Which granular fertilizer can I give to potatoes now

This is my first year with an allotment, so I am obviously keen to do well, or at least do well with some crops.

My potatoes are looking good, but I feel I should add some fertilizer to them.

I think I spaced the rows a bit too close together (16 inches) so it's a tight squeeze if I was to walk in between.

Surely there must be a good granular fertilizer which I can throw over the top, either on a rainy day, or followed by a soaking with the hose (to prevent any leaves being damaged by dry fertilizer)

Would Growmore be suitable ?

Or is there a better option

thanks

John's advice here

http://www.allotment-garden.org/vegetable/potato-fertiliser-program.php