Please some export advice on a issue I have.

Hi all I have not posted anything in a wile on here but comes a time where you need a little advice from the stoners around the globe.

Right down to the point-
I have a plant growing in a 20 litre air pot with canna stable ph soil and using cannas bio range + boost.
Its 3.1/2 weeks into flower and the buds are joining up nicely but I have noticed yellowing starting from the bottom what I thought was just not getting much light down there so I ignored it but now the bigger fan leaves at the top of plant are yellowing and drooping and dieing.
The tips of leaves are burnt so it has nute burn and possibly cause of nute burn it's looking out nutes for plant consumption.
So my question is should I wait for the plants soil to dry and then flush or flush ASAP?
Then after flush wait for soil to be dry again then very light nutes?
I have only started doing soil plants recently as i mostly do hydro and get nice results with that but I think the weed is better quilty with soil and bio nutes. So ATM I have 1 soil and two scrog hydro under 600 hps and 200w
cfl.

Thanks in advance

Sorted it now buddy I was using bio canna Flores and none bio boost and it was counteracting the bio floures so I'm have flushed and will nute with bio boost in future