David Moseley and Leandro Vieira, LSU AgCenter Scientists; and Carrie Ortel, Virginia Tech Extension Soybean Agronomist

Potassium (K) deficiency in soybean can be both visible and hidden, and both forms can reduce yield. Severe deficiency is often easy to identify, appearing as yellowing along the leaf edges. However, a more subtle form (hidden hunger) occurs when K levels are insufficient to maximize yield but do not produce obvious visual symptoms. This is especially common in fields with low to medium soil-test K levels, making it critical for producers to proactively manage K fertility rather than relying solely on visual cues.

Soil-K Concentration

The first priority in K management is ensuring an adequate supply of soil-K concentration. Soil testing using Mehlich-3 extraction provides a reliable baseline for determining K status and guiding pre-season fertilizer recommendations, with application rates varying based on soil-test K levels.

Table 1. Interpretation of Mehlich-3 soil-test for potassium (K) (Vieira et al., 2025)

In-season Tissue Testing

Monitoring plant K status during the growing season is essential, especially when soil-K concentration is low or medium. Tissue sampling should begin at the R1 (beginning bloom) growth stage to ensure that the crop has sufficient K to support reproductive development. For a representative sample, at least 18 random trifoliates should be collected across the field. It is critical that the correct plant tissue is sampled to ensure consistency with interpretation guidelines. Specifically, the third fully open trifoliate from the top of the plant should be collected, excluding the petiole. Consistent sampling methodology allows for reliable comparison to established sufficiency thresholds.

Figure 1. The third open trifoliate from the top of the plant should be collected without the petiole for tissue samples.

Leaf-K Concentration Curves

Leaf-K concentrations should be evaluated relative to established critical concentration curves, which describe how plant K requirements change as the crop develops. These curves indicate that K concentration naturally declines as the plant progresses beyond R1. If measured leaf-K levels fall below the expected range for a given growth stage, a corrective in-season K application may be warranted. The potential to recover yield depends on both the severity of the deficiency and the timing of the application relative to R1. Research indicates that visible K deficiency can be corrected within 20 days of R1 and hidden hunger can be corrected within 44 days of R1.

Figure 2. The critical concentration curve for potassium in soybean from the R1 growth stage to 70 days after R1. The percent relative yield for each curve (95%, 85%, and 75%) assume no potassium fertilizer is applied in-season to correct potassium deficiency (Ortel, 2025).

Corrective Application for In-season K Deficiency

For cases of hidden K deficiency, a general recommendation is to apply approximately 60 pounds of K₂O per acre. However, ongoing research by the Science for Success team (a group of soybean agronomists) aims to refine these recommendations by better defining optimal application rates based on the degree of deficiency and growth stage at application. These efforts will help improve the precision of in-season K management decisions in soybean production systems.

More information can be read in this Science for Success Potassium Management in Soybeans article.

References:

Ortel, C. (2025). Potassium management in soybeans. Science for Success.

Parvej, P. and Moseley, D. (2024). Tissue testing helps in determining hidden hunger potassium deficiency in soybean. LSU AgCenter Louisiana Crops Newsletter 14(4).

Vieira, L.O., Tubana, B., Padgett, G.B, Moseley, D., Gravois, K., Levy, R.J, & Kerns, S.P. (2025). Fertilizer recommendations for field crops in Louisiana: N-P-K-S. LSU AgCenter. P4004