During bloom is the ideal time to test your grape leaves and petioles (leaf stems) for any nutritional deficiencies. This analysis can be really helpful, particularly if your vineyard has suffered from low production or illness/disease. And you can use the results to add fertilizer or minerals if needed. Here is some info from FGL, one of the testing companies that I have used. ETS Labs and other testing companies perform this test as well.
For the leaf/petiole analysis, I have used FGL. Request the comprehensive petiole group. They will bill you around $60 for this. Follow instructions for sampling at bloom. Make sure to collect your samples in the morning, and cut the stems (petioles) of the leaves right away otherwise nutrients can flow from petiole to leaf, and brown bag them in separate bags instead of using ziplocks. I send them in one usps priority mail box instead of overnight because overnight is so expensive. If you use priority mail, get them to the post office before noon on either monday or tuesday so they get to the lab before the weekend, and send them to the Santa Paula address.
http://www.fglinc.com/documents/SamplingProcedures-AG/Grape_Leaf_and_Soil.pdf
https://www.fglinc.com/documents/FGLAgChain.pdf
