By Bev Johnson

Master Gardener

Read the book “An Air That Kills” by Andrew Schneider and David McCumber, and you may be afraid to start  seeds. The danger to most hobby gardeners is falling off a ladder, tripping over a hoe or cutting a toe off when digging a hole. We never think about what we might be inhaling.

This book is what happened to the miners who worked with vermiculite.

Vermiculite is a very odd mineral. It was discovered when a chap went into a mine with a lit candle. He scared himself spitless when he noticed the shiny ceiling moved when the candle got close to it. The vermiculite in the ceiling was expanding in the heat of the candle, making the ceiling appear to be moving.

This characteristic, makes it perfect for many applications, one is to hold water and nutrients and add air space in potting soil.  It is exploded to increase the usable area of the mineral. It has also been used as packing material in the padded envelopes used in sending seeds by mail.

Most of the vermiculite sold in Minnesota came from a mine in Libby Montana, and was contaminated with several kinds of asbestos. When seen under an electron microscope, asbestos fibers look like thin two ended needles. When inhaled, they lodge in the lungs and can cause scaring and/or cancer of the lung.

Most seed starting mixes and potting soil mixes used to have this vermiculite and or perlite as aerators. If the mix was kept damp, there was no problem. However, many horticulture workers got the stuff dry to do their own mixing. Studies show that some of them had the same lung problems as the miners.

Since it is a great medium to support cutting until they root, many home gardeners used this stuff. The vermiculite used now is supposed to be free of the asbestos  but it is a good idea to look at the label on your seed starter and potting mix. If it does have vermiculite, wet it in the bag, if for nothing else, to make it easier to use. And you won’t be sneezing all over your seeds. Those small ones are hard enough to handle, worse if you have sneezed them all over the room.

Perlite, a volcanic rock that is also exploded like the vermiculite can be substituted or added to the mixes. Do read the labels. With a seed starting mix, you want perlite, peat, maybe compost and a wetting agent. 

Since peat has been overmined, this is a good substitute. Coir is also supposed to keep the soil cooler in warm weather and warmer in cool weather. By itself, it usually comes in a brick that you add water to expand it.

Whatever seed starter or potting soil you get, do look at the back of the bag for the ingredients it contains. One ingredient you don’t want to see is soil. All the other stuff is sterile, dirt is full of all kinds of “bugs” that can kill your baby seedlings.

The moral of this story is—don’t inhale your potting mix. It’s bad for you.