RE: Fertilizing Crab apple trees??
Duckwhacker, pruning and fertilizing any apple tree will definitely help them bear fruit if it is done correctly. Since I don't know where you are from I would suggest that you do the pruning and the fertilizing at the same time. It should be done in the late winter before the sap starts to rise from the roots. In Virginia I would do it in February.
You would want either an all purpose 10-10-10 or similar fertilizer. The middle number is the amount of phosphorous, and this is what they need to flower and develop fruit. Use at least a middle number of 10.
As for application it should be sprinkled around the outer edges of the limbs; also known as the drip line, this is where the feeder roots are located. Do this part before you prune the limbs. Just don't overdo the fertilizing or pruning.