Have you done a soil test in that area ?
When in doubt , fertilize with phospherus and potassium or plant vigor and heath .
Grapes can get quite demanding in their need for minerals and trace elements.
I can not recall if he did a soil test before he planted the vines or not. My thought was that if it wasn’t a cow or horse pasture, he should do so and plan on some soil improvements - he does as he wishes - it’s his darn grapes - he thought he’d make some wine.
I’m still waiting for a sip. Good thing I like margaritas and Irish Coffee better than wine - LOL