According to University of Illinois assistant professor of crop sciences Fabián Fernández, most of the nitrogen applied last fall is probably still in the soil. However, some fields are showing what appears to be a nutrient-deficient crop.
He said that, at the end of February, most of the nitrogen was still present, mainly in the ammonium form, in fields in central Illinois that received a late November application of 150 lb. of nitrogen per acre with NServe.
Between Feb. 27 and Mar. 29, a large amount of ammonium was transformed to nitrate because of the warm soil temperatures. The ammonium concentration at the location of the ammonia knife declined by 39%, and nitrate levels in that location increased from 11 to 44 mg/kg. The data between Mar. 29 and April 8 indicate that nitrification is continuing.
The potential for nitrogen loss depends on the quantity and frequency of rain this spring. While some rain is needed now, the dry conditions have allowed nitrogen in the form of nitrate to be retained in the soil.
Fernández said, “Unless we get unusually high precipitation from now on, I do not anticipate a large potential for nitrogen loss. Because soils are dry, it takes a large amount of water to recharge the soil profile.”
Once the soil is recharged, nitrate normally moves approximately five to six inches for each inch of rain in a clay loam or silt loam soil. In sandy soils or heavily tile-drained soils, it is possible to move nitrate as much as a foot for each inch of rain.
“However, it is important to remember that between rain events, nitrate will likely start to move back up,” said Fernández. This movement is due to water evaporation from the soil surface that creates an upward suction force that moves water and nitrate closer to the surface. If crops are already growing actively, evapotranspiration will cause a similar suction force in addition to some nitrate uptake by the crop.
Fernández said that crops in some fields are showing nutrient deficiency. “The most common symptom I have observed this spring is pale green leaves, but I also received reports of some that have observed a purplish tint in the canopy,” he said.
The pale green is probably due to nitrogen deficiency. Nitrogen is important in chlorophyll, which gives leaves their typical dark-green color.
The purple coloration is due to lack of phosphorus in the plant, which impairs the formation of complex carbohydrates. The sugars that accumulate in the leaves are converted to a red-purple pigment (anthocyanin).
These symptoms are due to cool, dry soils that reduce nutrient availability. Also, the crop is not yet actively growing, which also reduces the capacity of roots to take up nutrients.
Another common symptom that is often observed early in the season when soils are dry is potassium deficiency. “I have not seen or heard reports of this symptom so far this year,” said Fernández.
Whatever the nutrient deficiency symptom may be, it is important to remember that, if the soil has adequate nutrient levels or has been adequately fertilized, these symptoms are temporary and have no negative impact on yield. Once the crop starts growing actively, these symptoms will quickly disappear.