I have worked with a couple growers that have made 100-plus bushels of soybeans per acre and numerous other growers that have made 90-plus bushels per acre. The one nutrient that has consistently been deficient in the tissue samples as the seeds begin to swell has been potassium (K).
The soybean plant requires huge amounts of K beginning at full bloom and especially during seed development (see the table below).
In our replicated on-farm trials, the key time to apply K is just prior to seed swell (see table 2). When we applied additional K at planting, we did not see a yield increase. The timing of the K application appears a key factor in ensuring the plant has adequate K during its peak demand for K.
Potash levels in the leaf rapidly decline during pod fill as the K is translocated from the leaf to the beans. K levels in the leaf need to stay above the adequate level of 1.75% in the leaves for as long as possible.
The increased yield when supplying adequate K during seed development appears to be due to larger seed. Cherokee County farmer Nick McMichen broke the 100 bushel soybean yield barrier with 102.5 bushels per acre in 2017 and credited his seed size as one of the primary contributions to his high yield. Our normal soybean seed count is 2,800 to 3,100 seeds per pound.
He averaged 2,140 seeds per pound with his record setting yield. He applied 100 pounds per acre of 0-0-60 and 40 pounds of ammonium sulfate per acre over-the-top of his soybeans at R3 stage (when the pods began to fill).
Potash is highly mobile in the plant so K deficiency symptoms first appear on the lower leaves and work their way to the top of the plant as K deficiency increases. The most common visual sign of K deficiency is yellowing along the margins of the leaves followed by, in severe cases, the leaf edges becoming brown. We often see deficient K levels in the tissue samples without seeing any visual symptoms.
Chicken litter before or at-planting also gave us a yield increase in our trials. This may be due to the slow release of nutrients. In reviewing the production practices of 100+ bushels per acre soybean growers in the SE over the last three years, most have applied 2-2.5 tons of chicken litter per acre to their soybeans.
Rutgers University’s Dr. R. Flannery Nutrient Requirements for 101-Bushel Per Acre Soybeans Study
Nutrient Uptake per Day (lbs/acre)
Soybean Stage Days N P2O5 K2O
3rd Trifoliate 40 0.8 0.3 0.7
6th Trifoliate 11 1.5 0.6 2.7
Full Bloom 16 7.8 1.8 5.8
Early Pod 15 9.1 2.3 9.6
Soft Seed 21 11.4 2.8 2.4
Total 548 136 344