plant food for self watering pot
 

hi, I have some self watering pots ( the ones with the reservoir of water
below the soil).

what is the best way to add plant food, please? I have been told that you
can just dilute the plant food into the water and it will evaporate and it
will then get to the plant o.k.

however I have since been told that phostogen is not good for this, even
though I understood it to be water soluble.

any suggestions on how to add plant food please. also what might be the
best plant food to use, in this case a small orange tree. many
thanks.