I thought I'd share the results of a little semi-scientific experiment I carried out. Substrate composition is one of those topics around which there is a tremendous amount of lore, and sorting out meaningful information can be somewhat challenging.  Like many of us, I switched to a inorganic substrate a number of years ago.  But living in a very hot climate and using a soils with low water retention means frequent watering (three times a day in the summer).  I've been very happy with the results, but the hobby has definitely become more of a lifestyle choice than a pass time.  That's not a complaint, but I've become sensitive over the years about water retention.  I had questions, lots of advice, and very few facts:

What inorganic substrate is the most water retentive?
How does grain size influence the amount of water retention?

I purchased several 500 ml beakers and drilled a small hole in the bottom of each:

I filled the beaker with 500 ml of the sieved substrate and poured through 500ml of water (red color is food coloring to help see the water better), measuring the amount that drained out the other end.  The water that fails to drain measures the field capacity (the saturation after the gravitational water has drained away).  Here's an example:

Here are the results for the 3/8"-1/4" size fraction for the various inorganic substrates.  Seramis was by far the most water retentive at 30% saturation field capacity.  Turface, akadama, and pumice were similar at about 13% saturation field capacity.  I tested decomposed granite as well - as long as it was rinsed thoroughly it had very low water retention (~6%).

Nice project  .  Now you only have to do 5000 more repetitions using different components in various ratios to find "the perfect" one.....  I've never used Seramis.  What is it designed for?

The real kicker is how much each tree takes in in a given season and how to compromise on a mix that works all year where you live.  Where I study, we go for faster draining mixes; there are also apprentices here who check water sometimes 5 times a day and water any given tree 1-3x a day.  It must be nice having "watering drones" as Mike Hagedorn calls them. 

Those tests are a very nice start.  Different containers have different zones of saturation and some pots (especially old Chinese pots and converted suibans) tend to keep more water in the bottom. 

I look forward to hearing more of your results.  It would be great if you combined different components and see how it affects the results. 

Nice project  .  Now you only have to do 5000 more repetitions using different components in various ratios to find "the perfect" one.....  I've never used Seramis.  What is it designed for?

The real kicker is how much each tree takes in in a given season and how to compromise on a mix that works all year where you live.  Where I study, we go for faster draining mixes; there are also apprentices here who check water sometimes 5 times a day and water any given tree 1-3x a day.  It must be nice having "watering drones" as Mike Hagedorn calls them.  

Those tests are a very nice start.  Different containers have different zones of saturation and some pots (especially old Chinese pots and converted suibans) tend to keep more water in the bottom.  

I look forward to hearing more of your results.  It would be great if you combined different components and see how it affects the results.  

Seramis is a German fired clay product designed for plants - http://www.seramis.com/.  Its $17 for 7.5L.  There are some videos posted on their site of a similar experiment to what I undertook here.  It's where I got the idea for doing this.  And to do 5000 more tests - yikes - I'd need my own set of watering drones!  I just have 4 kids and a wife who all think I'm a bit off and want no part of my beakers and ziplock baggies filled with gravel.  

In the summer here our climate is pretty tropical - it gets to one thousandty degrees outside (according to my daughter). And the only creatures that seem to like it are the mosquitoes.  During that time, water retention in the soil is a concern for us working folk whose employers frown on their employees taking off every few hours to water our trees.  In the heat of the summer I water before I go to work, have a sprinkler that comes on at mid-day, and water again in the evening - it's usually pretty dry by then.  The good news is that summer only lasts 14 months per year.

Also, its interesting that you mention pots.  I''ve started to look at water retention as a funtion of the pot shape.  I haven't finished that yet, but directionally it appears that an equivalent volume of soil has a higher field capacity in a shallow pot than it does in a deeper pot.  It also appears that adding a drainage layer has the same effect (it makes the mix more water retentive).  I'll post more on that later.  Not sure why old Chinese pots in particular would make a particular difference - if you send me a few, I'd be happy to test them for you!

I have tested some mixes as well - I'll post those results later.

Scott

Yes I was meaning CEC. 
I did a test between haydite and turface this summer, it wasn't as practical as yours however.  I measured equal cup size amounts, added equal amounts of water to each (done in buckets not in a pour thru situation), then I compared the dry weight of each sample with the wet weight after I drained them for an equal amount of time.  The turface was a bit more water retentive, but only slightly by my test.  I was looking for comparison results with your method.  Please post them if you do another test. 

<it gets to one thousandty degrees outside (according to my daughter). And the only creatures that seem to like it are the mosquitoes....The good news is that summer only lasts 14 months per year.>

Having lived in Houston for over ten years, I can attest to this...very funny Scott!

And Thanks for posting your results, quite interesting results at that.  Nice to know Turface and Akadama are about the same. 

Are you sourcing seramis locally or do you order it online?