Give about 9 oz (1 cup) of balanced fertilizer (10-10-10) every month and every 2nd month give 1 cup of potassium sulfate.
Look at the forum WiKi above for info on Fertilizer.
Info:Fertilizer - Bananas Wiki
Mature banana plants need 1 lb of nitrogen and 1.5 lbs potassium, from this you can calculate how much is needed from the N-P-K ratios.