What in-season plant tissue tests can tell you about fertility

Plant tissue tests complement soil tests to help producers fine-tune their fertility programs, which is especially important in the wake of higher fertilizer prices and low commodity prices. The tests can help growers monitor the presence of nitrogen (N), phosphorous (P), potassium (K), and other key nutrients, says Nick Ward, president of Ward Laboratories, Kearney, Nebraska.

When to sample tissue

Taking tissue samples at least three times throughout the growing season is recommended so farmers can identify nutrient needs, correct deficiencies, and plan for next year's crop.

Purdue University advises how to take tissue samples and at what stages:

• V4 or V5. Sample the entire plant above ground, cutting the stalk off about ½ inch above ground level.
• Between V9 and V12. Sample the first fully developed leaf from the top (or the first leaf below the whorl). Cut the leaf at its base, where it joins the sheath.
• Tasseling or early R1 (silking). Sample the leaf below the top ear, cutting the leaf at its base where it joins the sheath.

Obtain samples from at least 20 randomly selected plants in the field, or the area to be represented by testing. Combine these into one composite sample. Plants should be washed with distilled water, or however the testing laboratory suggests. Air-dry the samples. Don't freeze them, but they can be refrigerated for a few days. The plant samples need to be put into a paper bag and shipped to your laboratory of choice as soon as possible; results should be back within a week.

Expect to pay about $30 for routine plant tissue testing. Add between $10 and $15 for stalk nitrate testing, which can help determine whether your nitrogen fertility program is working effectively.

What to expect in return

The lab report will provide findings of several nutrients.

Nitrogen, phosphorus, potassium, calcium (Ca), magnesium (Mg), sulfur (S), and chloride (Cl) will be reported in percent.

Copper (Cu), iron (Fe), manganese (Mn), zinc (Zn), boron (B), and molybdenum (Mo) will be noted in parts per million (ppm).

If plant samples are taken early in the season, steps to remediate some nutrient deficiencies can be taken in a sidedress fertilizer application, fertigation (nutrient application via irrigation), or foliar top-dress.

The numbers in a plant tissue test can complement those from a soil test to help diagnose problem areas in the field. However, the test won't provide a prescription for nutrients needed to maximize crop production in-season, says Dan Kaiser, Extension fertility specialist at the University of Minnesota.

"Foliar application of most nutrients should be viewed as a last resort — only in cases where severe nutrient deficiencies are seen. Research on foliar applications of micronutrients has not shown that yield can be increased in corn and soybeans, even if tissue results indicate a deficiency," he says.

An agronomist or Extension agent can help you determine sidedress or foliar rescue treatments to maximize yield and plant health. Some words of caution from Ray Ward of Ward Laboratories: tissue testing should not replace soil testing when developing a fertility plan. And, environmental stresses such as too much or too little water will skew the results of a tissue analysis.

How about soybeans?

Growers can take plant tissue samples of soybeans, but chances are that growers won't be able to take any corrective action during this growing season, according to South Dakota State University.

However, the information can be useful for the following cash crop, or the next time soybeans are planted in the field.

Plan on taking soybean tissue samples at early bloom and again at R3, when there are still blooms but pods are forming on the fourth node down, says Ray Ward.

"These are the stages that influence yield as long as nutrients are adequate," he says. A sample should include the top trifoliate leaves. Grab and pull the trifoliate leaves, but don't include the petiole. Take leaflets from at least 15 plants per sample.