In terms of pot plants, how effective would it be to put back into soil dried out or broken parts of a plant? Is it worth the effort? It would seem logical, but I don't know how much of the actual plant (sans water) are substances suitable for use as fertilizer and if it's even a significant amount.
Not a good idea really - although leaves contain nitrogen when they're green, as they break down, nitrogen is robbed from the planting medium by the organisms which assist the break down process, which means your plant will have less nitrogen available. This effect is even worse with woodier parts of a plant, even if you have chopped them up. In the garden, the thing to do with green/woody parts is to compost them separately, then once the compost is ready, spread or dig that back onto the ground. I wouldn't even recommend that for house or potted plants unless the compost so produced had been created using a hot, aerobic system, which will kill off various pathogens (which can remain in cold, anaerobicly produced compost).