Perlite in Container mixes
 

I've been using Perlite in my container mixes for the last 3 years with excellent results in regards to eliminating aggregate compaction, improving aeration and moisture retention. I just broke down my Belmonte plant and took pictures of the aggregate. You hear people saying it floats to the top of the aggregate or ends up at the bottom of the pot. Proof is in the pudding so here are the pictures. Ami

Ami, what is the overall composition of your container mix? Is it just potting mix and perlite?

There was a time, and I don't know if they're still doing it in cheap potting mixes or not, that instead of Perlite in potting mixes, companies were using tiny Styrofoam beads. The beads would work their way to the top of a container over time. I wonder if that's what gave some folks the idea that it floats to the top?

I used to pick up one of the white bits of a potting soil before I bought it and squeezing it to see if it squished or crumbled before buying.

I use Rhododendron Mix that I get in 70 liter bags. I make batch in a tub using 2 bags of the mix and add my perlite and some organic ferts. Ami

Perlite will 'float'...but what collects on the top is usually just the stuff that is in the top fraction of an inch of the mix. In other words the very top layer will end up with the lightest stuff on the surface. This could give the appearance of the perlite working its way 'up'...and this usually takes a bit of time to happen, more than one season. I 'fix' it by stirring some 'amendments' into that top fraction of an inch of mix (sprinkle on a little and mix with my fingers).

I've never found significant migration of it in any direction...unless the pot/container is disturbed or flooded...then you have other problems than worrying about what your soil mix is doing.

Compaction in container mix seems to matter a lot, hence
the benefit of perlite and similar amendments in container
mix (it is not only a drainage issue).

This year I used a homemade mix that was mostly
well-composted wood waste mixed with fine sand
with a little clay in it, with some fresh homemade
compost from a compost pile mixed in (maybe 10-15%).
There were some larger chunks in the composted wood
waste and homemade compost, but not like you would
have if you mixed in 25% or more perlite.

In conventional containers, varying from 5 to 20 gallons,
watered from the top as needed, plants started out well
after transplant, when the mix was still loose from
transplanting and filling the container, but they basically
came to a near stop by mid-July. Growth was slow and
unimpressive for the rest of the summer. Moisture levels
were checked regularly with a moisture meter, and the
mix in the conventional containers was not allowed to
drop into the dry zone, nor were they over-watered.

Plants in self-watering containers in the exact same container
mix with the same fertilizer continued to thrive all summer.

The difference was soil compaction: the mix in the conventional
containers packed down and lost large pore air space as the
plants were watered over the first couple of months, while the
mix in the homemade earthboxes soaked all of their water up
from the bottom reservoir, producing no comparable soil
compaction over the summer.

(There was not enough rain over the summer for that to
produce any soil compaction in any of the containers.
Plants in the ground have the benefit of soil aggregates
created by fungi that produce more large pore air space
in the soil than the soil particle sizes alone would account
for. Those soil structures do not exist in containers filled
with container mix, so the mix itself must provide that
air space for the plants to thrive.)