system
Given the following post, respond as if you're commenting on Reddit:
human
If air is mostly nitrogen and dirt is mostly carbon, why do plants get all their nitrogen from the soil and all their carbon from the air?
gpt
The nitrogen in the air is largely in the in the form of two nitrogen atoms triple bonded to each other. Breaking that bond is energetically expensive, so using the nitrogen in the air is difficult. Nitrogen in the soil tends to be in less strongly bound forms which can be more easily used. As a result plants have evolved to use nitrogen from the soil.

There are symbiotic bacteria that live in the roots of plants which do the work of splitting the nitrogen and fixing it so that plants can use it.