Growing season conditions may be the biggest contributor to poor crop appearance today, rather than inadequate soil fertility, says Fabián Fernández, University of Illinois Extension specialist in soil fertility and plant nutrition.

"Environmental conditions play an important role in nutrient availability," Fernández says. "Plants obtain most of their nutrients and water from the soil through their root system. Any factor that restricts root growth and activity has the potential to restrict nutrient availability."

Fernández says four factors may be causing this season's observed deficiencies.

  • Excess water in the soil depletes oxygen and builds up carbon dioxide levels. While oxygen is needed by the roots to grow and take up nutrients, high carbon dioxide levels are toxic and limit root growth and activity.

  • Temperature influences how nutrients are absorbed. Under cool soil temperatures, chemical reactions and root activity decrease, rendering nutrients less available to the crop. Plant nutrients are taken up as roots extract soil water to replenish water lost through the leaves. Cool air temperatures can lower evapotranspiration and reduce the convective flow of water and nutrients from the soil to the root.

  • Light intensity affects nutrient availability. Many days in this growing season have been characterized by low light intensity due to cloudiness. Low light intensity reduces photosynthetic rates and nutrient uptake by the crop.

  • Immobilization of nitrogen (N) occurs when plant-available N becomes temporarily unavailable as microorganisms breakdown crop residue. This has been observed in cornfields planted on previous corn. As crop residue and soil organic matter start to mineralize, N will become available to the plant. If the full amount of N has not been applied yet, a sidedress application of urea ammonium nitrate (UAN) can help provide the N the plant needs at this time.

"As growing conditions improve, most nutrient-deficiency symptoms will disappear without additional fertilization," he says.

In Illinois there are instances in which calcium, magnesium, sulfur and a few micronutrients may be deficient, but these deficiencies are not widely seen, he says.

"The use of micronutrient fertilizers should be limited to areas of known deficiency, and only the deficient nutrient should be applied," Fernández says. "Exceptions to this are situations in which producers already in the highest yield bracket try micronutrients experimentally in fields that are yielding less than would be expected under good management, which includes an adequate N, P and K fertility program and a favorable pH."

If a nutrient deficiency is suspected, he encourages producers to collect plant samples and send them to a laboratory for nutrient analysis.

When diagnosing a fertility problem through plant analysis, select paired samples of comparable plant parts representing the abnormal and normal plants. After collecting the samples, deliver them immediately to the laboratory. Samples should be air-dried if they cannot be delivered immediately or if they are going to be shipped.

Soil factors (fertility status, temperature and moisture) and plant factors (cultivar and development stage) may complicate the interpretation of plant analysis data. The more information provided concerning a particular field, the more reliable the interpretation will be.

"Because growing season conditions accentuate problems that might not be as evident in other years, this is a good time to learn about field conditions or management practices that should be adjusted to prevent or lessen problems in the future," he says.

For more information, check out The Bulletin, an online publication written by U of I Extension specialists in crop science.