Did you ever wonder why potting soil disappears from houseplant pots? It disappears because it’s organic, and it breaks down over time.
I used to wonder why the amount of potting soil in a house plant pot seems to decrease as times goes by. If you leave a plant in the same pot for years, you’ll notice that it eventually appears to have no soil any more – how can that be?
The confusion comes from the fact that we tend to think of soil—or dirt—as being a mixture of ground and powdered rocks, the results of millennia of erosion. Sand, gravel, and clay are all words for various textures of broken up rock, and they don’t tend to disappear. Soil, however, is something different.
Soil, and particularly potting soil, is mostly the remains of plants that have partially, but not completely, decomposed. A list of ingredients for potting soil might contain peat moss, shredded hardwood bark mulch, composted plant material, perlite (a volcanic glass), and sand. Only the last two ingredients come from rocks and they account for very little of the total mass of potting soil. The rest is organic and it breaks down over time.
Of course, potting soil for indoor house plants is usually sterile because people don’t want insects, earthworms, and other soil dwellers living in their plant pots. This will significantly slow down decomposition of organic material in the soil—and also make it considerably less interesting.
Read about living things in natural soil:
