Source?

There are many ways in which organic and non-organic affect the environment other than land use (which is likely a marginal difference). Big agro, most associated with non-organic farming, does lots of bad things for the environment. Pesticides that are linked to decline of bee populations, pesticides that yield local imbalances in what animals/bugs are around to eat each other, synthetic fertilizers that thrash the land from being able to grow diverse crops into the future, chemical run offs into local water sources, exposure of workers to high concentrations of pesticides and their subsequent-related illnesses, GMO seeds spreading farther than the intended land mass mating with other species producing unknown effects (and lawsuits), etc.

This makes intuitive sense but I have never seen good evidence for it. I personally do not believe that:

  - organic food is bad for the environment 
  - organic food is less space efficient to grow

I do believe that:

  - organic food may have lower crop yields

Uh...

> - organic food is less space efficient to grow > - organic food may have lower crop yields

These are the same thing. If you have to generate X calories, and you generate fewer per acre, then you need more acres of crop.

Think outside the box. Think 3D agriculture. Aeroponics.

But if you're willing to do that for organic you're also willing to do that for non organic.

Organic food raised like industrial food will often take more land. But organic food raised in polyculture can sometimes take dramatically less space. Look up Land Equivalent Ratio and intercropping.