"The earth does not taste good."
Translation:La tero ne bongustas.
Gusti means only to give the sensation of a taste. To get the sensation of a taste is gustumi. Gusti as a verb takes an adverb. Nothing special about that. You can put them together to form the word bongusti (= bone gusti).
The earth does not taste well. = La tero ne bone gustumas. :-)