Growers in some lucky regions are now considering late, in-crop applications of nitrogen. Crops in Western Australia, south-west Victoria, and parts of Tasmania still have above average yield potential. Given current grain prices, the economic reward from late N application to these crops is likely to be high.
But late application of N is only really an option for crops with very high potential. The soil profile needs to be very moist to depth and there must be confidence surrounding September and October rainfall.
As a rule of thumb, the later N is applied in the season, the greater its contribution to protein % over yield. There is little evidence to show positive economic returns from chasing protein in low to medium rainfall environments.
When to apply
In the high rainfall zone where crops are still before flag leaf, extra N can be profitable. Earlier applications will boost yield while later applications will possibly increase protein. But not all in-crop applications of nitrogen are a good investment. Crops must be actively growing to take up applied N. Any plant stressors, including potassium deficiency, will reduce fertiliser uptake.
A decision to apply N should be based on a reasonable expectation of a deficit in the N budget. When the N status of a crop is in doubt, look at the oldest leaves (canola) or tillers (cereals) – if they are green then N is not likely to be limiting. You can do deep soil cores to test for nitrate , but the results will probably take too long to obtain to enable effective N application.
By mid-season, crop roots are established and most active deeper in the soil profile. Therefore rainfall is needed to transport applied N from the soil surface deeper into the soil. Infiltrating rainfall moves through soil pore spaces, carrying the N fertiliser to 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.
Another rule of thumb from Chris Dowling, Back Paddock Co, is
“Apply urea to a dry soil getting wetter, not a wet soil getting dryer.”
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