While visiting my customers in Southwest Illinois over the last few weeks, I have noticed pronounced yellow-green areas in the wheat fields. You’re now thinking, “What do problems in wheat have to do with soybeans?” Well, these areas (see photos below) are showing signs of sulfur (S) deficiency that have been confirmed with tissue and soil tests. Remember, S deficiency shows up on the young tissue because it is non-mobile in the plant, unlike N which shows up in the older tissues.
Sulfur is a secondary nutrient required for plant growth and is essential for: two amino acids which are building blocks for proteins, chlorophyll formation, N-fixation in legumes and gives the characteristic odor to some crops like garlic and onions. Sulfur comes from decomposing organic matter and rainfall. As a rough rule-of-thumb, about 15 lbs./acre/year of sulfate is released to the soil per percent of organic matter. Rainfall deposition is tricky. We used to receive 5 – 30 lbs./acre/year of sulfate. However, those numbers have steadily declined after the introduction of the U.S. EPA Clean Air Act and improved scrubber technology at coal-fired facilities (Figure 3). With the decrease in atmospheric deposition, we are now counting on the soil to supply the needed S.
The supply of S provided by the soil is a function of organic matter levels and rates of decomposition. However, S mineralization can be slow and is a function of temperature, moisture, residue levels and the carbon to nitrogen (C:N) ratio. The C:N ratio drives how quickly residues will decompose. Corn residue has a wide C:N ratio and decomposes slowly. In addition, the genetic packages in our corn hybrids have resistance to many fungal pathogens that inadvertently influence decomposition. This resistance can also affect the same groups of primary decay fungi, thus further slowing down the residue decomposition.
Sulfur chemistry in the soil is complex and soil or tissue results report “S” but the plant can only take sulfate. Therefore, one must understand what they see on a soil or tissue test report. I like to see soil test values for S to be in the range of 10 – 25 ppm (20 – 50 lbs./acre) and plant tissue values in the range of 0.2% – 0.5%.
The tissue results can vary depending on crop, crop growth stage and plant parts. Here is some data I recently collected from wheat fields (Table 1). Note that some fields both appear yellow and tissue tests confirm that S is low to deficient.
Table 1. Wheat Tissue and Soil Test S Results following Corn, spring 2016*
*All samples were from wheat fields following corn. Nitrogen values are adequate to excess for wheat. Sulfur sufficiency range for wheat: 0.2-0.4%.
So, “What do these apparent S deficiencies in wheat mean for soybeans this season?” There is the potential to experience S shortages or deficiencies in soybean fields in some areas. As I spoke with customers and discussed these areas in their fields, they admitted to noticing slightly decreased yields and differences in plant appearance across all crops for the past couple of years but really didn’t think much about it. You can have areas that may be slightly off in color while not showing a “true” visual deficiency symptom. These can be signs of “hidden hunger” in a crop.
If we are low in S in the soil, we can reduce soybean nodulation by B. Rhizobium japonicum, the soil bacteria that supplies N to soybean. (Proper soil pH and boron can affect nodulation.) This reduced nodulation could lead to a shortage of N to the soybean plant during pod set and seed fill. If we have an N shortage or are borderline low in S we may not reach the maximum yield potential of that soybean crop. For producers that feel their soybean yields have stagnated, I suggest taking some in-season tissue tests to look for these “hidden hungers.”
So, “How do we correct S deficiencies or low test values?” Correcting S in corn or wheat can be accomplished relatively easily by using ammonium sulfate (AS) or ammonium thiosulfate (ATS) of calcium sulfate (gypsum). Soybeans can be a little more challenging. I don’t like to use AS or ATS for soybeans at rates over 100 lbs./acre as too much N can also reduce soybean nodulation.
One can also use elemental S, but bear in mind that it is very insoluble in the soil and takes time to release and realize the added benefit. Synthetic gypsum from power plants is another option but not everyone has a local source—plus there may be some ease-of-application issues. Lastly, there are foliar supplements but they may be expensive and burn foliage. Therefore, consult with your CCA to start looking for S shortages in the soil and plant tissue. Then weigh your options and costs if you do in fact find shortages.
More information about sulfur deficiency is available here.
Terry Wyciskalla is an independent crop and soils consultant based out of Nashville, Illinois. He specializes in soil sampling, fertility recommendations, precision ag services and crop problem diagnoses. He serves a 12-county area throughout Illinois. He earned his 4R Nutrient Stewardship certification in 2015 and is happy to be a part of the 2016 Soy Envoy team.