I would say cut the bottoms off your pots and let the plant root
into the soil below it (some drought protection), but I do not know
if you have a soil floor in your greenhouse, and I do not know if
you are in root knot nematode country. Root knot nematodes love
sandy soils. They will infest the roots of your plants, cause little
galls to form on the roots, and the plant will not be able to root
normally and take in nutrients. In RKN country, containers (that
actually have bottoms) are one way to combat the problem.
(The other way is huge amounts of organic matter in the soil,
spreading out the sand particles.)
Expect to have to water daily once the plants gets some size
in 4-gallon containers, maybe twice a day in hot weather. Since
you are watering more often, fertilizer will leach out of your
containers faster. You might want to use something like Osmocote
or Nutricote, that is polymer coated. It sits on the top of the
container mix and releases some fertilizer every time you water.
(Far from organic, of course; these are synthetic fertilizers.)
You could also try the Mittleider method. It seems a little too
fiddly for me, and it also does not use organic fertilizers,
but if you are out there watering every day anyway, adding
a fertilizing pass once a week is perhaps not that much extra
work. It worked pretty well for Caroline Phillips (see this
thread:
http://www.tomatoville.com/showthread.php?t=18309 ).
Physical properties of container media:
http://oregonstate.edu/dept/nursery-...roperties.html