Hi Scott, I was part of this conversation over there, and used haydite this summer. I found that although in my test it was pretty close to turface in it's uptake of water, the plants did not stay as hydrated as long as with turface based soil. Could be that uptake is not all the information needed to base all results on. Perhaps it's not only uptake, but availability as well...
What I was wondering with my question, is the difference between "hard fired akadama", and "double line akadama". I don't seem to be able to find comparisons on these, but seems like the thought that it is basically a lava when hard fired, doesn't make it worth the money that it costs.
On the bag for double red line akadama it describes it as "hard quality".
I've worked with it and the bag with the big green slash across it.
I see no appreciable difference between the two from a water retention perspective. I've never worked with any other brand of akadama, so I can't really comment more on that question.
In terms of haydite, I hadn't included it in the tests I wrote about on the other forum. I have tested it, however, and, in short, turface holds more water than haydite. Now turface is quite a bit finer grained than haydite, but even eliminating this variable by sieving an equivalent size fraction of each, turface is still more water retentive. There are a couple of possible reasons why.
First, some background. Water storage in the substrate is in one of two types of porosity - intra- (within grains) or inter- (between grains) granular.
For the water storage in the intergranular pore space you basically have a trade-off between drainage and water storage. Better drainage = worse water storage and vice-versa. There are three factors that control the amount of water storage between the grains in the intergranular pore space - the grain size, the grain shape, and sorting. Finer grained substrate will have about the same porosity as a coarse grained substrate, but the water saturation in the pores after the graviational water drains away will be much higher. Rounded grains will have a higher porosity and lower intergranular pore saturation than angular grains. And a poorly sorted substrate will have higher water saturation and worse drainage than a substrate with a uniform grain size. Here's a chart summarizing all this:
For the water storage in the intragranular pore space, you're basically dependent on the type of grain that your dealing with. But this is the good kind of water storage because it won't muck with the drainage.
Now with the simple tests I wrote about in the previous post, there's really no way to tell whether the water storage was of the inter or intragranular sort. But I have my suspicions. Turface tends to run pretty angular and haydite tends to run pretty rounded. I suspect that this is probably accounts for much of the difference and that neither turface or haydite holds much intragranular water.
- Scott
