Carrie Ortel, Extension Soybean Agronomist, Virginia Tech Tidewater AREC

Mark Reiter, Soils and Nutrient Management Extension Specialist, Virginia Tech Eastern Shore AREC

Joseph Haymaker, Postdoctoral Associate, Virginia Tech Eastern Shore AREC

Many of the full-season soybean fields in Virginia are beginning to flower, which is a great time to start tissue testing if there is a concern for crop nutrition. Tissue testing is an effective way to monitor crop nutrition when done correctly. To get reliable results, choose the correct, uppermost fully expanded soybean leaf during optimal conditions and carefully interpret the results. Tissue testing may begin as early as V4 but is most reliable during flowering and can continue through pod filling.

Step 1: Plan Your Sampling During Good Field Conditions

Time It Right – Field Conditions Matter

Field conditions at sampling time greatly affect the accuracy of tissue tests. Aim to collect samples when plants are actively transpiring, which usually means:

• Adequate soil moisture (not drought or waterlogged).
• Moderate temperatures and healthy plant function.

Avoid sampling during:

• Drought, which can limit nutrient uptake even when nutrients are present in the soil.
• Saturated soils, which can temporarily inhibit root function.
• Shortly after foliar nutrient applications—wait at least a week and ensure a rain event has occurred to allow nutrients to be absorbed and leaf surfaces to clear.

Address In-Field Variability

Soybean nutrient levels can vary within a field due to differences in soil texture, drainage, or previous management practices. For meaningful results:

• Divide fields into management zones based on known variability.
• Take one composite sample per zone, collecting at least 18 trifoliolate leaves randomly throughout that area (Ortel et al., 2023).

This helps identify localized deficiencies and supports more precise nutrient management.

Step 2: Collect Your Leaf Sample

Choose the Right Plant Part

To get consistent and accurate results, it’s critical to sample the correct, uppermost fully expanded trifoliate leaf, as nutrient concentrations differ between leaves. The uppermost fully expanded trifoliate leaf is typically located on the second, third, or fourth node from the top of the plant (Figure 1, shown below).

Figure 1. Soybean plant at the full flower (R2) growth stage. The uppermost fully developed leaf is shown as the leaf on the third node from the top of this plant.

• Look for the highest leaf that is dark green, full-sized, and has a coarse texture.
• Avoid leaves with a velvety feel or are lighter in color than others—these are still developing and can falsely indicate nutrient levels.

Check with your testing lab for specific guidelines. Some labs also recommend sampling the whole plant during vegetative stages or including/excluding the petiole (the stalk attaching the leaf to the stem). Be sure your sampling method agrees with the lab’s interpretation standards.

Collect the Sample Properly

Once you identify the correct leaf, follow these best practices:

• Collect 18–25 leaves per sample for a good composite.
• Take samples across a consistent management zone (based on yield history, soil type, or other field characteristics).
• Place leaves in a paper bag (not plastic) to allow drying and prevent mold.
• Note the growth stage of the soybean crop.

If your lab uses critical nutrient thresholds without the petiole (e.g., Virginia Tech), remove the petiole before bagging.

Step 3: Interpret Results with Context

Getting accurate lab results is only half the process—understanding them correctly is essential.

• Use growth-stage-specific critical concentrations when available, such as potassium in soybean (Slaton et al., 2021). These values indicate the threshold below which yield may be affected.
• If no critical values are available, use sufficiency ranges cautiously—they are less precise and based on broader surveys instead of replicated research.

Also, consider nutrient mobility:

• Mobile nutrients like nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg) can move within the plant and are often relocated from leaves to developing seeds and pods (Bender et al., 2015). Lower concentrations in leaves during reproductive stages may not signal a deficiency.
• Immobile nutrients like calcium (Ca), sulfur (S), and most micronutrients remain in their original locations and offer more stable indicators.

Although some nutrient deficiencies may not be visually apparent (hidden hunger), only take action with a corrective application of fertilizer if a deficiency occurs. Prophylactic foliar fertilizer applications have not been shown to increase yields (Matcham et al., 2021). When a nutrient deficiency does occur, a corrective application of granular fertilizer should be used to correct macronutrients, while a foliar fertilizer may be used to correct micronutrients.

Take Home Points

1. Collect a composite sample of at least 18 of the uppermost fully expanded soybean trifoliate leaves from each management zone.
2. Only collect tissue samples during favorable field conditions and adequate soil moisture.
3. Consider the plant part collected (petiole included or excluded), growth stage, and nutrient mobility with interpreting results.

For more information please contact Carrie Ortel, Extension Soybean Agronomist, at carrieo@vt.edu.

References

1. Bender, R. R., Haegele, J. W., & Below, F. E. (2015). Nutrient uptake, partitioning, and remobilization in modern soybean varieties. Agronomy Journal, 107(2), 563–573. https://doi.org/10.2134/agronj14.0435
2. Matcham, E. G., Vann, R. A., Lindsey, L. E., Gaska, J. M., Lilley, D. T., Ross, W. J., Wright, D. L., Knott, C., Lee, C. D., Moseley, D., Singh, M., Naeve, S., Irby, J. T., Wiebold, W., Kandel, H., Lofton, J., Inman, M., Kleinjan, J., Holshouser, D. L., & Conley, S. P. (2021). Foliar fertilizers rarely increase yield in United States soybean. Agronomy Journal, 113(6), 5246–5253. https://doi.org/10.1002/agj2.20889
3. Ortel, C. C., Roberts, T. L., Hoegenauer, K. A., Poncet, A. M., Slaton, N. A., & Ross, W. J. (2023). Mapping variability of soybean leaf potassium concentrations to develop a sampling protocol. Agrosystems, Geosciences and Environment, 6(4). https://doi.org/10.1002/agg2.20439
4. Slaton, N. A., Drescher, G. L., Parvej, R., & Roberts, T. L. (2021). Dynamic critical potassium concentrations in soybean leaves and petioles for monitoring potassium nutrition. Agronomy Journal, 113(6), 5472–5482. https://doi.org/10.1002/agj2.20819