Nitrogen Fertiliser as we have talked previously is the Nutrient that stimulates vigorous growth, in excess, to the detriment of the strength of the plant.
So use sparingly especially on Lettuce, Cabbage and the likes as it can stop them from hearting up.
Nitrogen is best applied in conjunction with other Nutrients such as Phosphorus and Potash to balance the growth.
It is best applied by itself early in the growth of the plant.
Blood Meal is the only real Organic fertiliser with just Nitrogen approx 12%
Alfalfa Meal has small amounts of P and K with Nitrogen approx 4%
Balanced Fertilisers with useful amounts of Nitrogen are
Blood and Bone,
Fish Meal,
Pelletised Chicken and Sheep Manure,
Cow Manure (more balance the above)
Hope this helps,
Be Happy,
Barry.