Recycling potting mix with perlite: Would I get garden soil with perlite dust?

Question

I'm a fan of recycling, and it's not about money.

Chopped plant material is disposed into the compost pile. Even old potting mix does not go to the landfill, but rather to that same pile. I often use the old peat/coir based potting soil to balance the greens and the browns, though I don't get much kitchen waste for that. I avoid attracting rats, so I would rather use peels and things that are of low nutritive value to them. Recently, I am disposing of a large amount of old (too degraded for further use) coco/perlite soil, which is equal parts coir and perlite. The perlite does not biodegrade, but its downside is the tendency of the grains to crumble and make more dust. I sift the mix to remove all the grains that are large enough for reuse. The filtered material is mostly the fine particles of degraded coir. Unfortunately, there is lots of white "sand", which is perlite that crushed to over time.

As you may have guessed, by dumping into the compost pile this potting soil with crushed perlite, perlite dust would accumulate in the garden over time. I normally sift the perlite when it is moist, so I get no dust. However, the summers are hot and dry. That means the same garden patch which was composted in the spring may release dust as I work it in the summer. Do I have to worry about that and throw away the old potting mix with perlite dust, or can I be rest assured and just dump it into the compost pile?

Answer 1

I don't think you have to worry about the perlite. It is very close to whatever is already in the soil, you are adding small amounts relative to what is already there, and the dust isn't harmful.

Perlite is just expanded volcanic glass, which is pretty close to what you would already find in your soil. Per wikipedia, perlite chemical composition:

Chemical Formula	Name	% of Perlite
SiO2	Silicon Dioxide	70–75
Al2O3	Aluminium Oxide	12–15
Na2O	Sodium Oxide	3–4
K2O	Potassium Oxide	3–5
Fe2O3	Iron Oxide	0.5-2
MgO	Magnesium Oxide	0.2–0.7
CaO	Calcium Oxide	0.5–1.5
other	Organic/Water	3-5

As it breaks into smaller pieces, it integrates into the mineral content of the soil that's already there. And the perlite you add is not as much as it seems, perlite density is 30–150 kg/m³, 3-15% the density of water... that's really not much!

Regarding the dust's effects on humans, Midwest Perlite Inc. offers a material data sheet that makes it sound pretty darn inert.

OSHA has classified Perlite as a Particulate Not Otherwise Regulated (PNOR), which has the same Permissible Exposure Limit (PEL) as that used for nuisance and inert dusts. Although there are no published reports of adverse health effects from exposure to Perlite dust, dust levels should be maintained below the OSHA PEL for PNORs and respirators should be used when airborne dust is present, Excessive inhalation over long periods of time may cause harmful irritation.

I put all of my used potting mix through the compost pile, the same as you do. I don't even screen out the larger perlite, it becomes regular soil eventually.