First I just now found your  question - my apologies for the delay. Your Windmill palms can use two tablespoons of Osmocote for each one foot of height so depending on their size you can adjust the amount. The yellow leaves may be due to cold nights as well as lack of nutrients. I predict they will green up by mid summer. Congrats on growing such a tropical looking plant in Oregon - the hardy Windmill Palms do well in Western Washington as well. Keep growing, Marianne Binetti