It's just simple botany. Plants don't pee or poop and roots only allow fluid to flow in one direction. Flushing is just an old stoner myth.
Plants have no mechanism for excreting nutrients, nutrients are not 'consumed' in violation of the first law of thermodynamics, and the only gas they produce is oxygen via photosynthesis. There is no way for the elements that compose a plant to be flushed out.
If we spent as much time overthinking tomato plants as we do pot plants, ketchup would be $100 a bottle.
Feed 'em til ya pick 'em.