Question

Although nitrogen gas makes up 78% of the atmosphere, we can't use it because _____.

Answer 1

Although nitrogen gas makes up 78% of the atmosphere, we can't use it because nitrogen atoms must be broken apart first before. This nitrogen is very much packed and bonded with its atoms and would be useless to use since it can't be used for plants. 

Answer 2

The answer is - it can not be taken out of the atmosphere.

The nitrogen, even though makes for about 78% of our atmosphere, and is by far the most abundant gas in it, we can not use it because it can not be taken out of the atmosphere as it is. If we take nitrogen in the form as it is in the atmosphere it is useless for both, humans and plants. It has to go through a process of ''fixation'' first before we can actually use it in our food.