Nutrient Status of the Soil by Fabián G. Fernández

In the first issue of the Bulletin, I noted that most fall-applied nitrogen (N) was likely still present in the soil. Table 1 shows that for a late November application of 150 lb N/acre with N-Serve in central Illinois, most N was still present at the end of February, and most of it was in the ammonium form.

The warm soil temperatures during much of March resulted, as expected, in a large amount of ammonium being transformed to nitrate (Table 1). Between February 27 and March 29 there was a 39% decline in ammonium concentration in the location of the ammonia knife as N was transformed to nitrate. The transformation to nitrate is obvious, as nitrate levels in that location increased from 11 to 44 mg/kg. The data between March 29 and April 8 indicate that nitrification is continuing.

Table 1. Soil nitrate and ammonium concentrations in 2012 for the top 12 inches of soil in a field that received 150 lb N/acre with a nitrification inhibitor (N-Serve) at the end of November 2011.
Date in 2012 Position Nitrate (mg/kg) Ammonium (mg/kg)
Feb 12 Knife 11 145
Between 9 15
Mar 29 Knife 44 89
Between 11 3
Apr 8* Knife 34 67
Between 10 5
Data graciously provided by the Illinois Council of Best Management Practices.
*Soil was tilled and the knife track was no longer visible. Eleven samples were collected at 3-inch intervals perpendicular to the knife track direction and kept separate to “find” the knife track with the soil analysis. The knife values represent the average of the three highest adjacent values; the between values are the average of the remaining eight samples.

I also mentioned in the same article that potential for N loss would be dictated by the quantity and frequency of rain this spring. While some rain is needed now, the dry conditions have allowed nitrate N to be retained in the soil. Unless we get unusually high precipitation from now on, I do not anticipate a large potential for N loss.

Because soils are dry, it would take a large amount of water to recharge the soil profile first. Once the soil is recharged, one can expect nitrate to move approximately 5 to 6 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 12 inches for each inch of rain.

However, it is important to remember that between rain events, nitrate will likely start to move back up. Between rain events, water evaporation from the soil surface creates an upward suction that moves water and nitrate closer to the surface. Similarly, if crops are already actively growing, evapotranspiration results in a similar suction force, in addition to some nitrate uptake by the crop.

On a different topic, some fields are showing what appears to be a nutrient-deficient crop. The most common symptom I have observed is pale green leaves, but some producers report a purplish tint in the canopy.

The pale green is most likely due to N deficiency. Nitrogen is important in chlorophyll, which gives leaves their typical dark-green color. The purple coloration is due to lack of phosphorus (P) in the plant. As formation of complex carbohydrates is impaired due to lack of P, the sugars that accumulate in the leaves are converted to a red-purple pigment (anthocyanin). These symptoms are the result of cool and 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 (K) deficiency. I have not seen or heard reports of this symptom so far this year. Whatever the nutrient deficiency symptom may be, remember that if 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 to actively grow, the symptoms will quickly disappear.–Fabián G. Fernández


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