Nitrogen Management – Avoiding Ammonia Injury

 

A lot of anhydrous ammonia is going on this spring, and in many fields the hope is to plant as soon as practicable after NH3 application. This brings up the question about potential for NH3 damage to seeds and seedlings.

 

Seed and seedling damage from spring-applied NH3 is relatively rare in Illinois, but it can be quite damaging, and we want to minimize the chances of it happening. Such damage is rare is because NH3 converts readily in soil to the ammonium form (NH4+) which is held on soil exchange sites and is not damaging to plant tissue. If soils are moist at the time of application and there is normal rainfall (or at least an inch or so) from NH3 application through the time of crop emergence and establishment, chances of damage are close to zero.

 

A small amount of NH3 remains as free ammonia instead of converting to ammonium right away, due mostly to the large increase of pH that accompanies conversion of ammonia to ammonium. If placement is shallow or if soils dry out, some ammonia can end up in the seeding or rooting zone. If you can smell ammonia at the soil surface near the row at or after planting and soils are dry, there may be enough to cause damage. Free ammonia is very toxic to young plant tissue, and if seeds are planted into, or roots grow into, a soil zone where there is ammonia, damage can result. The most common damage is death of young roots, and this can affect yield if root systems don’t fully recover.

 

The best way to avoid the potential for damage is to physically separate the NH3 and the seed by placing NH3 between rows or row locations. This is possible using GPS (probably RTK) and autosteer, but it means that NH3 needs to be applied parallel (not at an angle) to the rows, and application and planting need to be precise in order to avoid placing any rows right over the ammonia band. If this can be done accurately, planting can take place right after, during, or before NH3 application.

 

Physically separating NH3 from the seedling zone by placing NH3 deep can help, but does not eliminate the possibility of damage. Deep placement (8 to 10 inches deep) takes more power and it can be difficult to maintain uniformity of depth across wide bars. Deep placement in the spring also means placement into wetter soil. With its very high solubility, NH3 moves less distance away from the point of release in wet soils than in drier soils. This increases the concentration of ammonia in the soil, and increases the amount that might move up if soils dry to that depth. The “path” left by a knife running in wet soil is more open for upward movement of NH3, and this can increase potential for plant damage.

 

If it’s not possible to apply NH3 between (the eventual) rows, then separating application from planting by time can reduce damage potential. The idea is to apply NH3 early enough so that enough rainfall will occur to keep NH3 out of the seedling zone. This means relying on weather probabilities, but not certainties; there have even been some instances of plant damage from fall-applied NH3. But the chances of such damage are low, and if this is the only option, then the longer you can wait between application and planting the better. The old rule of thumb – to wait 1 to 2 weeks between application and planting – is better than waiting 1 to 2 days, but not as good as waiting a month. So as long as we understand that waiting a week or two decreases but does not eliminate the odds of injury, it’s a guideline we can live with.