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Sulfur (S) deficiencies are becoming more commonplace due to less deposits from the atmosphere and higher crop yields. I think it is time to add sulfur to the list of important nutrients so the Big 3 now becomes the Big 4: N, P, K and S. Of course the amounts required differ considerably, but all four are important in soybeans.

Nutrient requirements for 50-bushel soybeans

http://www.ipni.net/ppiweb/ppinews.nsf/0/2aa6a1b33813f7cb8525691c005bd1ae/$FILE/98105-NRU-Soybeans.pdf

Sulfur is an important component in proteins that comprise metabolic enzymes in the plant and storage proteins in the seed. Sulfur is essential for plant protein formation. For a long time it was recommended that S be supplemented on sandy or light soils or soils low in organic matter, and to apply it on previous grain crops like corn or wheat and let soybeans scavenge for what they needed.

How much S do soybeans need and when do they need it?

Soybeans require between 20 and 30 lbs. of S per acre and the majority of that S is needed during pod development and seed fill.

Sulfur is a critical nutrient to make required proteins. Total uptake of S in soybeans is about 0.5 lbs. per bushel produced. While this pales in comparison to the total nitrogen (N) requirement, the crop can’t maximize its yield potential unless S is available at the right time in non-limiting concentrations.

Dr. Fred Below, crop physiologist with the University of Illinois, has studied nutrient uptake in soybeans. Sulfur uptake picks up at R2 as soybeans are just beginning to form pods and continues through R6 as pod fill ends. This is a similar pattern to nitrogen. Both nutrients are important components of protein.

When is the right time?

S uptake occurs over the entire growing season, with relatively constant uptake beginning with pod set (R3), very similar to nitrogen. And S is also immobile within the plant, which means the plant is unable to compensate for low levels of S that may occur late in the season by cannibalizing S from older leaves to new leaves.

What form does sulfur need to be in to be available for uptake?

Plants take up S as sulfate (SO4). The two common forms of S available are sulfate (AMS, calcium sulfate, thiosulfates) and elemental form. Sulfate can be mineralized from organic matter (about 1 lb. of S for 7 lbs. of N). Elemental sulfur is broken down over time by microbes into sulfate.

HJ Baker and the ILSoyAdvisor are sponsoring a webinar on sulfur requirements and management in soybeans. Wesley Haun, agronomist with HJ Baker, will present the webinar in early March. Stay tuned to ILSoyAdvisor for further announcements on this webinar.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.