A crop’s nitrogen (N) requirement rises when rainfall increases the yield potential. Applying more N fertiliser to meet greater plant N demand is often appropriate, but crop uptake is not guaranteed. Understanding why can help tailor topdressing practices to increase the chances of the applied N boosting crop yield and profit.
What can go wrong?
Not enough rain after topdressing
Topdressed N fertiliser starts at the soil surface, but by mid-season crop roots are established and most active deeper in the soil profile. Rainfall is needed to transport the applied N deeper into the soil. Infiltrating rainfall moves through soil pore spaces carrying the N fertiliser with it. Without further rain the applied N will not reach the active root zone where crops can take it up. The amount of rain needed to move N into the root zone depends on the soil texture and structure.
“Apply urea to a dry soil getting wetter, not a wet soil getting dryer”
Chris Dowling, Back Paddock Co.
Surface-applied N fertiliser can be subject to volatilisation losses. When urea is applied to a wet soil that is drying the urea dissolves, and is converted to ammonium, but is not well bound to the soil particles. Some N is lost as ammonia gas. Topdressing is better conducted on dry soils that are likely to get wetter, rainfall is more likely to get the ammonium into good contact with soil surfaces.
Crops need to be actively growing to take up N. In a wet paddock check you can see active white roots when you pull out a plant. If not it is too wet for the plants to benefit from extra N. Continue to monitor these crops and soil conditions- future N applications may still be feasible.
Once converted to the nitrate form in the soil, nitrogen from fertilisers can be lost from excessively wet soils via leaching or denitrification. Leaching is the downward movement of nitrate from the root zone, while denitrification results in gas loss (di-nitrogen and nitrous oxide) to the atmosphere. For denitrification to occur requires three factors:
- Anaerobic soil conditions (lack of oxygen, common in waterlogged soils)l
- Soil nitrogen in the nitrate form
- Available carbon for microbes (labile organic matter)
If all three of these requirements are present there is a strong chance that a significant amount of applied N will not be available for crop uptake. Losses of 25 to 90% of applied N have been recorded from soils that meet these conditions.
Too much rainfall following topdressing can also wash nitrogen off the paddock. Wet soils are also vulnerable to compaction, so there can be damage to the soil structure from machinery passes applying the fertiliser.
Applying fertiliser N after booting in cereals or bolting in canola is unlikely to produce a yield benefit. There may be a boost to grain protein, but the returns may not justify the cost of the fertiliser and its application. Topdressing late generally results in lower N-use efficiencies.
This article was prepared by eXtensionAUS Crop Nutrition staff following discussions between growers, advisors and researchers in the paddock at the MPCN II Northern region roadshow in July / August 2016.
eXtensionAUS Crop Nutrition