On the surface, this just seems like a simple song about what plants need to grow. However, I think it also makes a rather profound statement about selfless love.
During the bridge section, Ernie sings about how he doesn't reap any tangible/observable benefits1 from caring for the plant. But even though he doesn't necessarily "get anything" out of it, he still continues to care for the plant, because the plant needs him and because he knows that the plant would express appreciation if it ("she") were able.
Jim Henson was very concerned about ecology and environmentalism. I feel that he put a lot of heart into the performance of this sweet, deceptively simple little song.
1Although the song doesn't mention it, of course plants do give us oxygen, which is pretty important. However, the process of photosynthesis isn't grossly observable, and it's entirely possible that Ernie isn't aware of it yet.