Final answer:
A plant obtains carbon from the air, specifically in the form of carbon dioxide which is used in photosynthesis. Nitrogen is also essential for plants, but it is usually not absorbed directly from the air but rather converted by soil bacteria into a usable form.
Step-by-step explanation:
The substance that a plant obtains from the air is carbon. Plants absorb carbon dioxide (CO2) from the atmosphere through a process called photosynthesis, which is vital for their growth. Nitrogen from the air is also critical for plants, but it is not directly absorbed from the air by plants; instead, nitrogen-fixing bacteria in the soil convert atmospheric nitrogen into forms that plants can use. In contrast, elements like magnesium and other essential nutrients are typically absorbed by plant roots from the soil – not the air – whereas water can be absorbed from the soil through the roots, and also directly from the air under certain conditions.