The appropriate experimental procedure that would provide evidence that plants need nitrogen to grow is:
Identical plants are placed in two airtight containers with sufficient exposure to sunlight. One container has nitrogen gas, and the other has room air. The scientist measures plant growth.
This setup allows for a direct comparison of plant growth under conditions that vary specifically in nitrogen availability. One container has a controlled high concentration of nitrogen (nitrogen gas), while the other has a normal atmosphere (room air) with a lower nitrogen concentration. By measuring the growth of the plants in each container, the scientist can determine the effect of nitrogen on plant growth.