Water Retention in Hugel on Slab

Hi! I'm a urban gardener.  I am building a raised bed out of old concrete blocks on my back patio.  The bed will be 22" high.  The actual growing space is long and narrow (7'x 15").  First, I will be using heavy plastic sheeting as a liner to prevent possible chemicals inside the blocks from leaking into the soil.  Then I want to use hugel-type filler before adding the soil for planting.  Since both the hugel and plastic are known for retaining water, AND this is all built inside basically a large concrete planter ...

MY QUESTION: Will this type of raised bed cause root problems in my plants?  Will it be too much water retention going on?  Would it help to leave out to plastic sheeting?

BTW: I've read the great posts by Daron Williams on building different types of hugels and the thread Hugel Bed on Concrete.  But I could not find what I needed.  Thanks in advance for the helpful advice!

I dont think you have to be too worried about the contaminants in concrete blocks. Also you are in Georgia that gets alot of water so you don't really need the wicking bed setup.

S Bengi wrote:I dont think you have to be too worried about the contaminants in concrete blocks. Also you are in Georgia that gets alot of water so you don't really need the wicking bed setup.

Thanks for the input!  Just a couple of notes.
1. I want to use the Hugel base as a filler so I will not have to purchase so much compost.  The Hugel will be more beneficial to the plants than rocks.  
2. Georgia has been having the worst drought and heat-wave conditions for the past few summers.  Mmmm...can we say "Climate Change"?  

Just to add - Husband is worried that the soil will leak out without a liner.  Now the blocks solid side faces the soil.  Maybe this will help and I can go without the liner??

You can put some cardboad going up the sides. But I think that isn't needed because lots of folks go without. You are probably going to top dress the 'raised bed" and add mulch to it yearly to add back some of the fertility that you extracted with your greens and to cover the soil during the off season. So what little soil that do seep out will be replaced.

There will also be a good amount of settling too. so after watering the 1st week you might have to add more soil again. It would be cool if you could make two raised bed one without the liner and one with and see how they end up performing over two years.

I would think landscape cloth is your best option to both hold the soil and allow drainage.