Topdressing decisions have to be made or revised, as the season progresses. The best nutrient use efficiency is achieved when nutrient supply matches crop demand. Splitting N applications through the season can better match crop demand than applying all N upfront, and allows some of the expense to be deferred until more is known about the season. Not all in-crop applications of nitrogen are a good investment. Crops must be actively growing to take up applied nitrogen. Any plant stressors will reduce fertiliser uptake.
A decision to topdress nitrogen should be based on a reasonable expectation of a deficit in the nitrogen budget. When the nitrogen status of the paddock is in doubt reassess the situation in-crop (including any nitrogen applied at sowing) by taking deep soil cores to test for nitrate. If it’s been very wet, test wetter areas separately to drier parts of the paddock as denitrification can be extremely variable.
Here are some midseason conditions that can be considered good, offering a solid chance of yield benefit, bad, not much chance of yield benefit and ugly, where you might wish you’d decided ‘no’. If you do apply N and it’s not taken up by the target crop some of the fertiliser N may be available the following season.
- Soil moisture status medium
Good basis for promising yields. The paddocks predicted yield potential may be higher than at the start of the season.
- Good chance of following rain
Following rainfall of 15 – 100 mm will move the applied N into the crop’s active root zone, promoting uptake.
- Crop stage between GS20 and GS39
A boost to tillering gives the most prospect of yield increase.
- Soil moisture status very low
Low yield potential limits the chances of a response.
- Soil moisture above moderately wet
Any following rain might create conditions for nitrogen losses to the atmosphere.
- Slim prospect of rain
The majority of the applied N will stay at the soil surface, not accessed by crop roots.
- Crop stage after GS40 (booting)
Additional N will be used by the plant to increase grain protein rather than yield. The price premium for higher protein grain may not justify the nutrient application.
- Soil moisture above field capacity
High risk of conditions for denitrification and nitrogen losses. Waterlogging might limit the crop more than nitrogen availability.
- Hot weather
Increases severity of nitrogen losses if it’s combined with wet soils. Heat stress for plants might reduce growth and nutrient uptake.
- Chance of lots and lots more rain
Nitrogen might end up washed off the paddock or deep into the soil below the active root zones – although this rarely happens in clay soils even after significant rainfall. Waterlogging might limit the crop more than nitrogen availability.
This article was prepared in response to discussions between growers, advisors and researchers at the More Profit from Crop Nutrition Roadshows 2016
ANZ IPNI Nitrogen mid-season: yes – no – wait