Some of the older yellow leaves droop and then fall off of the plant. Lower leaves also start to show brown on their tips. Fruits from nitrogen deficient plants such as cucumber are light green in color. Cantaloupes are smaller in size and there is a smaller yield. 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.