With this (unexpected?) warm November weather, fall fertilizer applications are probably better done now than back in October — from a loss potential perspective.
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, as losses compared to a time-of-seeding application can be 30-40% in moist and warm conditions.
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.
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.
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 actually 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 or controlled release products like ESN 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.
On losses for fertilizer applied in October: The combination of warm weather and moist soils is almost ideal for losses. The combination of warm weather and moist soils is almost ideal for losses. Growers may want to reassess the nutrient situation with a time-of-seeding soil test, then top-dress if amounts are less than expected.
Manitoba extends 2016 fall-application deadlines: Manitoba’s winter nutrient application ban has been extended until November 22 as soil temperatures have not yet reached the freezing point. This means nutrients can be applied until midnight November 21, but certain conditions must be met:
-Nutrients can only be surface applied if they are immediately incorporated; and…
-Nutrients should not be applied in instances where the weather outlook is unfavourable, such as if snow or an appreciable amount of rainfall is expected that would result in run-off.