By Darren Hefty

Sulfur is critically important in all crops, yet there are consistently a large number of fields deficient in it across the country.  In just the last few years, we have seen good yield increases from applied Sulfur in many cases, especially in crops that use lots of sulfur, like corn and wheat.

You are likely familiar with a few of the reasons sulfur is less abundant in our soils than it once was.  Here are a few more that may surprise you:

  • Less air pollution.  Partially as a result of less sulfur in emissions, scientists are claiming sulfur deposits in our soils are decreasing by 5 pounds per acre per year.
  • Less manure application.  This is one that we saw on our own farm.  There are a lot of good things in manure in addition to N, P, and K.  Sulfur, micros, biologicals, etc., are big benefits that often go unmentioned in manure applications.
  • Less availability.  There is less sulfur in commonly applied fertilizers than there used to be.
  • More corn acres and less tillage leads to less mineralization (due to cooler soils when covered by residue) and less sulfur release.

Today, I want to focus on that last point.  I love reduced tillage!  Whether it’s no-till or strip-till or whatever, I believe it’s great to leave more residue in your fields in order to build organic matter over time, reduce erosion, and increase the holding capacity of your soil for water and nutrients.

One downside to reduced tillage is that more residue leads to cooler soils, which in turn means slower breakdown or mineralization of the organic matter.  For each 1% of organic matter in your soils, you’ll typically get two to three pounds of sulfur coming free each year.

Here’s where the carbon to sulfur ratio comes in.  If the carbon to sulfur ratio is

  • Less than 200 parts carbon to 1 part sulfur = your sulfur will likely be mineralized and made available.
  • Between 200 to 1 and 400 to 1 = your sulfur will remain in the form it’s in until the ratio changes.
  • Greater than 400 parts carbon to 1 part sulfur = your sulfur will be immobilized and unavailable for plant uptake.

In the grand scheme of things, you don’t have a lot of immobilization in your soils, nor do you get a huge amount of sulfur most years from mineralization.  Your crops are USING quite a bit of sulfur each year, and the consequence of that is that you need to fertilize with enough sulfur to supply each season’s crop.  Chances are you haven’t heard of the carbon to sulfur ratio before, so here are common results from testing the residues of corn, soybeans, and wheat.

Corn Residue = 350 parts carbon to 1 part sulfur (350:1)
Soybean Residue = 125:1
Wheat Residue = 300:1 

What we’re saying here is that if you have a lot of carbon (corn & wheat residue, for example), you may need to apply just a little bit more sulfur than normal.  Please make sure you consider applying sulfur in ANY crop you are planting, and keep the carbon to sulfur ratio in mind.