely under all circumstances.  The greatest risk for yield loss in corn is found in low organic matter soils or in continuous corn systems on poorly drained soils.  Applying 10-15 lbs of broadcast sulfur provides the best value if your fields fit with the criteria listed above.  Total yearly uptake of sulfur in a corn crop can range from 20-30 lbs of S per acre with roughly half being removed by the grain.  Rates higher than 15 lbs of S are typically not needed for medium and fine textured soils.  Some sulfate will be mineralized from the soil so take advantage of this free source of plant nutrients.  If sulfur fertilizer needs to be applied, sulfate containing fertilizers provide a source of immediately available sulfur to the plant.  Elemental sulfur may work in the long-term but oxidation of this material to sulfate is slow and is greatly impacted by soil temperature.  Having some sulfate applied in the spring in fields prone to sulfur deficiency can be critical to ensure a crop can get off to a good start.

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