does anything come OUT of the roots ?

[Mr Greenjeans]

I have a general plant/gardening question. Does anyone here know if anything comes OUT of roots ? I realize a mixture of water and dissolved nutients go IN the roots via roothairs, I believe. Besides needing to keep the nutrient level adequate is there anything that the plant puts back into the solution which might be harmful to the plant and worth removing ? i. e. drain the water/nutrient mix regularly and refill with a fresh batch. Use the waste/drainage to water the houseplants or soil-grown.