Soil testing is the primary way growers plan fertility applications for their crop and they usually use this information to decide how much phosphorus (P) and potassium (K) to apply; then they apply it before corn and let the soybean crop scavenge for the remains. This approach might work if you regularly test your soil and keep the soil test levels in the moderate to high range.

However, you will never know for sure if P or K are limiting if you do not pull tissue samples and analyze for the level of all macro and micronutrients. And remember even if soil test levels measure high, the plant might not be getting all that it needs because the soil can interfere with availability and uptake.

Regardless of how you manage your soybeans, fertilize corn and soybeans together, fertilizer soybeans separately or only fertilizer corn, running a tissue test is a good check on whether the plant is getting enough of both nutrients or one is deficient and limiting productivity.

Today many growers are applying nutrient packages to the foliage to make sure that none are limiting. This decision may or may not be a good investment unless you assess how the crop is actually doing by collecting tissue samples.

Soybeans can be sampled at any time, but R1 to R2 stage (full flower) is best for analysis. Just pull off the recently expanded trifoliate leaves at the top of the plant and collect from 20 to 30 plants.

Growth Stage Plant Part to Sample Number of Plants to Sample
Seedlng state (less than 12″) or  All the above ground portion. 20-30
Prior to or during initial flowering  Two of three fully developed leaves at the top of the plant 20-30

Send your samples off to a laboratory for testing. They will measure the concentration of nutrients in the leaves and compare it to established guidelines such as those below. If tests are low then make plans to correct. If excessive, try to understand why.

Soybean Tissue Testing Guidelines
Nutrient Low Normal High Excessive
Nitrogen (%) 3.1 4.01 5.51 7.01
Phosphorus (%) 0.16 0.26 0.51 0.81
Potassium (%) 1.26 1.71 2.51 2.76
Calcium (%) 0.21 0.36 2.01 3.01
Magnesium (%) 0.11 0.26 1.01 1.51
Sulfur (% ) 0.16 0.21 0.41 2.00
Manganese (ppm) 15 21 101 251
Iron (ppm) 31 51 351 501
Copper (ppm) 5 10 31 51
Boron (ppm) 10 21 56 81
Zinc (ppm) 10 21 51 76
Source: Penn State University

Do you routinely pull tissue samples? Have you found the information useful, and have you used this information to change the fertility program in your soybeans?

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at or by leaving a comment below.

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About the Author: Dan Davidson

Soybean agronomist Daniel Davidson, Ph.D., posts blogs on topics related to soybean agronomy. Feel free to contact him at or ring him at 402-649-5919.