Target soybean growth stages

BLOOMINGTON, Ill. — Over half of Illinois’ soybean acres were planted by early May and the focus now turns to assuring the plants have adequate nutrition for the season.

Jason Haegele, WinField United agronomist in Illinois and Illinois Soybean Association CCA Soy Envoy, gave nutrient management recommendations in a recent ISA-hosted podcast.

With growing season underway, Haegele said there are soybean growth stages that should be targeting in order to provide proper plant nutrition.

“We know that a lot of the yield potential for soybeans is affected during the reproductive stages later in the growing season. But I would start to think about some type of application based on a yield goal or maybe even using a practice such as tissue testing around R1 or R2 when the flowers are forming,” Haegele said. “We want to make sure that crop has adequate nutrition at that point so that flowers stay on the plant and don’t fall off the plant prematurely.

“The second time that I would think about looking at nutrition and ways to supplement nutrition would be at the R3 growth stage as we transition from flowering to pod-set. We should do anything we can from a nutritional or overall management perspective to keep those pods on the plant, ultimately provide adequate amount of nutrition to fill those pods out and realize full yield potential at the end of the season.”

The amount of nutrients a plant needs varies with the crop. Haegele used a goal of 100 bushels per acre for soybeans and what nutrients that yield would require.

“We’re looking at anywhere between 400 and 500 total pounds of nitrogen per acre in the crop, phosphorous is going to be about 72 pounds per acre, potassium is going to be 283 pounds per acre, sulfur nearly 30 pounds per acre and the key micronutrients of zinc, manganese and boron anywhere between 7 and 8 ounces of the nutrients per acre,” he said.

Crop Feeding

Dry or foliar products can be use to feed nutrients to the crop during the growing season, depending on the product.

“Soybeans need 4 to 5 pounds of nitrogen per bushel. Granted, with soybeans, a lot of that is going to be absorbed into the plant through the process of fixation with that rhizobia bacteria on the roots. But the amount of nitrogen that we might potentially need to supply through a foliar application would be a larger volume of product and anytime we think about large volumes of nutrients there’s a potential for damage to the leaf tissue,” Haegele explained.

“For micronutrients that are required in smaller amounts, we might only need to apply a pint to a quart per acre of the fertilizer product to supply the amount of nutrition we want to apply.

“The other consideration when deciding between a dry and a foliar-applied nutrient is the timing of when that nutrient is required. If it’s nutrient like nitrogen, phosphorous or potassium that’s going to be required in larger amounts and uptake is going to occur over the entire duration of the growing season, we really want to apply is as a dry soil-applied nutrient so it’s there in the soil for uptake over time.

“Whereas with a micronutrient or foliar nutrient that might be used to supplement key times during the growing season like around flowering, around pod-set we can get by with the smaller quantities liquid foliar feed.”

A wide variety of nutrients can be applied in foliar form. They’re typically either liquid sources or dry nutrients that easily dissolve in water for spraying on the crop.

Haegele said the foliar-applied nutrients will either benefit the leaf directly or go through a transformation process and move to other parts of the plant such as the developing pods and soybeans.

“Micronutrients are commonly foliar-applied on soybeans and other plants, as well. The reason for that is micronutrients are required in small amounts by the crop, so we don’t have to apply large amounts of fertilizer product to provide that requirement that the crop has,” he said.

“In the case of micronutrients, as well, manganese as a specific example, micronutrients are often involved in supporting photosynthesis which occurs in the leaves and so we can apply those foliar nutrients directly to the where they’re going to have the biggest impact when we make that application.”

Management Changes

In looking back at past soybean fertility programs, it was a common practice to apply nutrients, primarily phosphorous and potassium for the corn crop and then hope there was enough nutrition left over for the soybeans the following year.

“As time has gone on and farmers have gotten more and more excited about managing soybeans for higher yields, now we’re oftentimes applying fertilizer annually for both corn and soybeans individually and targeting fertilizer rates that are appropriate for each crop individually and not trying to do it all at once for both crops in a two-year type of cycle or program,” Haegele noted.

“Some of the things that are important to consider in soybeans are the amount of individual nutrients for a specific yield goal. There is the timing when nutrients are needed most. That’s going to vary somewhat from nutrient to nutrient and could potentially influence when we apply nutrients either before the crop is planted in-season with a foliar or dry application, and finally thinking about what the function of an individual nutrient is or where that nutrient is located in the plant that can also influence how we apply that nutrient as a foliar or as a soil-applied nutrient.”