I remember your posts. I can give you a recipe that will break down slowly & last for many years if you're willing to invest the effort in finding the ingredients, but it's the same thing I suggested last time. The stuff should be easy to find in CA.

If you'd like to see a photo of the soil after 5 years, send me an e-mail & I'll attach a couple of photos in the reply. You'd be surprised at what you can tell about a soil by a photo.

Al

Finding a mix that will drain at an appropriate rate while on an automated irigation system is probably darn near impossible.

I am thinking that if it were me I would probably forget about water retention and go with the fastest draining mix I could make and then use the automated system to water daily or twice daily to compensate for the rapid drainage. I would probably go with something like Al's mix and possibly even increase the inorganic content to further increase drainage. You will need some organic content to hold onto nutrients though. Given the frequent watering I am suggesting I would probably use a slow release or organic(pretty much slow release by nature) fert on top of the mix and then suppliment with more fert when you are present.

The frequent watering would be to prevent drought stress and the super fast drainage would be to prevent waterlogging on cool, cloudy, rainy days.

Unfortunately sharp drainage and water retention are usually mutually exclusive. Drainage refers to water draining away. Water retention refers to retaining water. See the problem? ;-)

Your choices would seem to be super, ultra fast drainage with frequent waterings or slow drainage with greatly reduced watering. Given that you have an automated watering system and can't control the weather I would go with the former.

Here's the trick to developing a soil that holds water well, but drains quickly:

Use soil materials that have large, irregular particulates so there are plenty of macro-pores (larger air spaces) in the soil. The particulates themselves should be full of micro-pores and do the job of holding water (why I'm so sold on bark).

Username is correct in his assessment of the need for an inorganic componant in your soil if you wish it to be a long term thing. I think your soil should be at least half inorganic. Don't be put off at the thought. Garden soils are usually 90% or more inorganic, and plants do quite well. I grow some pines, junipers, and some succulents in Turface only - nothing organic at all - completely sterile but for what feeds on the nutrients I supply. It drains lightning fast, holds water well, and because it is a clay product, is great at nutrient retention/delivery.

In your mail, you have pictures of a soil similar to what I would suggest you use & another pic that shows the Turface & crushed granite. You only need decide if you're willing to invest a little time in acquiring the componants. You could use pumice instead of Turface, too.

We can fix you up in grand style by wicking your containers to eliminate the PWT. With a good soil and wicking in place, it would be extremely difficult to over-water.

Al