Potassium deficiency can decrease soybean yield more than 50% across soil types that range from sandy loam to clay loam, says LSU soil fertility specialist Rasel Parvej.
"Potassium deficiency also decreases phosphorus uptake and reduces soybean seed quality by decreasing seed oil and protein content and increasing purple seed stain," Parvej says.
He explains that potassium, the second most yield-limiting nutrient in soybeans, requires the most attention since the plant meets its own nitrogen needs.
He says soybean potassium deficiency is more common on Louisiana upland soils that are coarse-textured (light soils) with low nutrient-holding capacity, such as loamy sand to silt loam soils. "However, potassium deficiency can occur in any soybean field that has a very low to low soil test potassium level and is not fertilized with potassium."
He recommends soybean producers check early for signs of potassium deficiency, which may first appear as irregular yellowing on the edges of deficient leaves.
"As the growing season progresses and severity of potassium deficiency increases, the entire leaf edges turn brown, and eventually the whole leaf dies," he says. Those symptoms can occur as early as at the V3 vegetative stage (three trifoliate leaves) and primarily on the middle older leaves.
Symptoms may occur later, as well. "Symptoms often occur on upper younger leaves during the reproductive stages as late as at the R5 stage (beginning seed stage)," Parvej says.
Early management easier
He says soybean fields with potassium deficiency symptoms early in the growing season are easy to diagnose and manage. "However, many soybean fields often suffer from potassium deficiency and exhibit yield losses without showing visible deficiency symptoms, or at least not until the later reproductive stages (beginning seed, R5 to full-seed, R6)."
He calls this phenomenon "hidden hunger" and says it mostly occurs in fields that test low to medium in potassium, have not received potassium fertilization, have high leaching potential due to low cation exchange capacity (CEC) and experience excessive rainfall or undergo severe drought conditions.
Low pH factor
"Soybeans grown in low pH (<6.0) soils also suffer from hidden potassium hunger effects because low pH decreases soil potassium availability even after fertilization."
Hidden hunger diagnosis is difficult early in the season, Parvej says, and "requires thorough scouting, along with additional information such as fertilization history, soil texture, soil pH, soil-test potassium level, crop rotation, rainfall amount and distribution after fertilization and during the growing season and drought periods."
He recommends testing in-season to evaluate deficiency and to determine appropriate response. Tissue (leaf, including petiole) sampling, he says, "is the best and perhaps the only tool to diagnose soybean potassium deficiency during the growing season."
He says soil-applied dry or liquid potassium fertilizer is the best bet before the R5 stage. "Top-dressing or flying 100 pounds granular muriate of potash (0-0-60; 60 pounds K2O ) per acre would be the best bet for correcting potassium deficiency during the growing season. However, if the soybean canopy is not closed, liquid potassium fertilizer at the same rate can be injected 6 inches from soybean rows."
Early response is best, Parvej says. Producers can apply potassium as early as the deficiency is diagnosed visually in the field or when detected by tissue testing in the lab. They also may apply potassium as late as at the early R5 stage when seeds just start to form.
"Early potassium application will be better than late application in recovering yield losses. Producers can apply potassium as late as at the early R5 stage (about six weeks after blooming) when seeds just start to form and still rescue some potassium deficiency yield losses."
Parvej emphasizes the importance of identifying and responding to potassium deficiency early. "Potassium deficiency can easily be corrected by applying potassium fertilizer during the growing season," he says. "However, the effectiveness and economics of applying potassium to rescue yield loss depends on soybean growth stage and the severity of potassium deficiency.
"The earlier the growth stage at potassium application the more effective and economical it will be in recovering yield loss."