Final answer:
The two natural ways nitrogen can be fixed for plant use are nitrogen fixation, where bacteria convert atmospheric nitrogen into organic forms, and nitrification, where ammonia is further converted into nitrates that plants can utilize.
Step-by-step explanation:
Two natural ways that nitrogen can be fixed for plant use are through the processes known as nitrogen fixation and nitrification. During nitrogen fixation, specialized bacteria in the soil or in symbiotic relationships with the root nodules of legumes convert nitrogen gas from the atmosphere into ammonia or other organic compounds that plants can utilize. Following this, nitrification occurs, where bacteria oxidize the ammonia to nitrites and then to nitrates, which plants can directly absorb as nutrients.