(Suggested by a 'basil problem ' post)
Why does dirt sink in a pot? I *expect* a certain amount of settling
and compaction over the first year, but I've seen the soil level in
plastic pots sink and sink year after year. I know plants use soil
nutrients, but surely that would be a tiny amount in terms of volume.
Even 'erosion' through water draining can't be *that* much, can it? A
large plastic pot had sage in it for about 5 years. Finally, the sage
became woody and unproductive and I started some new plants. When I
drug out the pot from surrounding foliage, the soil level was about
halfway down the pot! What's going on?
the soil is used up by the plant the organic matter breaks down and is
smaller as a result and the plant uses up the matter in five years a
plant grows and uses a lot of material also soil and matter is washed
out the bottom of the pot from watering and rain when I leave my
potted things out side some times they can not drain fast enough for
the rain and they fill up and flood a little and some soil is lost
that way too.
Primarily by continued oxidation (rotting) of organic material in the soil...
most potting soils are at least 1/3 organic matter. As the pieces get
smaller, the soil gets more compact -- and as soil organic matter completely
decomposes to CO2, the soil structure really collapses, leading to problems
with low oxygen availability to roots.
BTW, you probably should repot with fresh or mostly fresh soil every year.
Salts from fertilizer and hard water build up in the soil, leading to that
lovely crust you sometimes see on the surface, edge-burning of leaves, and
sometimes the salts concentration of the soil gets so high that the soil
may be moist to the touch, but water is unavailable to the plant (due to
osmotic pressure problems).
Kay Lancaster firstname.lastname@example.org
Oops. Forgot to mention I was speaking of *outdoor* pots, which are
mostly watered by rain.
I guess I can accept the decomposition of organic matter as a big
contributer to disappearing soil. Hadn't considered that factor. Must
go make some more compost. :-)
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