Nitrogen is one of the main nutrients all plants need to thrive. Deficiency of this macronutrient causes mature foliage to turn yellow and young plants to fail to develop a deep green color. But while all plants require nitrogen, some really crave it. Green leafy plants, in general, need more nitrogen than flowering and fruiting vegetation. Many of them are found in your cool-season vegetable garden.

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Nitrogen promotes leaf development in such crops as cabbage.

Lettuce

As a cool-season crop, lettuce responds to intense sunlight and heat by becoming bitter and developing a stalk. In addition, even if you prepare an organically rich site for your heads of lettuce to grow, they require an additional dose of nitrogen in the form of calcium or ammonium nitrate while they're developing. Lettuce takes calcium nitrate at the rate of 2 pounds per 100 feet of bed and 1 pound of ammonium nitrate for the same area. If you have sandy soil, which doesn't hold onto nutrients, even more applications of nitrogen are necessary for healthy leaf development.

Rhubarb

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Rhubarb is a nitrogen-loving vegetable that most people think is a fruit.

If you've never seen a rhubarb plant, imagine a red stalk of celery with 2-foot-long leaves and you'll have an accurate image. Even though rhubarb is a vegetable, it's generally used as if it were a fruit, as a pie filling and as a base for jams. Rhubarb also is another nitrogen-loving plant. Even if you enrich your soil with manure, which has a high content of this macronutrient, an additional nitrogen application midseason helps ensure a healthy crop.

Brussels Sprouts

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Brussels sprouts need frequent doses of nitrogen.

Looking just like small cabbages, but sometimes tasting bitter, Brussels sprouts require rich ground, moisture, sun during the day and light frosts at night to develop their best flavor. In addition to amending the soil with organic matter, you also have to add a large dose of nitrogen early in the growing season to promote strong development. Each plant needs 1 tablespoon of nitrate fertilizer three weeks after you set it outside. Two weeks later, it's time for another application, and again in another 14 days. Since nitrogen burns the foliage it touches, it's best to apply it 3 inches from the base of the Brussels sprout plants.

Ornamental Leafy Vegetables

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Ornamental kale, which requires high amounts of nitrogen, decorates a fall garden.

Varieties of kale and mustard greens, as well as of cabbage, which you grow for their decorative effect on the cool-season landscape, require a high content of nitrogen in the soil. In fact, the University of Florida recommends feeding them a water-soluble fertilizer regularly that offers more than 70 percent nitrogen in the form of nitrate.