Canola under the stress of too much moisture will turn yellow, because roots starved for oxygen can’t take up nutrients. Wait until the ground dries up and roots can get working again before rescuing the crop. In short, save your money until you know the crop will survive.
Top dressing fertilizer may help when new green leaves start growing and when good growing conditions return. Sulphur in the form of ammonium sulphate can provide an economic return when applied up to the bolting stage. Nitrogen applied past the six-leaf stage is unlikely to provide an economic benefit.
Tissue tests to determine nutrient deficiency may be unreliable this year in areas suffering unusual moisture stress.