All text in block quotes (like this sentence) is a direct translation of the Kli Yakar's words.
So the Kli Yakar recalls how Hashem originally commanded the earth to produce fruit trees in which both the fruit and the tree would taste the same via an element that was "pure and fine." Instead, the land put forth an element that was "dense and coarse," causing the fruit trees to be as they are today (which are pretty nice, but they could have been even better).
And this is what caused the taste of the tree to be unlike the taste of the fruit because there was a coarse and foul component, which also instilled Man with a yearning for the most nonspiritual desires.
And if this component had been pure and clean, he would never come to sin.
Therefore, the earth was not cursed until the sin of Adam because that's when the damage was most substantial.
But with the fruit trees, the damage was not so great.
....Therefore, we find that in every place where the sin of illicit behavior [gilui areiyot] is found, so Hakadosh Baruch Hu recalls the sin of the land because that was what influenced Adam Harishon, who sinned with the Eitz Hadaat, which in turn instilled within him a great inclination toward illicit behaviors....
"....upon seeing the plagues of the Land and the diseases with which Hashem struck it."
"Sulfur and salt have burned all the Land...."
Because with boiling heat it was corrupted and with boiling heat, it will be judged by fire in all the land
According to the Kli Yakar's conclusion below, the predicted vomiting out is neither metaphorical nor miraculous, but a natural process, a sort of spiritual physics, that must happen whenever illicit behavior reaches a certain point.
And when it says, "and the diseases with which Hashem struck it," it is using an example of how every sick person's way is to vomit out that which is in his innards.
Likewise, it says here, too, in this section on illicit behaviors "and the Land vomited out its inhabitants" because it is like the sick person who vomits out the food within him.
And regarding that, it says, "And the Land became defiled and I recalled its sin upon it."
And this is a precious lesson.
When people give into their baser selves, it's not because they are innately bad at their basic core, but because of the fruit which we have no other choice but to eat (because all fruit trees ended up being produced from this baser component).
However, there is still a natural point at which illicit activities can no longer continue.
May we all merit to always actualize the best parts of ourselves.
Although I did borrow a couple of terms here and there from Rabbi Elihu Levine's translation, this is primarily my own translation and any errors are also mine.