Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: In order for nitrogen to be used for growth in organisms, it must be "fixed." When nitrogen is "fixed," it is combined into the form of ammonium or other nitrogen ions. During which type of fixation do organisms and nitrogen interact?
Biological fixation.