I'm guessing that they would have added some simple nutrients and tried with plants you would find in e.g. deserts that are used to grow in very low quality soi

I'm guessing that they would have added some simple nutrients and tried with plants you would find in e.g. deserts that are used to grow in very low quality soil. Over time, you could probably make the soil richer simply by supplying simple nutrients and chemicals and letting nature do its thing. The importance of that is that there would be no need to transport many tonnes of earth soil. Though using some of that would probably help bootstrap soil production. Chemical fertilizers are mainly produced using simple chemicals (hydrogen, ammonia, phosphates, etc. So, you could see that happening on e.g. Mars which probably has most of what you'd need in some form. I guess the Moon would be more challenging as it has no atmosphere and lacks elements like nitrogen and carbon.