Patio fruit tree feeding??

I have fig, apple, and plum patio fruit trees. As they're in pots they need watering very often as in quite a sunny spot. I'm assuming that is washing out all the goodness from the soil. What should I feed them with to encourage fruit for next year as opposed to just boosting leaf growth. They are babies but are due to produce next yr if Thompson and Morgan are to be believed. Thank you for any advice.

 

Tomato feed at the end of the season is good for most plants, its high in potash
Dont use high nitrogen at this time of the year, although a balanced feed would still be ok.

 

@pete thanks I will do that.