Leaves turning yellow on young apple tree

Dear gardeners,

I have a young self pollinating apple tree in my fruit garden which is 2 years. Last week I started noticing that leaves are turning yellow. Is this normal or something more sinister? Its blossomed very well but still too young to bare fruit. I give it liquid seaweed as a feed once every two weeks. Please advise?

Could it be drought stress?

How much watering has it had in dry periods - not just during the recent hot weather, but generally too?