Onions have small light green tops. Nitrogen deficiency can cause increases in some fungus diseases. It should be about 800 PPM for growing plants and about 500 PPM for older established plants. Extra nitrogen in fertilizer favors growth of green leaves. Nitrogen Toxicity Lettuce can be injured by high nitrogen in hot weather.
Cool weather can also cause slow growth and light green color of leaves.
Virus yellows can be mistaken for nitrogen deficiency because each cause a yellowing of leaves. However, virus is more patchy yellow and the nitrogen deficiency is more uniform. Nitrogen in legumes Legume plants including beans, peanuts, lentils and peas have root nodule bacteria that can fix nitrogen for the associated plant from the air. These types of plants have reduced nitrogen needs, if the root nodules are well established. For fast growing plants such as beans and peas, this is hardly of any benefit, because the root nodules take longer to establish than the crop is rotated, but it may be used in the growing of slower plants like lentils and peanuts.