Nitrogen is the most abundant element in Earth's atmosphere, but it is not prone to chemical interaction. This makes it impossible for plants to use in its gaseous state, though nitrogen is a key component in how plants produce food and chlorophyll. That's why nitrogen compounds are used in fertilizers. With few of the compounds acting alike, each has its own uses and qualities. For example, complex nitrogen fertilizer compounds have to be broken down by helper bacteria in the soil before plants can use them, but they are not as prone to washing out of the soil as simple nitrogen compounds with smaller molecular structures.
Urea is also known as carbamide. With a chemical formula of (NH2)2CO, urea has the highest nitrogen content by weight of all known nitrogen fertilizers at 54.7 percent. Because of the high nitrogen to low weight ratio, it is cost effective to transport and can be sold for a lower price and still remain profitable. This makes it a popular choice by consumers. It is mixed, bagged and sold with varying percentages of inert materials like soil, moss and vermiculite to accommodate the nutrient needs of different gardens. It comes in the form of dry granules that are sown in the soil. In the soil, urea reacts with the bacterial enzyme urease and breaks down into two ammonia molecules and one carbon dioxide molecule. With the application of water, the ammonia molecules react again to form ammonium, which is readily absorbed by plant roots. The roots are where it is broken down into nitrogen.
Anhydrous ammonia, NH3, is a gas at normal atmospheric pressure, so it must be contained under high pressure to change its state to a liquid. It is roughly 83 percent nitrogen, but because of the required high pressure containment, it can be difficult to apply. Application requires anhydrous ammonia to be injected a minimum of 6 inches into the soil. This ensures it does not escape. The inherent moisture in the soil converts the ammonia to ammonium, allowing plants to make use of it.
Sodium nitrate, NaNO3, is a naturally occurring salt that looks no different from table salt, and both are obtained by mining. It can be expensive because of the effort required to harvest it. Also, when broken down into its components, sodium nitrate is only 16 percent nitrogen. However, it is soluble in water and absorbed by plants immediately on contact with no need for chemical reactions in the soil. This makes it extremely convenient.