Pansies with root rot

Hi there
Well I'm not having too much luck in the garden at the moment, my flower seedlings are being devastated by ( what I believe to be ) rampant earwigs, my petunias are covered in white fly (only the pink petunias strangely enough,not the blue...) but now my pansies have developped what would seem to be root rot.
I bought them on the cheap (they were looking rather droopy in the shop but thought they would pick up when replanted) and they did pick up for a while and now they're drooping again,bottom leaves turning yellow and basically dying...
You can pull up the whole plant so easily as if there was nothing 'rooting it down' which is the case because after closer inspection the roots are non existant , brownish etc...
So I guess root rot is the cause...

Is this because I re-used last years soil in the same pots ?
(Probably is.... I was trying to save a bit on potting soil... oups....)

So should I just throw out the whole lot desinfect the pots and start again with fresh soil ?

And for future reference : should I only use fresh potting soil each year or is there a way to 'clean' or 'recycle' the previous year's soil without spreading disease or unwanted bugs ?

Thanks for your tips !
Let's hope my currently thriving basil plants don't suddenly decide to keel over, at least they're doing well !

 

I always look upon pansies as a short term plant any way, best for winter and spring.

Soon time to get the summer plants in.