By Carol Brown

When humans need to learn about ailments or afflictions, a blood test can reveal what’s happening internally. Test results, such as an iron deficiency or a high white blood cell count, can direct doctors to help cure what ails us.

Similarly, soybean tissue testing can reveal internal nutrient concentrations. Knowing what’s happening on the inside of the plant can help farmers determine if nutrient applications or soil amendments need to be made when striving for the best yields.

Illinois researchers are exploring the details of soybean tissue testing and how these test results can impact the crop in the future. Led by Connor Sible, crop production research assistant professor, and crop physiology professor Fred Below, at the University of Illinois-Champaign, the project is entering its second year and is supported by Illinois Soybean Association checkoff funding.

“There’s a lot of farmer interest in tissue testing and I receive many questions surrounding this topic,” Sible remarks. “The farmers who are aiming for record yields take tissue tests regularly throughout the season and make adjustments accordingly. In our research, we are addressing some of the frequently asked questions, such as when to take samples if they were to test only once a season. Another question I receive is how to collect a plant sample and we’re addressing this, too.”

When collecting soybean plant samples for tissue testing, it is recommended to take most recently matured trifoliates, which are no longer touching each other, and without the petiole, which is the stalk that connects the leaf to the stem. Photo: Connor Sible

Refining Tissue Test Guidelines

The team is doing intensive soybean sampling at research plots near Champaign to find answers to farmers’ questions. After the first year, they have clarified sample collection guidelines.

Because tissue testing can cost between $10 and $20 per sample — depending on the test’s comprehensiveness — a field’s worth of samples adds up quickly. By finding the best sampling methods, farmers can receive the most accurate results for their test investment.

“Testing guidelines direct that the ‘most recently mature trifoliate’ should be used,” Sible says. “But this is open to interpretation. Which leaves are the most recently mature? The standard method is to sample the trifoliates that are no longer touching each other, usually the second or third node from the top. All three leaflets without the petiole should be collected as others have found that including the petiole makes a difference, which is why there is a general recommendation to exclude this from the sample.”

Soybeans can be tested for sufficient or deficient thresholds of nitrogen, potassium, phosphorus, sulfur, boron and more. Some deficiencies could be remedied during the season to improve yield potential, while remedies for others may take longer to see results.

“If tissue testing indicates a specific nutrient deficiency, a farmer could make adjustments during the season, such as micronutrient fertilizer applications could help the newer pods at the top of the plant,” Sible says. “Other nutrients such as sulfur or phosphorus may take longer to be effective.”

If farmers conduct tissue tests only once a season, Sible says they should pull samples around late R2 or early R3 growth stage. If farmers want to test more often, he recommends taking samples about every two weeks, collecting them in the same general areas of the field each time.

Research in Phases

To achieve the long-term goals for this research, Sible has divided the project the into four phases. The project is currently in phase one, the mapping phase, which is learning when nutrients are going through the soybean canopy. But the team needed to first define the best way to conduct tissue tests to achieve the most accurate results.

“The soybean plant is making multiple decisions at the same time — pods are developing while the plant is still blooming,” he explains. “We want to know when the plant remobilizes nutrients from the leaves to the pods. Tissue testing can shed light on this through the different nutrient concentrations in the plant.”

This crop season, the team is adding soil sampling to expand tissue testing results. Joining the project is University of Illinois Crop Sciences Associate Professor Andrew Margenot, who manages the university’s Soils Lab. The team will track the nitrogen and sulfur in the fertilizer to better understand how and when the soybean uses these nutrients.

“The nitrogen and sulfur will be molecularly ‘labeled’ in the applied fertilizer,” Sible explains. “We’ll then be able to track these in tissue tests, which will help us understand which nutrients are being used and when. We’ll see if the plant preferentially uses the fertilizer during certain times of the season as opposed to using only what the soil supplies. We can see when the plant is pulling nutrients from the applied fertilizer.”

All of this work is still in the mapping phase of the research. The other planned phases include:

• Phase 2: matching the nutrient maps from the tissue tests to the established thresholds, which could impact the recommended deficient or sufficient test levels.
• Phase 3: exploring prediction of nutrient use patterns in the soybean.
• Phase 4: sap analysis, which is measuring the Xylem and the transport of nutrients through the plant. Sible says the sap testing is intended as a “three-day head start” on a tissue test with the idea that what is currently measured in the plant sap will be measured in the tissue test three days later. This could provide three more days to make corrections.

With these research results, scientists will have a much better grasp on soybean nutrient usage, which can protect and improve yield potential — all from the humble beginnings of collecting a trifoliate for a nutrient test.

Meet the Researcher: Connor Sible

Published: Aug 11, 2025