Stuck in a soybean yield rut? Part of the problem could be proper plant nutrition, says Brad Hammes and Alan Peachey, agronomists for Helena Chemical.

Potassium and phosphorus are both critical nutrients for root growth. Phosphorus plays an important role in photosynthesis and seed fill. 

Soybean plants require more P and K as the season progresses, Hammes explains, especially when soybean plants hit the R3 growth stage. At R3, soybean plants have a lot going on: vegetative growth, flowering, pod development and seed fill.

At the same time, soybean plants move into the second half of P uptake. And they have a substantial K requirement: more than 9 pounds per day during full bloom to early pod set, and more than 2 pounds per day during pod fill, Peachey adds.

It’s at the critical R3 growth stage, Hammes says, that soils may be running out of P and K.

Surveys done by the International Plant Nutrient Institute from soil testing labs like Midwest Laboratories, A&L and Way Point Analytical, show that in a lot of the Midwest, more than 40% of soils are P-deficient, and more than 30% are K-deficient.

And most fertilizer applications go out before corn, not soybeans, Hammes notes.

“Soybeans are left to scavenge for what’s left,” Hammes says. “And if it’s been a year and a half since fertilizer went out before corn, and soils are already deficient, maybe it’s time to add starter fertilizer before soybeans.”

R3 BREAKDOWN: Alan Peachey, agronomist with Helena Chemical, describes the critical R3 growth stage to Illinois SoyAdvisors field day attendees.

While there is no direct correlation between pounds of phosphorus and bushels of soybeans — like there is with pounds of nitrogen and bushels of corn — Hammes notes late-season phosphorus does impact pod development and seed fill.

Research conducted by the University of Illinois, University of Wisconsin and University of Minnesota shows that fertilizer applications before soybeans resulted in more pods per node, Hammes explains. More pods per node causes a chain reaction: More pods per node equals more seeds per node, and more seeds per node on five to seven nodes could add up to 5 to 7 more bushels per acre.

Hammes adds that an additional yield benefit from starter fertilizer can occur but varies according to soil types, variety maturity and planting dates. Late-planted or earlier-maturity soybeans will not respond to starter fertilizer as much as early-planted, full-season soybeans. Soils with low organic matter will also show a better response.

Application rates for starter fertilizer in soybeans are lower, Hammes notes. Two gallons per acre of a low-salt starter is ideal, as soybeans are more sensitive to salt.

Another benefit from in-furrow starter fertilizer is getting the crop up two to five days earlier, Hammes explains. “That translates into more calendar days and more time to grow the plant before it starts flowering,” he says. “Every additional node gives us the opportunity to add pods.”

Bigger, healthier plants are able to fend off diseases and canopy earlier to help with weed control, Hammes adds.