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as another anon said, soil test. you can do it yourself, or I guess ask someone with some soil science knowledge. There is also the online USDA web soil survey in America (and I would imagine most countries have their own service). You can use this website to find what soil types are on your land, and other features of the area and the land. It's not perfect, and certainly not the final and definite survey of the land you should conduct before buying. You actually have to go to the land and feel the soil. Try digging if you can. Kind of annoying to have rocky land, for instance. You'll break your back every time you want to do anything. You want a loam, with good drainage, and basically there are crops that prefer different types of soil and pH so there is that, too.
Another factor I would consider is water rights, if this is land for cultivation, or even just for living on. I would look specifically for land with water running downhill. This can be used for power generation even, aside from irrigation. Of course, I would consider flood plains and how floods will impact your land.
If there are mature trees on the land, that can be useful in a few instances, for instance, nut trees take many years to mature, and have a high protein, high fat output.
Being near a dump or a prison or some other nightmare is also a possible reason why good land might be cheap, so look out for that.
Zoning laws are useful to consider, for example, some counties have unrestricted land where you don't need to spend a lot or anything on getting permits, because many outbuildings might not need permits. But then in New Jersey or something, you're not going to have that luxury, so I'd look for unrestricted land or something like that.
Then I'd also say climate matters. Northern Idaho gets cold, and has a short growing season. The prairies also have high winds and cold winters. parts of the south are pretty good though, but ticks