I randomly use my hydro leftovers to soil plants. I would guess it depends on how old the mix is. Salt buildup would be the biggest concern. I change water weekly, keep about 2gallons per plant, and my nutes are prolly still good when i change em. Of course, thats on bullshit plants sitting around. For mothers, I mix a light new nute solution. Depending on if i want em to grow fast, I go big lighting and new nutes weekly, or low lights and nutes bi-weekly. If you are keeping very select gals for mothers, I wouldnt risk it. Happy growing