Pumice and Perlite for Dummy Gardeners

Pumice (red) and perlite (white)

One of the mot common shortcomings of self-appointed "freelancer journalists"; at least according to our research staff, seems to be a sort of intellectual laziness. The freelancer may have a PhD (although in a much different field) or a Masters degree (ditto), but makes the assumption that he or she already knows what's necessary to write the article. You know what they say about "assume," right? It's what happened to SFGate.com's Janet Bayer when she tried to explain the difference in "Perlite vs. Pumice" (Leaf Grooup stripped out her byline, but the words are still Bayer's).

That linguistics MA of Janet's helped her pass along information such as the derivation of the word "pumice" (from Latin pumex, "foam"), but it didn't help her understand the science of the two products. Bayer opened by telling her readers at the eHow site,

"Perlite and pumice are two coarse, pebblelike materials that are mixed into potting mediums to help aeration and water drainage for plant roots."

Well, not exactly, Janet. The form in which the two are marketed is "coarse [and] pebblelike," but that's not the actual form. The part about improving "aeration and water drainage" is correct, but – paradoxically – both also increase water retention. To understand why, readers need to know the difference between porosity and permeability. Sadly, Bayer couldn't tell them that difference because she didn't know it.

Bayer had a few problems in her definitions of the two substances. For instance, her description of perlite is limited to a "siliceous rock" without mention that it's a hydrated form of volcanic glass (obsidian). She blathers about "popping" perlite, but doesn't explain that the expansion is caused by vaporization of the water in its structure. Likewise, Janet claims that "Pumice results when tiny gas bubbles are trapped in volcanic lava before the lava cools"; which is substantially correct except that she should have said as instead of "before."

When she gets to the uses of the two as soil amendments, Bayer runs into more trouble, with mistakes like

1. "Pumice, formed from molten lava that cools quickly, is porous, letting water drain through it."
2. "Neither retains water, unlike vermiculite, another soil amendment."
3. "In the nursery industry, where many plants are grown in bark, rather than soil, pumice often is added to increase porosity."

Corrections follow:

Porosity does not mean "water drains through it" – that's permeability. Pumice is extremely porous, but not very permeable – it has lots of pores, but they aren't connected. As a result it retains water well. Both pumice and perlite retain water. Pumice can, over time, become water-wet; after which it slowly releases the water. The surface of perlite is very rough, retaining water via adsorption (as opposed to absorption). Bark's already plenty porous. The pumice is added to improve drainage and aeration.

Bayer managed to find reasonably honest references (except for the one on perlite), but it's clear that she just randomly extracted factoids and threw them together. Since she didn't understand them, she got them wrong; and when people pull that crap we're more than happy to call them a Dumbass of the Day.

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