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Issue No. 9, Article 2/June 4, 2010

What Am I Lacking in My Soil Fertility Program?

Most farmers are very pleased with the way planting went this year. Many, though, report that the crop is not looking as good or growing as quickly as they would like, and they are concerned that micronutrients or other fertilizers look to be needed. What is important to remember at this time is that environmental conditions have an important impact on nutrient availability. The fact that nutrients are applied or that they are normally present naturally in the soil does not mean they are necessarily available to plants.

Plants obtain most of their nutrients and water from the soil through their root systems. Any factor that restricts root growth and activity can restrict nutrient availability. This is not because nutrients are not plant-available in the soil, but because the crop's ability to take up those nutrients is restricted. This year different factors or a combination of factors may be at play and restricting nutrient availability. Following are some of the factors that I consider most likely to be contributing to observed deficiencies or slow growth:

  • Excess water in the soil depletes oxygen (O2) and builds up carbon dioxide (CO2). Oxygen is needed by roots to grow and take up nutrients, whereas high carbon dioxide levels are toxic and limit root growth and activity.
  • Temperature is another important factor regulating the speed of soil chemical and biological processes that make nutrients available. When soil temperatures are cool, chemical reactions and root activity decrease, rendering nutrients less available to the crop. Portions of the 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.
  • Another factor is light intensity. Many days 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. Since low light intensity sometimes occurs when soils are very wet or temperatures are cool, cloud cover can exacerbate the plants' capacity to take in nutrients.
  • Immobilization of nitrogen. Plant-available nitrogen becomes temporarily unavailable (tied up) as microorganisms break down crop residue. This has been observed in corn fields planted on previous corn. As crop residue and soil organic matter start to mineralize, nitrogen will become available to the plant. If the full amount of nitrogen has not been applied yet, a side-dress application of urea ammonium nitrate (UAN) can help provide the nitrogen the plant needs at this time.
  • A final factor that may be causing problems this year is soil compaction, though since compaction tends to be worsened by dry conditions, it is not high on my list of possible problems because we have not been extremely dry. Soil compaction can limit or completely restrict root penetration and effectively reduce the volume of soil, including nutrients and water, that can be accessed by the plant.

In most cases, the deficiencies we are seeing this year relate to growing-season conditions and not to inadequate soil fertility. As conditions improve, much of what you see as nutrient deficiency symptoms will disappear without additional fertilization. However, since growing-season conditions this year accentuate problems that might not be as evident in other years, this is a good opportunity to learn about field conditions or management practices that should be adjusted to prevent or at least lessen problems in the future.

In Illinois, nutrients (besides nitrogen, phosphorus, and potassium) that can most commonly be deficient include these:

  • Calcium--only in soils with extreme acidity (pH below 5.5).
  • Magnesium--only in isolated situations in Illinois. The soils most likely to be deficient in magnesium include acidic and sandy soils throughout the state and low cation exchange capacity (CEC) soils in southern Illinois. Deficiency is more likely where calcitic limestone (CaCO3) rather than dolomitic limestone (CaMg[CO3]2) has been used.
  • Sulfur--While the number of incidents with sulfur-deficient crops in the Midwest has increased, crop responses to sulfur applications in Illinois have been inconsistent. Routine application of sulfur fertilizer is thus not recommended. Most often deficiencies are observed in low organic matter soils (less than 2.5%) or soils with coarse (sandy) textures.
  • Micronutrients--Confirmed deficiencies of micronutrients for corn and soybean in Illinois have been limited to zinc deficiency of corn and iron and manganese deficiencies of soybean. To identify areas before micronutrient deficiencies become important, I suggest continual observation of the most sensitive crops in soil situations in which the elements are likely to be deficient. Given some of the limitations of this growing season, this year could be a good one to identify these areas. The most common situations in which zinc deficiencies are observed are low organic matter soils, soils with high pH or very high phosphorus levels, and coarse-textured soils. Iron and manganese deficiencies are more likely to occur in soils with high pH.

The use of micronutrient fertilizers should be limited to areas of known deficiency, and only the deficient nutrient should be applied. An exception would be situations in which farmers 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.

Given the conditions of this growing season, it can be difficult to know exactly what factor or set of factors are causing an observed problem. If you suspect a nutrient deficiency, I suggest collecting plant samples and sending them to a laboratory for nutrient analysis. Critical tissue-nutrient level (below which deficiency occurs) is the concentration needed for a crop to complete its life cycle. These concentrations are largely independent of soil or growing conditions, so the values typically apply across environments and provide a more reliable measurement for micronutrients and secondary nutrients than do soil tests.

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.--Fabián G. Fernández

Fabián Fernández

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