August 08, 2013 by Andrew Westhoven, CCA
Driving throughout the region and listening to grower conversations, one apparent observation is that corn is firing (losing nitrogen) in all soil types. I have received several questions about the timing of N deficiency and the repercussions it might have on yield, test weight, and overall plant health.
The fields at the highest risk of N loss include fields that applied N in the fall or prior to planting (especially without N-Serve) and/or fields with low holding capacities (sandy soils). Side-dressed corn may be at a lower risk. Some of the N loss is due to the transformation of the N molecule.
Figure 1. Nitrogen deficiency (firing) in a corn leaf.
For example, when anhydrous ammonia is applied to the soil (NH3), it goes through a process called nitrification. Nitrification is the conversion of Ammonia (NH3) to Nitrate (NO3). Immediately, when anhydrous ammonia is injected into the soil, the ammonia (NH3) ions react with soil moisture and convert to ammonium (NH4) ions, which are very stable in the soil. Ammonium ions (NH4) carry a positive charge which binds them to negatively charged soil particles such as clay and organic matter. Over time, depending on environmental conditions, ammonium is converted to nitrate (NO3) and becomes vulnerable to leaching and Denitrification (figure 2). Leaching occurs when the nitrate form of N as (NO3), is moved downward through the soil profile by water, rainfall, flooding, etc. Denitrification occurs when the nitrate form of N (NO3), is converted to gas (N2) and volatilizes into the atmosphere.
Figure 2. Process of conversion of anhydrous ammonia to nitrate.
Corn firing (N deficiency) has been a combination of N denitrification, leaching, excessive rainfall, cold spring temperatures (low GDU accumulation), and compaction. The N has been moved deep into the soil profile and our root growth is very limited due to some of the aforementioned factors.
N deficiency late in the season is a typical, naturally occurring event late in the season. As the corn canopy becomes denser, the sunlight cannot penetrate into the lower portion of the canopy and the lower leaves cannot photosynthesize; therefore, the plant takes the nutrients from those leaves and uses those elements elsewhere. However, if firing occurs too early in the grain fill period it may reach above the ear leaf and/or cause kernel abortion on the tips of ears. If firing occurs for a long duration, another concern is the plant cannibalizing the stored reserves in the stalk causing the stalk to weaken. This ultimately decreases overall late-season plant health and increases the probability of stalk lodging and harvesting problems.
Figure 3. Picture of the lower canopy showing dropped lower leaves due to N deficiency.
Unfortunately, at this point in the season, there is nothing we can do to significantly lessen the damage of N deficiency. Earlier around tassel emergence, several planes were buzzing spreading urea to help prevent N loss (and spraying fungicides). Regardless of the growing season we definitely are remembered that each year is different. If you feel you have a severe N deficiency problem, please contact your local AgriGold Corn Specialist or myself for help.