Enlist the sources of nitrogen fertilizer

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Explanation: organic sources of nitrogen include alfalfa meal, which supplies 2 percent to 3 percent nitrogen and lasts approximately one-third of the year. Dairy and poultry manure also can release suitable amounts of nitrogen fresh or composted manure can also be from an acceptable organic source such as small independent farms.

nitrogen is the main source of  fertilization to plants

Explanation:

the main and big source of nitrogen fertilizers are atmospheric nitrogen which has to be converted into nitrates because plants cannot pick up the atmospheric nitrogen directly but now genetic engineering has produced plants that can fix nitrogen directly from environment its main source is nitrogen cycle