because it does not have the root structure, evaporative capabilities, or need of its donor.
I use 200ppm as a start with clones
each day, I refill the missing water from the rez and check the ppm's
if the ppm has gone up (rarely) I will let it go as long as it isnt rising more than 50ppm a day.
if its rising more than 50, I take water out and add RO water to bring it down.
normally, the ppm drops each day. I add back the amount that dropped, plus that amount again
for example
day 1 - 200ppm
day 2 - 190ppm, raise it back to 200, then add 10 to bring it to 210
day 3 - 190ppm, raise it back to 210, then add 20 again, to bring it to 230
day 4 - 235ppm, levels went up, so I would leave it at 235 because they didnt go up 50.
day 5 - 225ppm, levels dropped 10, so I put 20ppm in to bring it to 245.
this way you are constantly monitoring and recording the changes, and when they eat, you ramp them up bit by bit.
be sure to balance the ph after you add nutes. don't do rez alterations while you are high either, I have had to drain and refill from spills while high that took the ppms to 2000+.
it is important to note that you should be comparing the previous days ppm's to the current days ppms only when the rez has been topped with water to the same mark. comparing the ppms in half the water won't give you an accurate mesurement of what the plants are consuming in terms of water absorbed and ppms used.
I dont know where some of the numbers come from that are commonplace to see thrown around here, each strain is different, I have seen strains in full bloom that cant take ppm's over 900, and others that can take 1800+