We’re using our own mix of inert materials to create the substrate. I can’t tell you exactly what we’re using, but it’s cheap and very light, with a bulk density of about 12% that of loose soils. Shipping something to space costs a LOT, so we’ve been trying to make it as light as possible.

While any fertilizer can be used, NASA did ask us whether the system could be used beneficially in any other part of the habitat or trip there. It is possible that the astronauts can turn their waste into fertilizer for use in the system, though it must be done safely and responsibly. In terms of decades or centuries long fertilization of the system, I’m not entirely sure to be honest. With our current technology, there’d almost certainly be some loss in terms of water filtration, so a perfect system can’t be achieved. How long would that be? I’m not sure because I haven’t seen the details of their water filtration system.

TL;DR: It’s unlikely that it’ll be able to run itself forever with our current technology, but we expect to get a good 3-5 years out of it.