No, Coca-Cola is not good for plants … and you can substitute the soft drink of your choice for Coca-Cola in that statement. Where this strange belief comes from, I have no idea, but it’s widely repeated on the Internet.
Soft drinks are essentially carbonated water with lots and lots of added sugar. Carbonated water in itself is not harmful to plants and could even be good for them under some circumstances, but that’s not the case for sugar or, should I say, sugars, as various kinds of them are used in soft drinks: glucose, fructose, sucrose, etc.
True enough, plants produce their own sugars and use them for their growth, but they can’t absorb them: the molecules are far too complex for roots to take in. And when you break down sugars, they give only products plants are already getting from air and rainwater: hydrogen, oxygen and carbon. So where’s the benefit?
Adding too much sugar to soil can lead to dehydration, as water will be pulled out of the plant’s roots to dilute any concentration of sugar (this is due to osmosis, something many of you probably vaguely remember from school). Plus the bacteria and fungi that break down sugars are not usually ones that are beneficial to plants in any way and may become so numerous they “gum up” the soil, preventing air and water circulation. Also, many sugar-loving soil microbes are actually harmful to plants, notably causing root rot.
So, watering plants with soft drinks does no particular good and can possibly harm them.
I suggest just watering plants with … water! It’s cheaper and better for them.