Question

almost all plants directly use atmospheric nitrogen ​

Answer 1

Answer:

Plants do not get their nitrogen directly from the air. Although nitrogen is the most abundant element in the air, every nitrogen atom in the air is triple-bonded to another nitrogen atom to form molecular nitrogen, N2. ... Plants get their nitrogen from the soil and not directly from the air.

Answer 2

Answer:

Nitrogen in atmosphere is found as a pretty inert molecule (N2) and it requires a lot of energy to turn it into something else; plants lack this ability because they do not have molecular tools for that, namely enzymes (nitrogenase) that some bacteria do - and they readily use it to fix nitrogen from the surroundings; the reason why the process is happening mostly in the soil and not in the air is that the enzyme is sensitive to oxygen, so the relatively anoxic soil is better suited for the task than the oxygen-rich atmosphere.