Ethically, if you are uncertain about attributing sources in a speech, your text suggests

when in doubt, document all of your sources in a speech.

Explanation:

It is the ethical responsibility of a speaker that he should provide honest and unbiased information to his audiences. And it is not a one-time event but a speaker should consider ethics while public speaking, every time. A public speaker who avoids plagiarism and mention the source of information increases his credibility. But in case of doubt, documentation of all the sources becomes necessary.

fertile good for trade etc