Here are a few suggestions that might help. The economy of this historic country depends solely on the diversity of the crops that are harvested. The word, ANCIENT is used for places that no longer exist as the did, however, the word, HISTORIC, refers to places that are well known in history and still important. People would need to be able to HARVEST crops as well as the PLANT to survive. The sentences structure needs to stress that DIVERSITY helps them survive. The word SOLELY stresses how completely they rely on this diversity. The word, ONLY, is not as strong. Hope this helps

I would say it sounds just a little off. A couple of things to address: (1) Is the 'ancient country' a country that existed in the past and does not exist anymore? If so, the past tense would be required. (2) 'only depends on planting' <- this section is where it gets a little awkward. First, it would feel more natural if the 'only' and 'depends' were switched around to read 'depends only on planting...'. Second, 'only' sometimes carries the meaning/nuance of sufficiency/simplicity ("It's only 2 hours away, so close!"), while what you want to express--indeed emphasize here is the single factor on which the economy of this country depends. In which case 'solely' rather than 'only' emphasizes that a little better. There, your sentence is up to snuff, and I've read much worse from native speaker college graduates. If we were to nitpick more though: -'rich' and 'diversity' sound a little repetitive--after all, it's not like the 'diversity' of crops would be 'poor', right? -Avoiding 'the X of Y' formulations in favor of 'Y's X' formulations are considered by some to be a better form of expression. So saying 'This ancient country's economy' rather than 'the economy of this ancient country'. This isn't a set rule though, it depends on what you want to emphasize, and your style of writing. -Making the last idea 'planting a rich diversity of crops....' into a phrase without a verb sounds more natural for some reason to me. Putting this all together, we get a couple of slightly more natural sentences: -The economy of this ancient country depends only on planting a diversity of crops -The economy of this ancient country depends solely on the diversity of its agricultural production -This ancient country's economy depends solely on the diversity of its agricultural production