Lots of different opinions on this, mine is that it's a bad idea. It breaks down into pudding-like mess before the season is over. Also retains too much water. This leads to root-rot.

If you're not up for making your own mix (pine bark, perlite, little bit of peat), I'd probably say to buy regular Miracle Grow potting mix and amend it with lots of perlite.

The free compost is awesome for raised beds or for dressing your lawn though.

There's a container forum here at GW too, you might want to read some threads there. This guy "tapla" knows a little bit about container plantings.

Compost in containers is generally a bad idea and leads to slow/poor growth for many plants.

I will say that of the plants that can be grown in it and still do at least OK tomatos are among them.

That you live in a desert climate means things will dry out faster than they will for most of us so I think you will be OK with compost. Not ideal, but OK.

Please skip the mulch in the mix though. That will just tie up nitrogen and lead to poor growth. You can use it on the surface to aid water retention, but not in the mix without large potential for nutrient problems.

If you do use compost in tomato containers, what sort of percentage would you recommend? 10% or less?

Alice

Well this is one of those open ended questions. Compost is organic matter. All (good) soil has organic matter in it. Most likely if you bought a commercial potting mix, it already contains the proper amount of organic matter to grow plants in a container. Adding more will throw it off and end up with poor results. If you make your own potting mix, well of course you need to add compost.
Then you will start getting into the different types of compost. If you make compost with peanut dust you can grow plants in just that. If you make it with leaf matter you need to add some wicking materials, like perlight or peanut hulls or the often dissed (bark) mulch. If you make it with discarded food, than I would have to hold it in my hand before I could tell you what to add.
It sounds like you are spatially limited, so you may not be able to experiment, but if you have the room for some failure, I highly recommend playing around with making your own soil mixes. You will learn a lot about how roots work, if you do it. Just remember you will have some failures along the way.

Do you think potting soil really has enough nitrogen to grow tomatoes? It's made for all plants, and tomatoes need a lot more than most things, right?