Plants without nitrogen are like Popeye without his famous spinach--frail, pale and stunted. One of the primary nutrients plants require for daily growth, nitrogen is essential for the development of vigorous, well-formed plants. According to Oregon State University Extension, signs of nitrogen deficiency in plants include lack of growth and yellowed leaf tips or stems.Testing your soil plays a key role in ensuring that you provide the correct amount of nitrogen fertilizer for your plants.
Scoop a handful of soil from the ground near the plants that you suspect need supplemental nitrogen and place it in a plastic sandwich bag. Take the bag of soil to your county Cooperative Extension office where one of the extension agents can test it for a small fee (or even free of cost, depending upon what state you live in). The extension agent should give you a printout of the exact nutrient levels in your soil, including nitrogen, and can explain the recommended levels of supplemental nitrogen that you should add to your soil in your area of the country to keep from leaching excessive nitrate into the soil and groundwater supply.
Give your plants a chemical fertilizer that contains nitrogen. Look for a fertilizer rated for use with the type of plant for which you're providing the nutrients. Check for the nitrogen level, which is the first number in the three-number series on the side of each fertilizer package; the other two numbers indicate phosphorus and potassium levels, respectively. For example, if a fertilizer package says 30-10-20, then you know that it contains 30 percent nitrogen. Select a fertilizer that has a nitrogen percentage within the limits suggested by the extension agent.
Add a nitrogen-rich organic soil amendment to your plant soil as a natural alternative to using a chemical nitrogen fertilizer. Sprinkle a 1- to 2-inch layer of composted cow or horse manure across the surface of your soil. Mix the amendment thoroughly into the top 3 to 4 inches of soil surrounding your plants with a garden rake. Add the compost in the spring, preferably before placing your plants into the ground.
Plant your garden with a high-nitrogen cover crop, such as clover or field peas, after harvesting your vegetables in the fall. Allow the cover crop to grow over the winter. Till the cover crop into the soil approximately one month before planting time to provide your spring garden crops with a steady supply of organic nitrogen fertilizer.