Nitrogen is one of the 17 elements that are essential to plant growth and one of the three macronutrients needed in large quantities. Though nitrogen is one of the more abundant elements, making up 78 percent of the air around us, it needs to be changed into the form of ammonia or another compound before plants can use it effectively. Fertilizers containing nitrogen are often applied to help plants produce quick vegetative growth.
How Plants Use Nitrogen
Nitrogen is absorbed by roots from the soil as either nitrate, which is easily absorbed but also easily lost from the soil, or as ammonium, more slowly absorbed but more persistent. It is then passed to the leaves for use in making chlorophyll, the pigment that gives leaves their green color and helps in photosynthesis, proteins and other compounds.
The Nitrogen Cycle
When plants decay, the nutrients held in their tissues are released by the action of numerous microorganisms. How fast this occurs depends on temperature and soil moisture as well as the balance of carbon and nitrogen. Soft, green tissue such as grass blades contain quite a bit of nitrogen and decay quickly, returning the nitrogen to the soil. Dry leaves have a higher carbon-to-nitrogen ratio and so decay more slowly. Woody material such as sawdust is mainly carbon, so the microorganisms must draw nitrogen from the surrounding soil, temporarily depleting it.
Natural Sources Of Nitrogen
Manure is a common source of nitrogen often applied to fields and gardens. The percentage of nitrogen it contains varies depending on the kind of animal and the feed it receives. On average, dried cattle manure contains 2 percent nitrogen and dried poultry manure contains 4.5 percent.
Fish fertilizer and blood meal are products that release nitrogen quickly, making it immediately available to plants. They are useful for getting perennials started in the spring and for greening foliage plants, but need frequent applications for long-term effect.
Cottonseed meal, feather meal and leather meal are all products that release nitrogen slowly, useful for long-term fertilization.
Cover crops are plants, often legumes, seeded to enrich the soil. When half-grown they are turned under to decay and release nutrients, especially nitrogen.
Artificial Sources Of Nitrogen
To make an artificial nitrogen fertilizer, heat and pressure are used to combine nitrogen from the air with hydrogen from natural gas. This forms anhydrous ammonia, a compound that can be applied directly to plants or turned into other compounds such as ammonium nitrate, ammonium sulfate and urea.
Using Nitrogen Fertilizers in the Garden
Since nitrogen is so closely tied to leafy green growth, it is essential to balance it with phosphorous and potassium for general fertilization. Quick or slow release materials that are high in nitrogen can be used for short-term growth enhancers, but too much will reduce flowering and fruiting and interfere with strong stem formation. Most flower and vegetable fertilizers are 5-10-10, with nitrogen being the first element and phosphorous and potassium being the second and third. The numbers indicate the percentage of each element in the compound.