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  commercial mixes-Add another factor or two
From: scottvergara at comcast.net> on 2005.12.18 at 12:00:50(13616)
An interesting discussion that has been a pain in the butt or an
"opportunity for improvement" for all growers from one person with 5 plants
to mega nurseries with 1000's of acres and millions of containers.

The one point that is quite important and not mentioned thus far is what
about the container? Many growers use what ever is cheapest and collectors
like myself recycle a mix of odd sizes. What needs to be considered is not
only the water/"soil" particle interaction but how it acts/interacts in a
container of different sizes and more importantly different depths.

Just about any growing mix will work fine if you have a big pile of it on
the ground and it is more than a couple feet deep and you plant directly
into the pile. This is more like the "real" world. To illustrate my point,
one can take the most fertile, well drained garden soil and put it in a pot
and the vast majority of the time you will discover that it doesn't "act the
same" and many if not most plants that grew well in that soil in the ground,
perform poorly or die quickly when grown in that soil in a container. Soil
physics comes into play and requires us to at least acknowledge that drain
holes (size, number, placement), depth of container, if the container is
porous (clay) or impervious, and the surface the pot is sitting on are all
part of the system. The latter point has to do with capillary movement of
excess water out of the container. As a visual example: water a pot, let it
drain for 15 minutes or so, then set it on a paper towel. Typically you
will get a great deal more water draining out of the container.

As an aside to this, I used to use the sponge demonstration to explain this
point and recently learned a new variation using one of those scrubbing
sponges that has a coarse side to it. Essentially take a new sponge and
soak it in water. Pick it up and lay it on the dish rack/ cookies cooling
rack so that the large flat surface is parallel to the counter top. Some
water will drip out. Now rotate it so that it is resting on one of the two
longest edges. A bit more water will come out. Now rotate to that it is
resting on the shortest edge. Even more water will come out. Now carefully
put the sponge, still resting on its shortest edge on a paper towel and see
how much more water is removed from the system. Nuff said.

The point of the scrubbing sponge is to make it easier to see what a perched
water table looks like and can be seen when the coarse side is on the bottom
and the water in the sponge collects along the interface (the bottom of the
spongy side and the top of the coarse scrubbing side. This helps to
illustrate the myth of putting rocks or broken crockery in the bottom of a
container to improve drainage. I always had to roll my eyes when people
would mention using rocks, charcoal etc. in the bottom of a container with
no holes to provide drainage. And the water went where?

Lastly to add another two factors in that impact water needs in container,
namely color of the container and exposure to sun. When I taught
greenhouse/nursery management courses one experiment I had students conduct
was to measure the soil temperature in containers. They typically never
correctly predicted that the soil in a 5 gallon container sitting in the sun
in a black plastic pot would register on the SW side, a temperature 1 inch
in from the edge and 3 inches down of 130 deg F. Commercial soil
pasteurization temperature is 140 deg F. Think about that point for a
moment. Most all roots function poorly in water absorption (never mind
actual growth) when the soil temps go above 100 deg F. Watering the pot
dropped the temperature to around 70 deg F for an hour or so but it was back
up over 100 deg F in less than two hours.

There was an excellent one page article in Science perhaps 20 years ago that
discusses Soil Physics very nicely and explained the soil/container depth
issues, perched water table etc. very succinctly. I couldn't find the copy
I had but someone surely has come across it.

Well enough for now. We are expecting a huge freezing rain event with very
high winds this afternoon continuing to tomorrow morning which means power
outages most likely. This means the weather is moderating after a couple
unusually dry weeks of sun and freezing temps that have frozen pots solid
and causing significant foliar desiccation. It is kind of odd to be wanting
more rain here in the PNW during winter as all we do is complain about it
when it falls.

Wishing everyone a Safe, Sane and Joyous Holidaze!

Scott Vergara

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