I've got two "hot lips" salvias. One is extremely full and green w/ many blooms, while the other is more woody and the leaves have yellowed over the last several months. Both receive equal sunlight. I can't figure out why the one isn't thriving. I've tried pruning it and have considered testing my soil for acidity, but don't know how. Any suggestions for what might be causing my problem, and how to treat it? Thanks.