For naturally slow growing plants the fish food/fish waste should be adequate. For fast growing plants in large quantities in the tank, I doubt that fish food/fish waste would be enough. For a mix of growth rate plants, which is probably typical, you would have to experiment to find out, if you really want to know. If it isn't enough I would expect to see problems with the least competitive fast growing plants. It isn't an easy thing to determine, because a shortage of just one of the nutrients slows down the growth, and that slower growth may reduce the demand for phosphorous enough that fish food/fish waste supplies enough. When hobbyists first began to dose phosphates they quickly learned that eliminating the shortage of phosphates often led to shortages in other nutrients, so they had to dose more of everything else once they started dosing phosphates.