Space

But isn't the point to make the regolith into something other than murder dust?

The point of these experiments is to find out what will happen.

Which btw is the point of all experiments.

Hydroponics requires biologically available nitrogen too and it has to come from somewhere. The point of these experiments is to explore a wide variety of scenarios. Hydroponics as a growing method has some drawbacks too. Any robust food production scheme in space will likely include a mix of hydro- and geo-ponics (fancy name for soil growing).

Also consider the non-food benefits of living plants and soil. Any long term habitat on the moon or Mars will need living plants for the psychological and air quality benefits. There is a massive body of research that shows the benefits of having trees around. There is even research that shows just smelling healthy soil can be antidepressant.

Let’s say we enclose a large lava tube, one seriously considered approach for long term habitats. Having some greenery would go a long way to staving off Space Madness.

I am not familiar with what is required for hydropony, but I would guess it requires more equipment. Plus growing them on Lunar soil means eventually you get some elements from the Lunar soil itself and do not need to have full recycling otherwise, which means you don't have to have a fully closed cycle for this.

There is still the issue of the closed cycle for air though (which is where Mars is easier than the Moon for medium term colonies).

I'm not an expert, but as a lay person I agree with you.

However, I'm guessing people running these experiments are just looking at all the options. Hydroponics is pretty much a solved solution, no? I suppose it makes sense to run these experiments to prove what we already know. I suppose there is a hyper slim chance that lunar soil was somehow beneficial for the plant and made growing easier.

I think the nitrogen issue is probably secondary to the water issue. Hydroponics use way less water. Water is heavy and expensive to ship to the moon. Anything to reduce weight will probably be the priority.

I don't see how shipping more soil and more water would be better than hydroponics, but sometimes we write papers for problems we don't have yet, but might have in the future. Some moon colony might be struggling for physical space and need to use soil to grow supplies in an emergency? Might be nice to know they can reserve some moon land and follow these guides to produce food in the soil.

My wife got her PhD in Physics and one of her favorite anecdotes is the critical piece of technology that made her project possible was derived from a mathematical paper written 200 years ago. The dude didn't have the technology or science to know where his math would be useful, but he published it anyways. Eventually, someone needed it and found it useful.

¯\_(ツ)_/¯