Is round or angular gravel better for wicking beds? If round is better, how much better? Where I am, round gravel is quite a bit more expensive then angular. I would imagine that the round gravel would hold more water.
This is a matter of capillary action, which means that the closer the open spaces are to each other the better. What this tells us is that rounded surfaces can get closer together than angular (sharp) edges.
Now, if you have or can get angular gravel at a much lower cost. You have two options; 1) Go ahead and use the angular gravel just as you get it and pack it as tight as you can by hand or machine.
This will put the gravels in greater contact with each other and that will help the capillary action.
2) Get the angular gravel and build a giant tumbler (rock polisher) with a blue plastic barrel. I have one that is made with a large blue barrel (one end opens and has a ring that holds it tight when buttoned up for turning) and sits on a 4x4 stand with wide, 6" wheel casters to support the barrel and let it turn.
I use a salvaged electric motor (came from an old washing machine, (but if you had something bigger it would work better,) and chain drive (from a wrecked bicycle) to turn it, I would think a belt and pulley drive set up would also work.
I use wet sand as the grinding medium when loading this contraption I go by shovel full measurements and never go over half full total.
I use a 1 shovel sand to 2 shovel rock ratio. I usually can put 8 shovels of rock into it before I start to think I may have to much weight in it for the motor to turn.
I don't know about suitability for wicking beds, but angular gravel can theoretically pack more tightly than round. It may be more open when placed loose, but when compacted the angles can fit into each other. Round surfaces can't do that. The best for wicking would be a range of sizes, as sand can fill the open spaces left in the gravel and there will be a continuous pathway of fine spaces. I'll leave it to the experts in this area as to what exact range would be best.
Yes Glenn, the angular gravel can be packed very tightly, by a road roller. I don't think any one building a wicking bed would want to use one of those to pack their gravel though.
The best wicking bed would actually be the opposite of a good draining bed. When I build a wicking bed I use river rock and three different meshes of sand, I try to get fairly small rocks ( around 1 inch to 1/4 inch diameter sizes ) as I lay the rock in the bed, I alternate rock and sand, my goal is to have no large gaps anywhere in the bed from the bottom all the up to the topping soil. the last layer of the fill is fine sand about 2 inches deep with an overall depth of the bed at 2 feet. below that is bed rock on my homestead. I even go so far as to break up the sand stone rocks I dig out of the bed area, I also use a coarse sieve to separate out almost all the local rocks from the soil and I use those "leavings" in the wicking bed after sizing them.
Capillary action is dependent on spaces that are small, usually from .5 mm down to what a fine sewing thread could occupy, will give the best results.