Issue No. 12, Article 9/June 12, 2009
Factors That Limit Nutrient Availability
Weather conditions this spring have caused some corn fields to appear nutrient-deficient in some portions of Illinois. Additionally, some farmers are concerned that since phosphorus and potassium applications were skipped this year (no addition of "fresh" phosphorus and potassium), their crops might not have enough, even though soil tests showed adequate levels. It is not unusual for crops in fields or portions of fields to show nutrient deficiencies even though adequate fertility and proper nutrient management plans are followed for phosphorus and potassium.
Under adequate nutrient management practices, these deficiencies are most often temporary and occur during early stages of development. What is important to remember is that in addition to inherent soil properties (parent material; amount of organic matter; depth to bedrock, sand, or gravel; permeability; water-holding capacity; drainage), environmental conditions have an important impact on nutrient availability. The fact that nutrients are applied does not necessarily mean they are available. Plants obtain most of their nutrients and water from the soil through their root systems. Any factor that restricts root growth and activity has the potential to 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.
Keep the following points in mind to avoid excessive concern about the need for additional phosphorus and potassium when a sound nutrient program is already in place.
- Soil water content is critical not only to supply the water needs of the crop but also to dissolve nutrients and make them available to the plant. Temporary nutrient deficiencies can be observed when the surface layer of the soil becomes too dry and the root system of the crop is small and shallow. On the other hand, excess water in the soil depletes oxygen and builds up carbon dioxide levels. Oxygen is needed by roots to grow and take up nutrients, while high carbon dioxide is toxic and limits root growth and activity.
- 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. To limit soil compaction, avoid entering fields that are too wet, and minimize the weight per axle by decreasing load weight and/or increasing tire surface area in contact with the soil. Planting when soils are wet can create a compacted wall next to the seed that will prevent the seedling from developing an adequate root system. Tilling wet soils will result in clods that become hard and dry out quickly on the surface, preventing roots from accessing resources inside the clod.
- Light intensity is low on cloudy days. Low light intensity reduces photosynthetic rates and nutrient uptake by the crop. Since low light intensity sometimes occurs when soils are waterlogged or temperatures are cool, cloud cover can exacerbate the capacity of the crop to take nutrients.
- Temperature is important in 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.
Diseases and pests can have an important impact on crop-nutrient uptake by competing for nutrients, affecting physiological capacity (such as reduction in photosynthesis rates), and diminishing root parameters through root pruning or tissue death.--Fabián G. Fernández