If there over 50% dead they go, or very brittle/crumbly they go. You don't want to take too many, but old, brown. brittle, or dried out leaves aren't doing your plant any favors so pull them. Especially if you had a PH problem recently addressed, get new growth going with happy healthy foliage
if your not in heavy flower you can foliar with nitrozyme - its pricey but I have tried a bunch of kelp products as foliar in the last year from samples and the nitrozyme is the bomb - lots of lush new growth
Thanks all! you all are great. Yea they are just the bottom leaves that are all crusty. I have new growth and they are coming back nicely. Will be triming today. These are my momma plants and want to be sure they are happy as I will be taking cuts at the end of next month.