How potting soil is made
Most potting soil contains little or no actual soil. It is mostly bog moss or coconut pith, lightened with puffed volcanic glass.
- stations
- 6
- things upstream
- 25
- raw sources
- 14
- steps from the ground
- 5
The short version
Bagged potting soil is usually soilless. The base is sphagnum peat scraped from drained bogs, or pith from coconut husks, which holds water. Perlite, a volcanic glass that puffs up when heated to 850 to 900 °C, adds air spaces. Ground limestone offsets peat's acidity and fertilizer supplies nutrients, and the ingredients are blended to suit different plants and bagged.
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Strip the bog surface
Peat bogs are drained, and the partly decayed sphagnum moss near the surface is stripped off and dried. As of 2013 the US got up to 80% of its sphagnum peat from Canada.
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Wash and buffer coir pith
Coir pith, the spongy dust left over from coconut fiber processing, is a common peat substitute. Raw pith carries too much sodium and potassium for plants, so it is washed and soaked in a calcium solution to swap those salts out.
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Pop the perlite
Raw perlite is volcanic glass with water locked inside. In a furnace it softens, the water flashes to steam, and each grain swells 7 to 16 times into a bright white, bubbly bead.
850 to 900 °C
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Dose lime and fertilizer
Peat is acidic, so ground limestone is weighed in to raise the pH. Dolomitic limestone is preferred because it adds magnesium as well as calcium, and slow-release fertilizer supplies what the bulk materials lack.
about 5.0 kg of limestone per cubic meter
Goes in -
Blend to the recipe
Water-holding and air-holding ingredients are tumbled together in proportions matched to the plant. Cactus mixes take much more perlite or sand, and seed-starting mixes stay light and fine.
Goes in -
Bag the mix
The finished blend is metered into plastic bags and sealed for sale.
Goes in
Worth repeating
- A harvested peat bog can take 1,000 to 5,000 years to grow back.
- Vermiculite, another potting-mix ingredient, takes its name from the Latin for breeding worms, after the wormlike way it expands when heated.
- The term potting soil first appeared in print in an 1861 issue of the American Agriculturist.
Back to the ground
Follow every input upstream and potting soil starts as these 14 raw sources.
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Sources
These sources say little about the factory steps themselves, so parts of this line are a general description. Treat the details with care, and tell us if you know better.
Cite this page: "How potting soil is made." Assembly Line, 2026-11-03. https://assemblyline.org/make/potting-soil