The key strategy of fall fertilization is to store nitrogen over the winter in the ammonium form – which is held on clay and organic matter – and is referred to as stabilized N. Urea and anhydrous ammonia are both considered ammonium-based fertilizers. When the ammonium form N is converted to nitrate by soil microbes, then it is vulnerable to losses. This is a legitimate risk: In wet and warm conditions, losses from a fall application can be up to 30-40% higher than from a time-of-seeding application.
To keep nitrogen in this stabilized ammonium form…
1. Apply when soils are below 10°C. Cool soils delay microbial conversion.
2. Place N fertilizer in tight bands in the soil. This will further delay microbial activity in areas of high nitrogen concentration. (See the important point below about broadcasting in the fall.)
3. Consider chemical inhibitors, which can reduce this microbial conversion. Urea as SuperU or treated with eNtrench or anhydrous ammonia treated with N-Serve will keep N in the stabilized form longer. Their forte is protecting N from converting to nitrate in the fall when farmers choose to apply N when soils are still warm. Read more about these products and how they work. 4. Band at least 2" deep. Nitrogen is further protected if bands are deep enough that they are not disturbed by spring tillage or seeding operations. 5. Note band separation. Nitrogen bands should be no more than 18” apart for cereals and oilseed crops. 6. Consider phosphorus. Dual banding of phosphorus fertilizer with the nitrogen is an efficient way to safely meet the phosphorus needs of crops such as canola. Dual banding with nitrogen increases phosphorus availability next spring. However bands should be no more than 12” apart so that bands are relatively close to next year’s seed. A seed-placed rate of another 10-15 lb./ac., or more, of phosphate may be required when dual banding if soils are cold or very deficient in P. Note that choosing to fall band nitrogen because soils are often too wet in the spring to support the equipment needed for a pre-seed application may increase the risk for losses because these same factors — warm, moist spring soils — also greatly increase the rate of nitrogen losses. The best soils for fall banding are well drained and tend to be on the drier side. On dry soils, there may be little difference in efficiency between fall- and spring-applied nitrogen. On broadcasting in the fall: Broadcasting N in the fall could lead to maximum losses. With broadcasting N on the soil surface, nitrification is not inhibited as it is with banding. Broadcast N is vulnerable to runoff and immobilization is greater with crop residue (although, as John Heard notes, could rescue some of the N from being permanently lost). If a grower “must” broadcast N in the fall, nitrification inhibitors such as SuperU or eNtrench can help. Research indicates that fall broadcast N requires about 20% more urea-N than spring broadcast or banded N to achieve the same yield. That extra N ends up in the environment.
-Thanks to John Heard, Manitoba Agriculture, for help with this content.