Don’t Forget Sulfur
While nitrogen (N), phosphorus (P), and potassium (K) may be your main fertility concerns, don’t forget other nutrients that could be costing you bushels at harvest.
“Sulfur (S) deficiencies are starting to show up more often,” says Ryan Van Roekel, Pioneer field agronomist. “It’s becoming a bigger problem.”
Sulfur is necessary for the synthesis of chlorophyll in plants. If chlorophyll is short, S-deficient plants will have yellow streaks between the veins of new leaves and general yellowing overall, says Van Roekel. S deficiencies can also stunt plant growth. Under severe S deficiencies, plants may not be fully formed, and they may not be able to pollinate well.
In soybeans, the plants will also appear yellow.
Since S is immobile, it cannot move from the older leaves to the younger leaves. This results in yellowing of the newer leaves toward the tops of the plant.
Alfalfa has a very high S requirement, says Van Roekel. Due to the high requirement, alfalfa will show signs of deficiency easily, he says.
Sulfur needs have changed
Historically, farmers never had to apply sulfur, says Van Roekel. This has changed in recent years due to environmental reasons and production practices. While S is naturally occurring in the soils, it’s declined due to two factors.
1. Less atmospheric S being deposited in the soil. That is the result of reduced power plant emissions.
2. Fewer fertilizer products containing S. Manure, which provides adequate levels of sulfur, is used less than it was in the past.
Unless your nutrient-management plans will soon include manure applications, neither of those factors will change. Identifying shortages and replacing crop removal of S can help you stay ahead of yield losses.
Another consideration for S is whether you remove the residue from the field. If you remove stover, you’ll want to compensate for the removal of S in the stover, says Van Roekel.
“You need to start incorporating sulfur into your fertility plans,” he says. “It needs to be considered like N, P, and K in corn.”
Many ways exist to get sulfur into your fertility program, he says. The trick is figuring out what works best for your management program.
First, examine fields and organic matter levels, says Van Roekel.
Calculate the sulfur removal with the grain yield to understand how much sulfur needs to be added back to the soil. In 200-bushel corn, 16 pounds of sulfur is removed per acre. Likewise, in 70-bushel soybeans, 13 pounds of sulfur is removed per acre. Add these amounts back into the soil.
Soil tests aren’t reliable for detecting sulfur, which may leave you questioning whether you need to apply S, says Peter Kyveryga, Iowa Soybean Association’s director of analytics.
Sulfate, which is like nitrate, can leach from the soil. That makes soil testing difficult, says Van Roekel.
Fall and winter moisture can lead to S leaching. In-season tissue testing for sulfur is an option for checking if the corn plant is short of S, says Kyveryga. It should be viewed as a diagnostic tool for a deficiency in the plant.
Tissue sampling is considered more reliable than soil sampling for sulfur, says Van Roekel.
If you see a sulfur deficiency, you’ll want to address it as quickly as possible, says Van Roekel.
“S deficiencies won’t be found throughout the entire field,” says Van Roekel. “That creates a variable-rate opportunity.”
Because a lot of S comes from organic matter, check sandier soils for deficiency along with the sidehills, suggests Van Roekel.
If you catch the deficiency early enough in the season, you can still save the crop.
If you’re going to try to save any yield, you have to do it as soon as possible before tassel, says Van Roekel.
Yield responses can be dramatic depending on how severe the deficiency is, he says.