Soil microbes for air quality?

dswsJanuary 6, 2013

I've just resumed having houseplants, so I'm on a learning binge. What got me to start having plants again was checking out the claims that they remove various pollutants from the air. It seems quite solid: they do, and pretty much every house or apartment that has any furniture manufactured this century has enough pollution to make it worthwhile.

But apparently, what actually breaks down the chemicals is soil microbes. Different plants in the studies removed different chemicals from the air according to what types of microbes grew in their pots.

The question is, what growing practices make what difference to the soil microbes?

My spider plants and "philodendron" (so-called by the person who gave it to me; actually presumably an Epipremnum, aka "pothos") are abuse-tolerant houseplants. The species were chosen by the people who did the studies for their ability to grow well under the conditions people commonly subject houseplants to: salty-pudding soil, light intensities normally found only in a cave, and desiccating humidity levels. So I'm not worried about them. I don't really care whether they do ok, thrive, or grow in great profusion. I care how effectively they remove stuff from the air -- stuff presumably present at concentrations I can't detect, so I have no way of judging how well it's being removed.

People talking about plants for indoor air quality almost always only mention the kind of plant, nothing about how to get the best effect from the same plant. Does anyone know anything more?

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pirate_girl(Zone7 NYC)

I don't know if it's improving the best of one's plant's effect, rather more if it works or doesn't, or maybe which plants are best at doing this. I've heard Spiders (Chlorophyutum), Pothos & Sansevierias (of which I grow many).

I've read NASA's done work in this area, have you read any of their materials?

    Bookmark   January 6, 2013 at 9:50PM
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Yes. That's most of what I was referring to when I said that the studies seemed solid, and that the people who did them chose to try plants including pothos and spider plants based partly on their abuse-resistance.

They didn't do anything special to grow the proper soil microbes. Here's a typical quote: "All plants tested were obtained from nurseries in our local area. They were kept in their original pots and potting soil, just as they were received from the nursery ... ."

It seems obvious to me that the populations of soil microbes won't be the same in a pot of peat-mud and rotting roots as in a pot of gritty mix and optimally-repotted healthy roots, just because the same species of plant is growing in each. But as far as I've seen, NASA has had nothing to say about it.

    Bookmark   January 6, 2013 at 11:15PM
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pirate_girl(Zone7 NYC)

I think you might be overthinking this. Frankly, I'd go w/ the plants mentioned, already known to excel at cleaning the air.

Then I'd be watching what kinds of products are entering the home.

New furnishings are well known to off gas chemicals like formaldehyde (sp?).

Also bizarre things folks think nothing of using which are chemically pretty significant, things like Glade-Plugins which get plugged into outlets, G_d knows what chemicals come off there. Even air fresheners the ones that don't plug in, just a myriad of chemicals to inhale. (Personally, this has always made me wonder if there's some connection to Autism.)

Folks seems to miss the fact that since these chemicals are in the air & inhaled, we really should consider them as drugs & regulate their manufacture to control such exposures (especially to pregnant women, young kids & the elderly).

    Bookmark   January 7, 2013 at 1:22PM
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Yes, Karen is right on!

Then many of us have the choice anything but natural natural products that work just as good if not better than the ones sold in the chemical isle at the stores, including , bleach, detergeants and the such.

I use a Melliluca products and I can bet you my air quality in my home is much better than a room full of plants.
Have you ever stood in the isle at the grocery store where all the household cleaning products are stored for more than ten minutes? Many people actually store the same chemicals throughout their whole house, especially in the cubbards. I feel bad for the poor children exposed to all those, including myself in my kid days.


    Bookmark   January 7, 2013 at 4:05PM
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pirate_girl(Zone7 NYC)

Don't know what Melliluca products are, but I will say, I sometimes put out a saucer of baking soda just to absorb any odd, stray odors or just freshen things up.

As an ex-smoker who smoked for 42 yrs., I can tell you btwn the plants & the baking soda, my home didn't smell much of tobacco, kind of astonishing really (tho' I did keep all my windows open an inch or 2, even in the dead of winter).

    Bookmark   January 7, 2013 at 4:29PM
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Overthinking stuff is what I do for fun.

I use unscented cleaning products, as much as possible. I find those "air freshener" perfume dispensers kind of nasty.

Melaleuca is the genus name for tea tree. It's also the name of a company that sells various wellness products.

    Bookmark   January 7, 2013 at 5:09PM
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dellis326 (Danny)

Posted by dsws; "It seems obvious to me that the populations of soil microbes won't be the same in a pot of peat-mud and rotting roots as in a pot of gritty mix and optimally-repotted healthy roots, just because the same species of plant is growing in each. "
Soil microbes are going to need soil to grow a large population. The less organics you have in your mix, you will likely limit or change the varieties of microbe species you have in the pots, meaning, for example, a species that consumes benzene that requires a certain organic substrate may not be present at all in a gritty mix or in hydro-culture. Plus the higher use of nutrients might be damaging to the microbes with either method. With less organic materials in your gritty mix or hydro-culture you would probably build a microbe population (as long as it never dried out) closer to an aquarium filter then a typical house plant pot with an average soil mix.

It may not be the way favored in this crowd but folks have kept healthy potted plants in peat based mixes for hundreds of years. All you have to do is go to an old conservatory to see these older plants kept this way.

    Bookmark   January 7, 2013 at 7:01PM
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nancy_pnwzone8(OR zone 8)

since this seems to be science week on the houseplant forum...

if you (OP) are really interested in this topic, i suggest that you read "Eco Friendly House Plants" by B.C. Wolverton cover to cover. most of the 'summaries' of this book online are simple listings of the house plants and completely miss most of the important points in the book. Dr. Wolverton also has more info on his website. i'll list some links below.

the major points that are never mentioned:

1) if you live in a house or apartment that was built before the mid 1970's, all of this plant cleaning 'data' is irrelevant because you have so much air flow through your place that the amount of chemicals removed by plants is trivial (unless the structure has had a *major* weatherization type of remodel).

there is a fair amount of debate whether typical housing built since then could benefit from plants. most homes built today will have a complete air exchange with the outdoors about every 3 hours. the amount of chemical removal by plants most likely won't make a dent compared to the normal air exchanges. this is the reason that the EPA has so far not changed its opinion that plants don't make a difference under normal circumstances.

the whole reason that this research was done by NASA, and not the Centers for Disease Control or National Istitutes of Health, is that space ships are sealed up super, super tight. so, again, the question isn't whether plants and soil bacteria *can* remove chemicals, it's whether the rate that they can do it is relevant.

one of the few real life tests that made a significant improvement in indoor air quality, and the 'study' unfortunately was very small and not really well documented, was a Katrina trailer in which a fan assisted planter (see below) was added for a couple of weeks. formaldehyde levels dropped dramatically.

2) if you are growing plants specifically to clean your air, the first choice growing method is hydroculture using expanded ceramic media. additionally, he waters his hydroculture plants from the bottom. the reason is that the dry surface and inorganic media won't harbor much if any mold, and the very airy media allows for maximum air contact between the room air and the bacteria in the pot.

3) the second choice growing method is bottom watered sub-irrigation using traditional potting mix. the main reason is mold reduction.

4) if you are really trying to 'clean air' then you pretty much need to add a fan. he has designed different planters over the years that pull air through the growing media. one sold in Japan was called the EcoPlanter. he also helped design one that is now for sale in the US called the Plant Air Purifier.

the set up he has in his own home (which he describes in the book) is a sunroom that has a built in planter the full length of two of the walls. an air return pulls air from the bottom of this planter and pipes it into his HVAC system in the main home.

5) the plants need to be "photosynthetically active" if they are going to be consuming many chemicals. most houseplants are kept in less than optimal lighting - to put it mildly.

most people read the book, or just the summaries, and translate the message into "i can take a top watered little african violet, place it in a dark corner of my 1950's house, and, like, wow, now i'll have clean air!" thoughts like that make us feel better. whether they're true or not is beside the point.

i'm not trying to rain on anyone's parade here. i think folks should enjoy their houseplants simply because they enjoy them... and if they need science to appreciate (or justify) their plants, well there is plenty of science that says house plants make people calm. their presence helps us heal faster, and they help us focus. these facts have been pretty well established, and more importantly, they have been tested with the types and numbers of plants found in typical homes and hospitals. and you don't need fans and HVAC contractors...

as i said, i'm only posting this since it sounds like you're interested in science. personally, i think that plants may have a future in indoor air quality, but i think that it's going to be at the level large, commercial installations that are tied into the HVAC system.



ps here are some links if you're interested.

Wolverton Environmental Services

Plant Air Purifier


Inside Urban Green - What did Bill Really Say?

Phytophilter Technologies

Plants are the Strangest People - Indoor Plants as Air Purifiers

Kamal Meattle: How to grow fresh air
(i would be remiss for posting this link without providing more info. in this TED talk, it sounds like they just added some plants to a big office building, like you would see in a typical cubicle farm, and - shebang - the air was cleaned. what he doesn't show is that they have a full on indoor grow op with high intensity lighting and full hydroponic plumbing. the grow rooms are tied into the building's HVAC. there was another video that showed the set up but i can't find it now.)

    Bookmark   January 11, 2013 at 5:16PM
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Thanks. Some stuff there I hadn't found.

If the chemicals are consumed by soil bacteria, why do the plants need to be photosynthesizing faster than they do in typical interior lighting? The first source I'm looking at mentions 1150 lux, which is about what Wikipedia describes as tv-studio lighting, or twice what it describes as typical office lighting. That's not extreme.

I don't put much stock in the person who couldn't find anything about removal of xylene and toluene. Lots of the sources mention them, including some cited by Wikipedia, as well as WP itself of course. Wikipedia isn't a source itself, but it's by far the world's easiest place to find cited sources. If someone didn't bother to look at most of the sources cited by WP, I have to wonder where they did look.

    Bookmark   January 11, 2013 at 7:41PM
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The Ficus Wrangler

Thanks Nancy, that was a lot of good info. I knew about the NASA Wolverton studies in the 80's, the interior foliage industry expected them to spark a plant renaissance. Didn't quite happen. I've read some of the links in your post already, don't have time right now to check out the others, do you know if any of them, or any other studies, spcifically focus on the types of soil microbes or actual plant processes that produce the "cleaning" effects?

    Bookmark   January 11, 2013 at 9:56PM
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Some time ago, (and I'm too lazy to look it up again, so I'm winging it from memory), I did up a three point bulletin about what you'd need to know about how to use plants to produce the best results as re: using plants to clean air.

1. pot size matters. Specifically, diameter, or rather, exposed surface area.

2. the number of plants recommended is far more than most people will ever put in a room. for a fifteen by fifteen room, you'd be looking at approx. ten plants in 8-10" pots.

3. use a variety of plants. Not all plants draw the same materials, or at the same rates. you'd have to research each and every plant and each and every chemical, and that research just isn't going to happen. so a few things are known, but many aren't. and that just isn't going to change, so use as great a variety as you can grow effectively.

    Bookmark   January 11, 2013 at 11:31PM
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nancy_pnwzone8(OR zone 8)

dsws... when i said that most indoor light is inadequate, i meant that indoor light falls off dramatically as compared to the light available outdoors. this is why everyone's photos are blurry when shot inside. our eyes have 'software' that automatically adjust for the lower levels, so it doesn't *seem* so dark. also, people then make things worse and put their plants off in dark corners.

i'm not saying that decent indoor light near a window is inadequate, especially for the lower light plants - pothos, aglaonemas, sans, parlour palms, etc. if the plant is healthy and growing, i'm assuming that's good enough.

in dark northern winters, plants are barely doing anything, even if they are right up near windows. this is why many voices on this forum will suggest being more careful of watering and fertilizing during this time. plants are transpiring very little water when it's dark.

also, i'd say that more than half (maybe 75%) of the "what's wrong with my plant?" threads on this and other house plant forums are caused by inadequate light. overly moist potting soils are not nearly as problematic when plants are grown outdoors in better light. the plants transpire the excess water away. and, the plants are better able to defend themselves from bugs because they are making more chemical defenses and can out grow a problem.

so, i don't think that you need special high intensity lighting, especially if you choose plants with lower light needs, but you may need some sort of supplemental lighting (fluorescents?) at least for the dark seasons.

theficuswrangler... i haven't seen much info on the 'bugs.' i ran across what i'd describe as a 'comment' - ie this wasn't a scientific study being written up - that you really do need the plant. i'm surprised in a way, because you'd think that once some dirt built up in the filter bed, some bacteria would take hold and grow. this is what happens in aquariums that have under-gravel filters. you can even buy bottles of 'bugs' at fish stores to help get your culture started faster.

anyway, the only other info on the bacteria i've found says that different plants support different bacteria types. also, the bacteria will evolve or gain new member types based on the types of food (dust and chemicals) they are exposed to. in other words, a fan assisted planter in a smoking lounge will get better with time in breaking down cigarette smoke chemicals.

strobiculate... yes, plant number and surface area is the reason that folks are using fans. when you add a fan to a filter bed that contains hydroponic ceramic balls (LECA) combined with some activated charcoal, you can effectively multiply the plant numbers. so, on most of these planters, they'll say things like "this one has the cleaning capacity of 100 plants, or 200 plants, or whatever." this to me seems very doable. in this case (a fan assisted planter), a typical 2,200 square foot house might only need 2 or 3 planters - depending on size - to take care of the house.

i've been sorta tempted to buy one of the fan assisted planters to see if i notice any difference in my household air. i'm not really concerned that it's 'toxic' because my house is a hundred years old and leaks like a sieve. but my air 'stinks'. for lack of a better description, i call it "old house smell" or "attic smell".

the other reason to do it is simply that i'm interested in this sort of thing. i mean, why have normal hobbies like normal people :-)



ps if i don't buy one, i'm tempted to try to make one myself. i already grow most of my plants in Hydroton media, so the only change would be to add some activated charcoal (from a pet store). the main problem i haven't figured out is how you keep the fan from drying out the media too much.

    Bookmark   January 12, 2013 at 2:02PM
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you'd think that once some dirt built up in the filter bed, some bacteria would take hold and grow. this is what happens in aquariums that have under-gravel filters

I think it would -- if the contaminants were present at high enough concentrations to serve as a major food source for the bacteria. But we're talking about residual solvent evaporating from glue months or years after it's apparently dry. The plants provide the main food source, and the pollution is just a little extra bonus (from the bugs' point of view).

    Bookmark   January 12, 2013 at 3:24PM
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The Ficus Wrangler

Well, it's all very interesting, and I think we can all agree there's opportunity for tons of research. Anyone know any kids open to inspiration?

Nancy, I would like to address your comment that 75% of problems people are having are due to inadequate light. It may seem like 'six of one half dozen of the other', but to me it looks like the problem is incorrectly dealing with soil moisture. The reason is this: water is the variable over which people have the most control. Often, for whatever reason,the folks who are looking for help can't really do much about the light, but what they CAN do is adjust the water they give their plants so that the plants, while not attaining the levels of vigor found under adequate light, can still look lovely and bring much enjoyment to their people.

    Bookmark   January 12, 2013 at 9:11PM
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nancy_pnwzone8(OR zone 8)


"The plants provide the main food source, and the pollution is just a little extra bonus (from the bugs' point of view)."



in your screen name, you list yourself as zone 9/10. this is a whole different ballgame than large areas of the US. first off, you're going to get more hours of winter light, and depending on where you live, you're likely to get more sunshine/clear skies.

i live in portland OR and it is so dark in the winters that we have a houseplant book written for our area called "Green plants for gray days." it can be summarized as "pretty much you can only grow spaths, sans, aglaonema and pothos unless you supplement your winter lighting." that is only a very slight exaggeration. no amount of cutting down the watering to adjust for plants that need higher light levels will keep them happy in the winter. the exception might be some of the succulents that can effectively go dormant for the winter. a sun room will make a huge difference because even a dark, overcast sky is still pretty "bright" compared to the light that comes in through a window.

    Bookmark   January 12, 2013 at 9:59PM
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The Ficus Wrangler

Good point Nancy. I should have said that people can adjust the amount of water they give their plants, and they can also make sure the plants they have are suited to the light they can provide. So while I still think that overwatering is the major cause of houseplant death, trying to have plants that are unsuited to the available light is a big one also.

    Bookmark   January 13, 2013 at 12:32AM
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With supplemental lighting, as I understand it, the biggest thing is to put the light really close to the plant. Otherwise, most of the light is going to waste providing illumination for the rest of the room. And there's no way you're going to illuminate a whole room at levels high enough to make an appreciable difference.

Of course, there's a whole 'nother forum for lighting.

    Bookmark   January 13, 2013 at 8:31AM
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nancy_pnwzone8(OR zone 8)

dsws... i've never done much supplemental lighting, but you're correct that the light needs to be close to the plant (as long as it's not an incandescent light that can burn the foliage). there are a lot of very good threads here on gardenweb and out on the web that can help you with this if you want to try it.

every year of living in the "land of the dark" puts me closer to trying it. maybe i should just give up and move to a sunnier climate :-)

    Bookmark   January 13, 2013 at 2:12PM
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