I'm referencing to a great transcript resource for the quotes below.
Surprising to me, no hint that Sandoval's colony/settlement on Omicron Ceti III is an agricultural one is even mentioned in the opening exposition/dialog until after the landing party beams down and Sandoval says, during the meet-and-greet scene in the house, "You see, Omicron is an ideal agricultural planet."
I guess it is one of those things that the audience was supposed to guess at purely from the visual setting.
On the other hand, the Macguffin Berthold Rays are expounded upon three time prior to this already, just to pound home to the audience how extremely deadly they are, but of course, conveniently, only after long exposure.
The next mention is by Sulu while investigating the environs, "When it comes to farms, I wouldn't know what looked right or wrong if it were two feet from me." Tellingly, this is spoken while two feet from an alien spore plant, which of course we don't know details about yet, but they get their own "strangeness" musical theme, so we know something is up. So here is some of the richness -- are the strange plants part of Sandoval's agriculture? Yes and no, as it turns out.
Sulu's partner Kelowitz develops the agriculture plot thread further with, "No cows. This barn isn't even built for them." To which Sulu replies, "Come to think of it, we haven't seen any animals. No horses, no pigs, not even a dog." Hmmm.
Later Spock chimes in hammering the same thing to us, "There seems to be a total absence of life on the planet, with the exception of the colonists and various types of flora." To which McCoy opines, "No animals. That's peculiar." And to which Kirk gets the capper, "Yes, especially in view of the fact that the records for this expedition indicate that they did have some for breeding and food purposes. Apparently, none of them survived." Alrighty then that's settled.
Finally, Sandoval makes his agricultural mission rather explicit to Kirk and McCoy, "You haven't seen our fields and crops. I'd like to show you and the Doctor what we've accomplished here." Only Kirk goes to the field with him, where he says, "This soil will grow anything we plant in it. It's a perfect world. We have a moderate climate, moderate rains all year round. It gives us all we need. It is perfect." But then Kirk gets the biology report from Leslie, "I heard Sandoval saying they could grow anything here. That's true, sir. They've got a variety of crops in. Grains, potatoes, beans." And, "Well, sir, for an agricultural colony, they have actually very little acreage planted. There's enough to sustain the colony, but very little more." And so the plot thickens.
Spock is also in a field, with Leila: "Nothing. Not even insects. Yet your plants grow, and you've survived exposure to Berthold rays." Leila knows something, but won't say, yet.
Kirk and Sandoval together again: "What about your animals?" "We're vegetarians." "That doesn't answer my question, sir. Why did all your animals die?" Sandoval also isn't talking, yet.
Sulu makes his final report to Kirk before the colony is to be evacuated, "Captain, we've checked out everything. It all seems normal, except for the absence of any animals." How many times do we need to be reminded?
Meanwhile, Spock had already encountered the spores and their effects. Since they're a major plot element, I am not going to "count" them in this agricultural subthread. Food for another post maybe. Except for this final entry from McCoy, "DeSalle said he was going to examine some native plants he found."
And the spores dominate the entire remainder of the episode. Except for fighting in the fields with farm implements, there is no agricultural reference until this final exchange about farm work between Sandoval and McCoy: "Well, Doctor, I've been thinking about what sort of work I could assign you to." "What do you mean, what sort of work? I'm a doctor." "Not any more, of course. We don't need you. Not as a doctor." "I am the leader of this colony. I'll assign you whatever work I think suitable." And then after they fight and Sandoval loses his spore influence, "We've done nothing here. No accomplishments, no progress. Three years wasted. We wanted to make this planet a garden." And, "I think I'd, I think we'd like to get some work done. The work we started out to do."
And that's it. They didn't cultivate the spores; the spores helped them live; but they kinda ruined their will to do the agriculture work they came to do. Overall, a tragedy.