All my compost has always come from my local garden centre and most of it is J Arthur Bowyers Multipurpose. I have several pots that are 3/4 full of old compost dating back two or three years. They are probably pots that I had planted up with summer displays, and come the autumn I would have just pulled out the shallow rooted contents and left the compost in situ. I'm wondering if it would be alright to leave about half a pot full of this old compost and top up with fresh new compost. I will only be planting annuals.
Also, I have a couple of opened bags with about 1/5th of the compost still remaining and unused. One was opened in February and some of the soil has a white substance on the side that is in contact with the bag. Is this still ok to use?