[al78]

I think one of the problems with carpet is that it isn't a solid barrier, it often consists of woven fabric which weeds can get through, meaning that if it is left down for a long time the weeds grow through the carpet, more weeds seed on the carpet after soil finds its way there and than you have a real job pulling the carpet up with a mass of weed roots sticking it to the ground. I had that problem with a half plot I took over where there had been woven black membrane laid which was covered with weeds that had grown through. Getting it up was like giving the ground a wax treatment.

The other issue with carpet is the possible leaching of chemicals into the soil. Carpet is banned at my allotment site.

[Christine]

I've been down the same problem route on a clearing plot as al78 and another plot holder spent a season pulling out carpet saying it was like strings and every time he thought it was clear his fork found some more. Neither of us were happy and the new tenancy agreement bans carpet on the grounds of the problems with clearing it off when you are a new tenant.

[oakridge]

I only use carpet to keep wind-blown seeds off the compost heap which is moved regularly on the land I use cardboard and mulch. 

[Yorkie]

Regarding using tap water for seeds. I think the idea is that tap water isn't going to have any fungal spores that could cause problems to seeds or seedlings. My view is you are sticking the seeds in what is most probably composted green material, that will be a huge reservoir of fungal spores anyway so can't see using water from a spring or water butt is going to make much difference.

Bought compost will have been sterilised so shouldn't contain too many nasties.  Home grown compost will, of course, and for this reason it's not advised as a base for sowing seeds / seedlings.

[Trikidiki]

The commercial hot composting process kills off weed seeds fungal spores and pathogens where the heaps get hot enough, it does not guarantee the compost is sterile.  PAS100 which is the British Standard for commercial composting doesn't mention sterility, it even allows for a detectable level of E.Coli.

Even if it were sterile at packaging, most compost bags have perforations which can allow ingress of rain water (you now how heavy the last few bags from a pallet can be when they've been stored outside), the rainwater that seeps into bags is the same rainwater that goes into water butts or the surface water spring that the OP was asking if it is safe to use.

[oakridge]

I don't buy any compost, but the compost I make does get very hot.  I sift the compost and add vermiculite for seed compost and now I hardly ever get weeds growing.  On one tray of leeks some green algae has appeared on the surface.