I'm curious if you add amendments to improve your soil?
I went down the KNF rabbit hole after I found this dude. His cannel is awesome. Shows how to make all the natural inputs for soil health and foliage sprays.
It improved my cannabis crop by orders of magnitude. Could do the same for your grapes.
Its all about enhancing your environment with your environment. The opposite of salt farming.
https://www.youtube.com/watch?v=H5p4x986Eb0&t=372