When spring field activities are delayed by unfavorable weather, growers may be forced to consider applying anhydrous ammonia and then planting within a few days, if not the same day. Planting shortly following anhydrous ammonia application can increase the risk of injury to developing seedlings in or near the NH3 retention zone in the soil. This article explains why seedling damage may occur and reviews strategies that can help reduce that risk.
Mechanism of Injury
When anhydrous ammonia is injected in the soil, it expands in all directions (2½ to 3 inches in most soils) creating a 5- to 6-inch cylindrical retention zone. Expansion can be even greater in dry or coarse soils. Free ammonia (NH3) is highly toxic to developing seedlings. Once in the soil, the NH3 molecules are converted to ammonium (NH4+) by associating with H+ ions. Most of the H+ ions come from the splitting of water molecules, leaving behind hydroxyl ions (OH-) as a product of the reaction. The hydroxyl ions increase the pH of the soil at the injection site, slowing the conversion of NH3 to NH4+. This allows some free ammonia to persist in the soil for a longer period of time, increasing the risk of injury and stunting to the roots of nearby corn seedlings (Figure 1). Several factors, including soil texture, temperature, and moisture, can influence the duration that NH3 will persist in the soil.
Figure 1. Roots of corn seedlings showing injury due to spring application of anhydrous ammonia.
Practices to Reduce Injury Risk
Reducing risk of injury involves separating the ammonia from the seed/seedling by either time or distance. Applying anhydrous ammonia well in advance of planting allows time for the NH3 at the injection point to be converted to NH4+. Standard recommendations are generally 5 to 7 days or longer between application and planting; however, free ammonia will persist longer in cooler and drier soils. Injury resulting from fall applications has even been observed in rare cases.
A more consistent solution is to create spatial separation between developing seedlings and the NH3 retention zone by injecting anhydrous ammonia 7 to 10 inches deep in the soil. Consider a scenario in which anhydrous ammonia is applied at a depth of 5 inches. If the NH3 expands 3 inches in all directions, it is now only 2 inches from the soil surface and will come in direct contact with the emerging root system. Injecting anhydrous ammonia deeper into the soil reduces the likelihood of injury (Figure 2).
In a study conducted by the University of Illinois, anhydrous ammonia injected 4 inches deep caused severe injury at high rates. However, anhydrous ammonia injected 7 to 10 inches deep caused little injury to corn planted the same day (Colliver and Welch, 1970). In fact, applying anhydrous ammonia 10 inches deep reduced injury even more than waiting 10 days to plant.
Figure 3. Plants with anhydrous ammonia injury to the roots will appear wilted and spindly.
Another common practice to reduce injury risk is to apply anhydrous ammonia at an angle relative to the direction the corn will be planted so that entire rows are not at risk. Crop injury can still occur, but the injury will more likely affect individual plants rather than long lengths of row.
If anhydrous ammonia applications are made when the soil is wet, the knives can create sidewall compaction, forming a direct channel for the NH3 to move up to the seed zone before it can be adsorbed to the soil. This can lessen the benefit of applying anhydrous ammonia deep in the soil.
Growers who are using high-speed, low-draft applicators do not have the option to place anhydrous ammonia 10 inches deep. Research has shown that applying high rates of nitrogen with these systems can result in significant seedling burn if planting directly over the injection zone (Fernández et al., 2011).
If a grower has RTK, it would be a good idea to run the application parallel to the corn row at least 5 inches to the side. This would be superior to the traditional practice of applying the anhydrous ammonia diagonally, which will result in seedling burn wherever the injection zone and seed row intersect.