Specialty Gardening: Container Soils - Water Movement and Retention III , 1 by RickCorey_WA

RickCorey_WA wrote:
Do I have the numbers right? (see image)
Perching starts when the grains get down to around 0.090 - 0.100 inches (2.286 mm to 2.54 mm) diameter.
3/32" is 0.094" is 2.4 mm.


Is it true that, if the mix has even 10-15% by volume smaller grains, perching can be expected even if 85% of the mix (by volume) is larger than 3 mm (0.118")?

I'm picturing 1 small grain lodging between many or most of the big grains, making the pore size linear dimension much less than half what it was, hence being very prone to perch water.

Would a rule of thumb be something like "if you have any small grains, the large grains should outnumber them by 2-3 to 1 if you want to decrease the amount of perched water significantly "?

In other words, I should KEEP 1/2 or 2/3 of all voids FREE of smaller grains.

Don't hesitate to say "it doesn't work like that at all". You won't hurt my feelings.

I'm starting to think that I can EITHER avoid perched water, OR have any upward wicking of water, but not both with the same mix.

I might change my goal to "maintain enough open air channels all the way to the bottom of a cell, and avoid excessive water retention near the top of a cell (to prevent damping-off).

I've been convinced of the value of bottom-watering seed-starting cells like inserts and propagation trays. But that requires some wicking. I was trying to provide enough wicking and sufficiently little perched water that roots would thrive all the way to the bottom of a 2" deep c ell.

Someone said:

>> "You'll need to be mindful of rain, because if you leave your gritty mixed plants out,
>> you can have a washout problem."

Speculating, maybe this person was concerned that some finer component in his mix, if any, would wash right out and away from the coarse phase.