Take a world with the same physical characteristics as Earth, but where biochemical development has "rolled the dice" differently in some cases.
- The simple chemistry are the same compounds in slightly different proportions. The air consists mostly of nitrogen, oxygen, argon, carbon dioxide. The water is ordinary H2O, with dissolved salts in the oceans and the trace organics one would expect from the ecosphere (see below). Sand and rocks are made from silica and carbonates, mostly.
- There is carbon-based life which looks much like life on Earth. Simple organic compounds like ethanol or butane are the same. The differences are in the more complex compounds.
- Life includes something like photosynthesis-based flora and mobile fauna. Details differ, of course.
What are the problems if humans want to raise crops and lifestock on this world outside a sealed greenhouse?
It seems obvious that humans could sterilize soil and bring it into a greenhouse, but that is not necessarily easier than raising crops in a space station.