Rhodopseudomonas: The Purple Bacteria That Helps Your Soil Cycle Carbon

There's a bacteria in your soil that's actually purple, photosynthesizes like a plant, and does more for your roots than most gardeners will ever hear about. Let me introduce you.

Okay, so. I get worked up about soil bacteria. My husband has heard about most of them over dinner by now, my daughter has started finding somewhere else to be, and I'm still not sorry about any of it.

That's the trouble with soil. Once you know what's happening down there you can't go back to not knowing.

This week's microbe is Rhodopseudomonas. Say it "row-doe-soo-doe-MOH-nas." Nobody gets it on the first try, myself included, and I've been saying it out loud for years. The short version is that it's a phototrophic bacterium, meaning it makes its own energy out of light — same trick your tomatoes are pulling up top, except this one's microscopic and lives in the dark.

What Is Rhodopseudomonas, Anyway?

It's a purple non-sulfur bacteria, or PNSB if you want the shorthand the researchers use. And yes, actually purple. Grow enough of it in a flask and the whole thing goes a reddish plum color, which is where the "rhodo" comes from. Greek for rose.

The species everybody studies is Rhodopseudomonas palustris. Microbiologists have a real soft spot for this one, and I think it's because it might be the most adaptable living thing anybody's ever put under a scope.

Most bacteria make a living exactly one way. This one has four:

• Make energy from sunlight, like a plant does
• Pull carbon dioxide right out of its surroundings
• Feed on organic carbon already sitting in the soil
• Grab nitrogen from the air and hand it off to roots

And it flips between them based on whatever the day hands it. Light's gone? Fine. Oxygen's gone? Also fine. Researchers have built entire careers on working out how one organism pulls that off, and for our purposes it's the reason this bacteria keeps earning its keep in gardens where conditions are less than ideal. Mine very much included.

Most microbes have one job. This one has four.

Why It Gets The Spotlight: Carbon

Your soil is full of carbon. But not all carbon is the same, at least not from a microbe's point of view.

Some of it's easy. Sugars and simple compounds, gone within hours of hitting the ground. Then there's the other kind — stubborn, ring-shaped molecules left behind by old roots and woody stems and whatever dropped off your maple three autumns ago. That stuff can sit there for years, because most bacteria simply don't have the equipment to break it apart.

Rhodopseudomonas does. It takes apart those tough aromatic compounds. Then, when there's light around, it turns the other direction entirely and pulls carbon dioxide back into the system.

Working both ends of the same cycle like that is a little unusual, and it's the reason this microbe earns a spotlight post.

If you want the fuller picture of how carbon and its cousins actually move through your dirt, I went through the whole system in our guide to the nitrogen cycle in your garden, and there's a companion piece on how fungi improve soil nutrient cycling. It all connects up down there.

What Rhodopseudomonas Does For Your Plants

Carbon's the headline. It isn't the only thing this bacteria is up to, though.

• Fixes nitrogen. It pulls nitrogen right out of the air and converts it into a form roots can use. No bag of synthetic salts involved.
• Makes a growth compound called ALA. Short for 5-aminolevulinic acid. Research ties it to stronger growth and better stress tolerance in crops, which is a big part of why these bacteria turn up in commercial biofertilizers.
• Helps plants take a punch. Heat waves, salty soil, a brutal stretch in August — purple non-sulfur bacteria have been shown to keep plants going through conditions that would normally set them back weeks.
• Cleans up around the roots by eating leftover organic gunk that would otherwise just sit there.
• Doesn't need oxygen. This is the one I find most useful. It'll keep working in soggy, low-oxygen ground where plenty of aerobic microbes give up and go quiet.

That last one matters more than it sounds like it should. Rice farmers across Asia have used purple non-sulfur bacteria as a biofertilizer for decades, and the reason is standing water — flooded paddies don't bother these microbes one bit. So that one clay bed of yours that turns into a swamp for two days after every storm, the one you've mostly stopped trying with? Prime real estate. Our post on anaerobic bacteria in soil gets further into what's happening in those low-oxygen pockets.

The research keeps stacking up too, if you're the type who likes to go read it yourself. There's a good pile on PubMed covering Rhodopseudomonas palustris and plant growth, and Cornell's soil health program lays out the basics of why living soil biology matters without burying you in jargon.

None of these microbes work alone, by the way. Rhodopseudomonas has coworkers, and I've written up a few of them: Desulfovibrio handles sulfur, Paracoccus keeps nitrogen in balance, and Bosea is another sulfur specialist. There's an entire economy running under your tomatoes.

How To Get Purple Bacteria Working In Your Soil

You don't have to buy a thing to encourage these microbes. Some of them are almost certainly in your beds already. They'll only do the work, though, if the conditions let them.

Four things that help:

• Give them carbon to eat. Compost, shredded leaves, worm castings. Phototrophic bacteria are carbon eaters, and thin bare soil starves them right out.
• Keep the ground damp. Not swampy. Just not bone dry. A couple inches of mulch does most of this for you and saves you the watering besides.
• Ease off the synthetic salt feeds. High-salt granular fertilizer is rough on soil biology. That's the whole reason I started making products the way I do.
• Put more back every few weeks. Microbe counts rise and crash with weather and watering and season, so a living liquid feed every two to four weeks through the growing season keeps the numbers where you want them.

That last one is the step everybody skips, me included, some Julys. Soil biology isn't a purchase you make once. It's closer to a sourdough starter — feed it or lose it. Ask me how I know. I've killed two starters and one very promising bed of dirt, so.

What's Actually In Plant Juice

Now I have to tell you something that does nothing to help me sell anything.

Our Plant Juice does not contain Rhodopseudomonas palustris. Not under that name, anyway. I won't put a species on a page if the lab report doesn't show it, and it doesn't.

What the report does show is three of its close purple non-sulfur relatives. Same family, same carbon habits, same general line of work. BiomeMakers ran the DNA sequencing — that's an outside lab, not us grading our own homework:

• Rhodoferax fermentans — ranked 69th most abundant species in the report
• Rhodobacter sp. — ranked 92nd
• Rhodoplanes sp. — ranked 154th

Three species out of the 291 that lab counted in the bottle.

I'm a chemical engineer by training, so "it's got good bacteria in it" was never going to be enough for me. I wanted somebody outside this building to name them and count them. That's the entire reason those reports exist and the reason I keep paying for them.

If you want the bigger picture on why any of this beats a bag of synthetic feed, start with living soil explained, then how beneficial microbes help plants grow and the science behind microbe fertilizer. Or if the science isn't the draw and you'd just like gardening to take less out of you, read this one instead.

One practical note before I let you go. Because soil biology wants feeding on some kind of schedule, a lot of our regulars just put Plant Juice on auto-delivery so the next bottle turns up before the last one's empty. You pick the interval at checkout and change it whenever you want. And if you'd like to give those carbon-eaters something to actually chew on, Ancient Soil worm castings are the obvious thing to pair it with.

What Gardeners Are Seeing

Plant Juice averages 4.65 out of 5 stars across 1,793 verified reviews.

★★★★★

"Then, a few weeks ago, I read on Facebook an ad for Elm Dirt Plant Juice. Its promise of healthy root systems seemed worth testing on my Gala, and, if it worked, I could apply it to other trees in my mixed-fruit orchard that have been struggling to develop wind-resistant roots. I bought three bottles… This morning, I was ecstatic to see that these new leaves are growing larger, more leaves are appearing on more branches, and leaf clusters are forming, too! … Elm Dirt Plant Juice has been this tree's savior, I'm sure."

— Jennifer N., Verified Buyer

★★★★★

"I started using the Plant Juice a couple of months ago in hopes to revive my African black olive tree, which had all its leaves drop at the end of April… About a week ago (3 mos) later I noticed new buds on the stems !! I almost burst into tears, my tree is coming back to life!!"

— Denise S., Verified Buyer

★★★★★

"I have never had tomato Roots grow out of the pot as strong as this this stuff is amazing it definitely makes root production much faster then miracle grow I love it"

— Timothy M., Verified Buyer

Quick Questions, Straight Answers

So: a purple, light-eating, carbon-cycling bacteria that almost nobody has heard of, running a cleanup operation under your feet while you're inside making coffee.

Feed the soil and the soil feeds your plants. That's still the whole game.

Happy growing, friends.

Lauren Cain — Founder & Chemical Engineer · Elm Dirt, Grandview, Missouri Lauren started Elm Dirt after her infant daughter ate a handful of backyard dirt. As a chemical engineer and a mom, she built her fertilizers around living soil biology instead of synthetics. Elm Dirt is now trusted by home gardeners, rose champions, and organic growers all across the country.