Fertilizer is any type of soil amendment. Used since ancient times, fertilizers may be either organic or inorganic. Organic fertilizers derive from something which was, or is, living; inorganic fertilizers are usually chemically synthesized. There are three primary macronutrients that soil and plants need: nitrogen, phosphorous and potassium.
Our human ancestors typically used organic fertilizers, such as fish meal and manure, along with mined fertilizers to increase the fertility of soil for farming and gardening. Inorganic synthetic fertilizers were not developed until some time during the Industrial Revolution. There are three primary macronutrients which soil and plants need: nitrogen, phosphorous and potassium. Soil also needs many micronutrients, but on a smaller scale.
Bags of fertilizer are typically labeled with three numbers. These numbers represent the ratio of how much of each primary macronutrient is present. The primary macronutrients are always listed in the same order: Nitrogen (N), Phosphorous (P) and Potassium (K). Nitrogen fertilizer might be called for when a soil test indicates the soil lacks a sufficient level of nitrogen. A label of 60-0-0 represents 60 g of nitrogen. Organic fertilizer usually clearly states it is organic on the label.
Soil amendments are important because they increase production and make plants healthier. Fertilizers do this because they enrich the soil with the proper macro and micronutrients, and sometimes trace minerals. Nitrogen fertilizer increases plant biomass. Latest statistics show one billion tons of nitrogen fertilizer are used annually. Unfortunately, most of this is inorganic. Inorganic or chemically synthesized nitrogen fertilizer is commonly used in large-scale commercial agriculture.
Organic or Inorganic: Does it Matter?
It does matter whether a fertilizer is organic or synthetic / inorganic. Here are two reasons why:
1) A major, long-term UK study by Newcastle University showed that produce and cattle raised organically contain higher amounts of antioxidants and nutrients than those grown inorganically. These antioxidants and nutrients can have a dramatic impact on human health.
2) Evidence is conclusive that inorganic or chemical fertilizers run off into ground water, rivers, creeks and eventually to the oceans--causing dead zones, fish kills and excessive algae bloom or red tide.
Nitrogen is present in the air. To make it accessible to plants and thus to human bodies, it must go through a process called nitrogen fixation. The artificial means of fixing nitrogen is typically accomplished using natural gas or coal as an energy source. The Haber-Bosch process is most often used to convert nitrogen into ammonia. Since supplies of coal and gas are limited, this process is not sustainable.
Cover crops such as clover and vetch, which are grown and then plowed under, can be used as a type of "green manure"--the common term for cover crops. The plants capture nitrogen from the atmosphere within root nodules. The main way that nature "fixes nitrogen" so it is available to plants is through bacteria. Farmers discovered that peas, beans and alfalfa were nitrate-fixing crops and nitrogen could be increased in soil using manure, guano and nitrate deposits. Another way nature fixes atmospheric nitrogen for plants and animals is through lightning.