Ammonia Injury in Corn
Ammonia Injury in Corn
One of the first implements that enters a field in the springtime is an anhydrous ammonia (NH3) applicator. Due to the cool, wet fall of 2014 the spring NH3 workload was much higher than normal. Anytime a large volume of spring NH3 is applied, growers ask the questions: “How long between application and planting should I wait to avoid injury?” and “How do I know if my plants have been damaged?” The following bulletin explains how and why NH3 injury occurs.
Diagnosing Injury in Corn
The first signs of injury to look for are burnt radicle roots and burnt nodal root tips. The first root of a corn plant, called the radical (Figure 1), should be 4 to 5 inches in length with lateral roots extending outwards closer to the seed itself. This root is extremely sensitive, especially in its first stages of development. When exposed to high amounts of NH3, the radicle root is burnt, sometimes all the way back to the seed in which it began (Figure 2).
The second diagnostic sign occurs when corn plants reach the growth stage of approximately V4-V5 (4-5 collars). At this stage, plants that may have been injured by ammonia as a seedling tend to get set back compared to other neighboring plants. The pattern resembles the same direction and angle that the ammonia was applied (Figure 3). This is due to plant roots being burnt in the knife tracks or injection points, while the plants outside of this high ammonia concentration take on a greener color and outgrow the injured plants. When observed from a perpendicular view, a field that was applied at an angle will show a wave pattern as the plants cross in and out of the knife tracks (Figure 4).
Movement of NH3 in The Soil
When anhydrous ammonia is applied to the soil, it can disperse approximately 3 to 4 inches away from the injection point in all directions. Most of this dispersal takes place within the first 24 hours of application. This means that a solid band of ammonia, 6 to 8 inches in diameter, may exist every 30 inches, or whatever the knife spacing may be. The travel distance away from the injection point can be affected by the following:
- Anhydrous ammonia has a very high affinity for moisture and will seek out water in the soil
- The dryer the soil at application, the greater the NH3 travel distance = less concentrated in the band
- The wetter the soil at application, the lesser the NH3 travel distance = more concentrated in the band
Depth of Application
- The deeper the application depth, the deeper the concentration band = more time for the radicle root to grow until it reaches the NH3
- The shallower the application depth, the shallower the concentration band = less time for the radicle root to grow until it reaches the NH3
Amount of Nitrogen Being Applied
- Higher rates equate to higher concentrations
- Lower rates equate to lower concentrations
Weather Conditions after Application
- Dry weather pushes the Nitrogen upward in the band
- Wet weather pushes the Nitrogen downward in the band and reacts with any free NH3 trapped in the soil
Plant recovery is very dependent on the development of the nodal root system (Figure 5) which grows at a 45° angle out of the crown. Once the plant gets to the V5 growing stage, the crop switches from growing off of kernel reserves to becoming almost completely dependent upon its nodal root system for water and nutrient uptake for the remainder of its life. This is a rough transition for the corn plant and is sometime referred to as “The Ugly Corn Stage”. A plant with a small, injured root system cannot support its aboveground vegetative tissue, and deficiencies are expressed. The crop can recover, but it will first have to grow a greater nodal root system so it can reach out and capture the water and nutrients it is lacking.