In the midst of a hot and dry summer my basil began to look feeble. It got yellow. The leaves become somewhat thicker and dryer. I fixed the problem with a dose of acid fertilizer (see growing basil). Two weeks later the leaves are lush and green. You can see the old yellow ones under the new growth.
Oddly, I don’t really use a lot of it anymore. I am pesto-ed out. Though there was pesto on the ratatouille I had at Pastiche and it was delicious.