Nothing says unsustainable more dramatically than a bunch of plastic bottles being toted to the landfill (okay- maybe there are a few more dramatic examples). I've always foraged the recycling can for better uses than future soil bulk. I've posted how to turn plastic bags into a weave-able material, now it's time for the bottles.
Need to
water slowly? If you aren't in sand, fill a nice juice container (can also be done with used glass bottles) and dig a shallow hole big
enough to shove the muzzle of the bottle in. Quickly flip and shove so that the bottle is up-side down with it's nose in the ground (like a dachshund on the hunt). put in a cork if you have trouble viewing water level. It
should slowly leek into the ground around it. Your soil type will determine the area of watering. If you're curious how affective shove something in the ground and check the moisture.
Dang slugs got my cabbage! Slugs 1, Amit 0. Time for plan B (or was it F?). Now you'll need plastic unless you have a way to cut through glass bottles. Cut off the bottom of your juice can and stick it over the little seedling (preferably before the slugs get to it and definitely while the slug isn't there). It's working so far. Also works against other animals, but not 2 year old humans. They tend to think it fun to re-arrange the covers.
Frost protection? Place the bottle, but put a thin piece of fabric and a rubber band (or some siding holes and the lid) so that you get a little air circulation, but not the frost. I think you can forget the frills on light frosts, but hard ones require extra support.
Need a watering container? We all know that the best way to water seedlings is with that light rainy pitter-patter. Well, a 50 dollar watering can wasn't in my budget so I guzzled the remaining
milk in the gallon container, made nail-sized holes at the neck opposite the top of the handle. I made a small, robin's egg sized hole above the handle, unscrewed the lid, filled her up, screwed the lid back on, and rained the water lightly on the seedlings. Even fits in a 5 gallon
bucket for faster (submerged) re-fills. No seedling complained of the pressure and the watering was done in just the amount of time I have patience for.
Supposedly there's ways to hook plastic bottles to hoses and turn them into sprinklers (haven't tried)
They have also been used for lighting & housing, but I haven't tried this either - yet.
They can be used for wicking pots or seedling pots (but I prefer used toilet paper tubes (and so do the earthworms)).
They make rattles and (when attached nose-to-nose with some food coloring and oil) they make good toddler toys.
Not that plastic bottles are
the answer, but while your still unable to survive off your back-yard, you might as well re-use