In natural circumstances plants get nutrients from rain water run off with organic matter such as rotting plant matter, animal feces etc.. all of these "nutrients" are what a plant survives off of in the great out doors. If you are growing indoors, in clean soil and feeding JUST water.. your plants will likely die after a short while.. and if they dont die.. they will not grow well at all.. Humans can survive off of just water.. but optimum health requires nutrients... your plants are the same way ;-)