Retaining moisture in soil

What can I add to potting soil that will retain moisture? I have a hanging basket that dries out in half a day and it isn't convenient to water when I'm not home. I was thinking of repotting it but need to know what mixture of soil to use. Thanks for any ideas.

You can buy moisture crystals to mix in--just be aware that they swell and become much bigger when wet so don't use too many or they'll be exploding out of the pot. Miracle Gro also sells a moisture control potting mix that has the crystals already mixed in so that may be your easiest solution. However...it might be worth exploring how you're watering. Even in super hot weather your container shouldn't be drying out that fast unless the plants are severely rootbound and there's hardly any soil left. So I suspect that you're not really watering the basket enough and if you gave it a really good thorough watering it wouldn't dry out nearly as quick. I know everyone's been taught that you water until water runs out the holes in the bottom of the pot, but what can happen if your potting mix dries out is that it doesn't soak up water very well the next time you go to water, so the water will find a few channels through to the bottom of the pot and run out. You think you've watered enough, but in reality the majority of the potting mix is still bone dry. If you're in a situation like this and you can bring the basket down and soak it in a bucket/tub that's a great way to ensure that it's really soaked all the way through. If you don't want to get the basket down, then you need to keep watering for a longer time to give the rest of the potting mix time to soak up the water. Check the weight--for my hanging baskets I just lift up on them a little from the bottom and it'll be pretty easy to tell if it's full of water or if it's still really dry. It's amazing how much less time I've spent watering containers since I started watering them properly--I've got containers that can go a couple days without watering even in 110 degree temperatures when I water them thoroughly and make sure the potting mix has actually gotten wet all the way through.

Thanks for your help. I do water until water runs out the holes and just assumed it was soaked, thanks for that info! :) I'll go soak my pot right now, it was my Mother's Day gift so I don't want it to die on me.

I would suggest adding vermiculite to your potting soil, maybe some extra perlite too. The perlite helps with the oxygen and the vermiculite helps with the water retention. These two with worm castings do great. Here's a little information on vermiculte. http://www.vermiculite.org/Reports/TVA-R001.pdf

Ditto on what ecrane said. Soak your baskets in a bucket or tub of water for a while. They'll soak up plenty of water. The moisture will reach the entire soil in the basket instead of running down the sides and out the bottom, which is what's happening when you water now.

Thanks for all your help.