[Freerider]

I have been planting seeds in individual cells, then after seedlings appear, put in small pots, afterwards plant them in my square foot plot.
Now I would like to start growing some veg individually in 6 inch pots.
Is there any reason why I can't just plant one seed in a large pot instead of potting up?

[Aidy]

yes and no, if you notice when the seedling first appears its a bit leggy, by potting on you transplant upto the the first leaf, thus giving the a stronger stem as it now grows. You could sow in say a three or four inch pot but leave room at the top so you can top up with compost as it grows.

[Aunt Sally]

Growing in pots restricts the root run (obvious I know).  

In a pot the plants roots rush out to the edge of the pot and start going round in circles.  If you pot up into successively larger containers the roots gradually fill the newly available soil at each potting.  When they have reached the edge in reasonable quantity and start to circle you should pot on again.  

If you pot on to too large a pot the roost rush straight out to the edge missing a lot of the new soil which they then never grow into and use.

The only exception is if the container is in excess of the final root run anyway  

[Freerider]

The only exception is if the container is in excess of the final root run anyway  

Do you mean that if the pot is big enough, it will be ok?
What happens when you plant a seed in the ground?

[jacnal]

Growing in pots restricts the root run (obvious I know).

Not so obvious, aunty. I didn't know that. Never even heard of root run before. That might explain why some of my plants haven't done well.

[Aunt Sally]

Do you mean that if the pot is big enough, it will be ok?

YES

What happens when you plant a seed in the ground?

The ground is a VERY large pot  

[Freerider]

I meant:

If you pot on to too large a pot the roost rush straight out to the edge missing a lot of the new soil which they then never grow into and use.

What happens when you plant a seed in the ground?

[Beanzie]

If you plant a small seed in a large pot then a lot of the compost is unused but still subject to a watering regime with the result that it can become stagnant. So by the time the seed(ling) is looking for the extra room and feed its not very good. Doesn't happen in the ground because there is a much more complex habitat ( actions of worms and bacteria constantly refreshing soil). Additionally other plants including weeds influnce the soil structure.

[Aunt Sally]

That's quite right !  A pot is a very artificial enviromnment.  The ground has a very complex micro flora and fauna structure which keeps things "sweet"