Wicking Beds-Where is the Wick?

Ok I have built a lot of SIP's(Sub-Irrigated Planters), and all of the designs I have used involved having some of the wicking soil extend down to the very bottom of the water reservoir.
Most of the bed designs I have seen here have a gravel filled reservoir , then a filter cloth of some kind between the reservoir and wicking soil on top.
So when the water level falls, how does the wicking continue? Does the gravel wick the water up to the soil level?
I think I will be using slotted pipes for the reservoir and a sawdust/ manure mix above that, with no gravel or filter cloth barrier. Any reason to think it wont work?

Yes the gravel wicks the water to ths soil. I use four inch corrugated drain pipe in the bottom of some of my beds and it works great.

Thanks! I think I will skip the gravel and weed barrier that any way, any downside to that?

I used the weed barrier on top of the drain pipe just to slow down silting of the resevoir. that sawdust compost mixture might compact but it should work.

Cool...I actually will be slitting the pipe with a sawzall, and I am hoping that will control the silting. If I can find something cheap or free I will use an actual filter.
I am still not sure about the wicking portion of the soil mix. I have free aged manure, but no source of free wicking material.
Coconut coir, peat, and compressed wood pellets are all candidates, what would you suggest?

The aged compost should work just fine as a wick.
Free is good. I picked up several rolls of 4x500 ft of weed barrier for a couple bucks a roll from a Habitat for Humanity Restore along with a bunch of the corrugated drain pipe for a grey water bed and have been using the left over for wicking beds.

Strait compost for the win!
Thanks for all your feedback.