Soil additives play big role in gardening

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While teaching a gardening class recently, it came to my attention that many people are confused about the kinds of soil texturizers and their functions. Let's take a look at a few, so we're aware of materials used and why they're important. For the most part, I'll be addressing potting soils, although some of these can be used in your clay soils, as you will see.

Perlite is made from volcanic rock. It's those small, white, round pieces you see in your potting soil. After it is mined, it is crushed and rapidly heated to about 1,000 degrees, which makes it pop like popcorn. This process makes the particles much less dense and very lightweight. The purpose of perlite is to ensure good drainage. Contrary to what some gardeners think, it is not meant to help retain water, but instead to help aerate the soil, allowing roots to have plenty of oxygen. Perlite is relatively expensive to produce, so only good quality potting soils will have a useful amount.

Vermiculite, on the other hand, has the opposite effect in the soil. It's the "brown stuff," often used for starting seeds. When you look closely at vermiculite, it resembles a pile of stacked plates. Its purpose is to temporarily trap water and nutrients, thus giving plants — especially seedlings — enough time to take up moisture and nutrients before they drain away.

A few years ago, vermiculite got a reputation as being dangerous to work with. That happened because vermiculite was largely mined in Libby, Mont., where it was discovered that asbestos also appeared in abundance in the same mine. For a while there was undoubtedly asbestos in the vermiculite, which is unhealthy if inhaled. Vermiculite is mined in other places now, and bags of vermiculite might even specify that it did not come from an asbestos mine. Look carefully at the source before using. Better yet, wear a mask when dealing with vermiculite.

Peat moss also is mined, and it is becoming more expensive because peat bogs are in shorter supply than they once were. Peat acts much like vermiculite in its water-holding capacities. It is a great soil additive, but is becoming cost-prohibitive to use in large amounts.

Pumice is solidified lava foam. You've probably seen large pieces of it if you've done any hiking near inactive volcanoes. It has such low density that a dry piece will float on water. While both pumice and perlite are volcanic materials, pumice is usually just crushed and used decoratively. It's especially favored by growers of succulents because of its resemblance to coarse sand.

None of the additives listed here have any plant nutrients in them to speak of, so that needs to be taken into consideration when using them.

Upcoming: The next Jackson County Master Gardener training session, which lasts for 13 weeks, begins Jan. 20. Classes are held from 9 a.m. to 4 p.m. on Wednesdays. The cost for this entire sequence is $175. During or after these classes, each participant will spend 70 hours doing hands-on learning in the greenhouses or in one of the 24 demonstration gardens. Trainees have until October to complete the "outdoor lab" part of the training.

The class has a capacity of 75 enrollees and is currently more than half-filled. For further information, call Bob Reynolds, Southern Oregon Research and Extension Center home horticulture agent, at 541-776-7371.

Carol Oneal is a past president of the OSU Jackson County Master Gardeners Association. E-mail her at diggit1225@gmail.com.

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