Fertilizer top dress: 2 questions

May 31, 2017 - Issue 10

We had two questions in the past week about top-dressing fertilizer after seeding.

1. How early is too early to top dress?

Liquid nitrogen fertilizer can burn leaf area it touches. Does that mean early is better because small seedlings have less leaf area to touch? Or is waiting until plants are past the 2-leaf stage better because plants that are hit will most likely recover?

We have no clear-cut answer. Canola crops will not need a nitrogen top dress until at least the 2-leaf stage and applying before the 5-leaf stage will have needed nitrogen present before significant update occurs. Driving a decision to go early would only be the risk that if weather turns wet for a few weeks, the window to apply before the 5-leaf stage could be missed.

Adding more water to a liquid UAN application would make it safer for application at any crop stage. Dry urea applied in dry conditions will roll off the plants and not cause any damage, but urea is the most susceptible to losses.

2. Does adding S stabilize N against losses?

If the crop also needs sulphur, by adding ammonium thiosulphate to the fertilizer blend could provide some (limited) benefit as a nitrification and urease inhibitor. But if stabilization of nitrogen against losses is the true goal, use a known effective stabilizer such as Agrotain or Super U.

NOTE: Plants hit by liquid nitrogen fertilizer will be burned and stressed. Spraying them with a herbicide right afterward can kill or set back these stressed plants. Give plants a few days to recover before going onto the next application.

Further reading:

Top dress tips for nitrogen and sulphur

Canola Watch