Its very common for us around here to pour the concrete "in the hole",
using the hole itself as the mixing container. Of course the amount of
rebar/cage construction will play a big part in that scenario. Just fill the
hole partway with water, add sacrete, and mix well, and then repeat. In a
lot of cases it sure beats using a wheelbarrow and/or a big tub. You just
need to be careful about loosening excess dirt etc during the mixing process.
Its also very common to just pour "dry" sacrete around posts for fences, or
support columns for sheds etc, and in a few days the sacrete has set up
solidly just from the moisture in the ground even tho no water was added.
Its the same process as having a partially open bag of the mix harden and
become unuseable just from the moisture in the air, but a lot faster.
Hope this helps....Good luck