Dirt in pots

(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?
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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.

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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 snipped-for-privacy@fern.com
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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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Ah, outdoor pots... you also have to add in wind erosion of dry soil, soil-splash from rain.
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