While sixteen elements may be confirmed as essential to plant growth, human growth needs more. Remember iodine? Others like Yttrium, strontium, cobalt, scandium, and who knows how many more, are vital for vital human health.
Don't forgot the most imortant element, carbon; as in sunlight and carbon dioxide produce plant sugars through photosynthesis. This is also important in lowering atmospheric carbon dioxide and carbon sequesteration.
Indy, plants use carbon dioxide from the air and also water and sunlight to make the sugars that they need to survive, like ecshroeder said. The plant then releases oxygen back into the environment. So therefore, plants help to decrease the overall level of carbon dioxide in the air on earth. Which is the main reason why deforestation is such a problem. I read somewhere that one full size adult tree produces enough oxygen to supply like 100 or 200 people with breathable air. And many thousands of trees are cut down each day.