Question

why did plant need to fix nitrogen

Answer 1

it is an important part of many cells and processes such as amino acids, proteins and even our DNA. It is also needed to make chlorophyll in plants, which is used in photosynthesis to make their food. As part of these life processes,nitrogen is transformed from one chemical form to another

Answer 2

N2 for plants is vital to the sucess of a garden. Without sufficient N2, plants will fail and will be unable to grow.
N2 is abundant in the world but most of the N2 in the world is a gas and most plant cannot use N2 as a gas. Most plants must rely on the addition of N2 to the soil in order to be able to use it. There are a few plants that love N2 gas, though; they are able to draw the N2 gas from the air and store it in their roots. These are called N2 fixing plants.