The large green bottle of fertilizer is for more flowers on the plant. The smaller green bottle is a general help for both flowers and foliage. The best one is the small yellow (for 150).
As for ranking, the water ratio bar does not change - I'm not sure why, as it's been that way for the years I've had the game. I played the game a few years ago, and now have come back to play again. You could play on easy, than try hard. Still, to come up with over 500 species of plants will take some time - enjoy the journey