I grow aparagus outside. Catstuff is toxic to them. I don't know if planting the cats add nutrients to the soil, but it does stop them from poisoning the dirt. Around here though, the red/brown seaweed though is a traditional additive to compost. My personal bias is to not use organic fertilizers in an indoor garden. Indoors I use sterilized soil and chemical ferts. It's too easy to get an infestation of mould or critters into an indoor ecosystem -- and it's a lot of work to eradicate them and sterilize the room once it's happened. I've got those darn black flies in my grow room right now. They didn't come from the dirt or fert -- another grower was inspecting my setup, and he brought the flies with him from his house. It only takes one . . . and this example shows how vulnerable an indoor garden is to pests, even when you do everything you can to keep them out. So, save yourself some potential problems and think carefully about making your own dirt or fert for indoor growing.