Petrichor: Smell of the Earth

Petrichor: Smell of the Earth

A heavenly scent: Do you love the smell of soil after a fresh bout of rain? Are you a fan of the earthy smell of beets? There is a word for that: petrichor.  It comes from the Greek petros, meaning stone and ichor, the fluid that flows in the veins of the gods. It is defined as “the distinctive scent which accompanies the first rain after a long warm dry spell”. 

Geosmin: After puzzling over the smell of soil for over a hundred years, scientists have pinned the source to Streptomyces, the soil bacterium that also gifts us with the most antibiotics. The bacteria release volatile compounds when disturbed, like the bicyclic alcohol, geosmin (named for “earthy smell”). Did you know that the human nose is incredibly sensitive to geosmin? We can detect as little as ten parts per trillion! 

One hump or two?: Bactrian camels are reputed to detect water from 50 miles away. The signature smell of Streptomyces is easily carried across the desert and picked up by the camel’s sensitive nose. In return, the bacterium probably benefits from having its spores spread around. The musty earth scent of some Cactus flowers is also due to a derivative of geosmin. It lures pollinating insects by a promise of water. This is known as floral mimicry. Unfortunately, fish that absorb minute amounts of geosmin from water don’t taste that great.

✿ This smelly chemistry post is a birthday present for our favorite Google+ chemist Siromi Samarasinghe! Check out other odoriferous posts in Sirome’s honor by Chad Haney (http://goo.gl/INUpXi , http://goo.gl/SN4WQs and http://goo.gl/XpVL9R). 

Image: Streptomyces coelicolor http://microbewiki.kenyon.edu/index.php/Streptomyces

Source and Ref: http://www.bios.niu.edu/meganathan/smell_of_soil.shtml

#ScienceEveryday  

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96 Responses to Petrichor: Smell of the Earth

  1. Azlin Bloor says:


    Another illuminating read, Rajini, thank you! Your posts are always fun & informative.


    Happy Birthday Siromi Samarasinghe! 

  2. Rajini Rao says:


    A small repayment for your culinary inspirations and carefully curated foodie posts for the G+ community, Azlin Bloor . I was so pleased to stumble across the English (even if Greek derived) word for this earthy smell. I grew up with the Hindi word as a child and searched for the equivalent in English for years 🙂 

  3. Uttam Sandy says:


    Very interesting post Rajini Rao 


    Thanks for the post… 🙂


    Have a great day

  4. Jeff Leaper says:


    Soylentcor make Soylent Green. 

  5. Rajini Rao says:


    Thanks, Uttam sanDy . This is better than science fiction Jeff Leaper 🙂


  6. Earth smells lovely and you rock Rajini Rao … =D

  7. Rajini Rao says:


    It’s Magnus Fahlén ‘s birthday too, people! 


    Happy Birthday, Viking ♫♫  🙂

  8. Jeff Leaper says:


    Rajini Rao  for all your contributions to G+ Science


  9. Ahahaha thanks again Rajini Rao … ^^

  10. Azlin Bloor says:


    You are too kind, Rajini. What’s the Hindi word? I don’t think I missed it in the post or did I?


    I find it interesting that Bear & Thomas chose “the fluid that flows in the veins of the Gods” for their portmanteau.


    I know you’re busy nomad dimitri but I’m sure you can come up with deeper motive behind their choice, I’m sure.


  11. Azlin Bloor ı am back early at the hotel wıth stomach cramps ı wısh ı had a jar of my petrıchorous Anne Ricci ınspıred beet kvass Rajini Rao. best wıshes to sagıttarıan Siromi Samarasinghe 

  12. Chad Haney says:


    That’s fantastic Rajini Rao. Can you comment about the ion channels on the TCA post? I remember hearing that dogs trained to track, not only follow the scent that they are given but also the scent of the freshly disturbed soil. I need to look for a reference to see if it’s related to geosmin.

  13. Rajini Rao says:


    Chad Haney , will do! The tidbit on doggy sleuthing is great, would love to know more. 


    There is so much good science on Google+ today, too bad that we were sidetracked by the pseudoscience stuff too. 

  14. Chad Haney says:


    Yes I’m still going at it with one anti-GMO guy. Argh. Time for a late lunch first.

  15. Rajini Rao says:


    nomad dimitri sorry to hear about the tummy troubles 😦 How about some ajwain or carom seeds (Trachyspermum ammi) as a digestive?


    I think the word is saundhi for earthy smell, Azlin Bloor . I need current Hindi speaker to check my memory. Hopefully, one of our readers will correct me if I’m wrong. 


  16. Do you watch Doctor Who? Many people learnt this word a couple of years ago, as it was used profusedly in the series.

  17. Rajini Rao says:


    Víktor Bautista i Roca that’s cool, thanks for the info. Doctor Who is a bit on the campy side for me 🙂 I watched the original series years ago but didn’t keep up with the various reincarnations. 


  18. Happy birthday Siromi Samarasinghe 

  19. Jeffrey Lan says:


    Also a significant component in the flavor of beetroot

  20. Rajini Rao says:


    Yes, indeed Jeffrey Lan . Some people hate the smell of beets! 


  21. Is Geosmin always in a planar structure?


    It looks like most often the carbon rings are on the same plane, having the Methyl and the Hydroxyl groups on one side bending away from the Methyl group on the opposite side.


    But could it ever be shaped like a butterfly? Could it have carbon ring wings bending away from the Hydroxyl and that opposite Methyl groups? Would this not work because the remaining Methyl group would be unstable?


    What are some other small molecules with dual 6 carbon rings?


    Thanks so much for sharing science!


  22. i think smell varies with places  Rajini Rao 

  23. Rajini Rao says:


    I’m sure it does, siddardha bonu . Do you know the word for earthy smell in Hindi (or Bengali, not sure where you are from). 


  24. in hindi we can call it as  “prithvi gandha”

  25. Rajini Rao says:


    Hmm thanks… that just translates to “smell of the earth”. 

  26. Chad Haney says:


    Still searching. I learned that tracking is different than trailing. Trailing is what most people think search-and-rescue dogs are doing, i.e., following a human scent. Tracking involves using vision and more than one scent, e.g., soil disturbances. Now if I can just find if someone has determined that it’s a specific soil scent.

  27. Bill Carter says:


    Very timely post for us – it just rained for what feels like the first time this year.

  28. Rajini Rao says:


    Good news, Bill Carter . The west coast really needs winter rains. We’ve just emerged from a soggy (and cold) week as well. 

  29. Marta Rauch says:


    Fascinating! Thanks for another excellent post, Rajini Rao!


  30. …parts per trillion!! That is fantastic!


    For a little perspective on that number, a billion seconds ago is 32 years – a trillion seconds is 32,000 years.


    I had no idea our olfactory sense was so sensitive. Makes perfect sense that our smell would evolve to detect geosmin, that signature scent of water, but parts per trillion blows me away.

  31. Rajini Rao says:


    Put that way, it is quite fantastic, I agree!  As animals go, we don’t even have a particularly good sense of smell, David Andrews .

  32. Mike Davey says:


    Yes and yes. I’ve always considered this one of the most distinctive smells and tends to evoke very particular emotions.

  33. Rajini Rao says:


    Mike Davey I love this smell. Takes me right back to my childhood and the monsoon rains in Calcutta 🙂

  34. Mike Davey says:


    Rajini Rao It always reminds me of summer rains when I was young here in Canada. They weren’t monsoons I’m glad to say. 🙂

  35. Rozni Yusof says:


    I remember watching a cooking program where the chef prepared a sauce using a bit of earth from the forest and truffles. I thought it was too strange at the time.


    But this post makes sense of it now.

  36. Rajini Rao says:


    Monsoons in Calcutta meant flooding and holidays from school, Mike Davey 🙂 Warm rains in a Canadian summer sound lovely too. 

  37. Rajini Rao says:


    Rozni Yusof: this is news to me!


    I laughed when I heard wine experts talking about earthy and “barnyard” flavors! 

  38. Chad Haney says:


    “barnyard” flavors?

  39. Rajini Rao says:


    Chad Haney aka “poop”. 

  40. Chad Haney says:


    That’s what I was afraid of. When you think of pie with a dessert wine, you don’t think of mud or cow pies.

  41. Rajini Rao says:


    You must have a special nose for pseudoscience then IOANA Datcu 😉 

  42. Norman M. says:


    The can be incorporated into a scifi story

  43. Gary Cox says:


    Thank you for the lesson !! 

  44. Rajini Rao says:


    Gary Cox I’m a regular school marm 🙂 

  45. Steve Thomas says:


    Excellent! This may also explain the “uncanny” ability of native desert peoples to locate water sources. Not quite as well as camels, but it’s not a big stretch from detecting a scent to locating it’s direction.

  46. Gary Cox says:


    Rajini Rao  I always try to learn something new everyday . Today you were the most informative !!!

  47. Smokey Brr says:


    I have Viens of the gods


  48. Oooh, oooh! Rajini Rao — thank you for helping me put words to sensory experiences that I never had words for, before.


    One night, many years ago, around this time of year, I was walking out of Schermerhorn Hall at Columbia, around 8:30 at night, with a bunch of researchers including Herb Terrace, the behaviorist/psycholinguist. And I sniffed the air and said ‘Smells like snow.’ (A smell my mother had taught me to recognize at a very young age.) Prof. Terrace barked: “How do you know what snow smells like? Didn’t you grow up in the City?”


    What’s that smell called? And what causes it — do they know?


  49. Thanks for posting this Rajini Rao, it’s my favourite post I’ve seen all day! Must show to the kidlet, she’ll love it.

  50. Rajini Rao says:


    Now that’s a challenge, John Jainschigg . I’m told that the smell before rain is due to ozone from electric discharges in the sky. Perhaps it’s ozone or some other atmospheric gas associated with a snowstorm. There may not be a word for it in English, but surely the Eskimo people have one 🙂


  51. I wondered if it might be the nerves in the cribiform plate detecting the presence of frozen water droplets in the air and interpreting their nano-geometry as a ‘smell.’ (This is how I like to do Science — by ‘making things up.’)

  52. Rajini Rao says:


    John Jainschigg  You’re good!! 😀


  53. Interesting! I love the smell of the rain, but I never wondered what caused it. Thanks for sharing.


  54. Amazing piece of article. Basically humans love soil 😉


  55. But I am puzzled by this – It smells same when I turn on my car air conditioner usually on a dusty day!!


  56. A very informative post.


  57. And that brings us to geophagy. My first wife loved to eat clay, it’s very common among pregnant women in Africa. She told me it tastes like the smell of wet soil after the first rain. The problem is that it is addictive. When she was craving clay in France because I couldn’t find edible kaolin, she was really miserable. So I wonder if it is the mineral itself which gives pleasure or if there isn’t some kind of biological evil inside, a biochemical compound which creates addiction. (And yeah, for a geologist, all these biological slimy thingy are by essence evil)…

  58. Anne Guillot says:


    Rajini Rao Great post! I am currently studying toxicology and bacteriology, and I love your insights here. I am passionate about bacteria, and not only in my kitchen when I make nomad dimitri ‘s favorite beet kvass or sauerkraut, but also in nature.

  59. Rajini Rao says:


    R Prakash Prakash it’s possible that bacterial spores are sucked into your car on a dusty day and that’s what you are smelling. 


    Olivier Malinur may be she was keeping you busy hunting kaolin since you are a geologist 🙂 But that is remarkable, to crave clay. Did they cravings go away post-pregnancy? 

  60. Rajini Rao says:


    Anne Ricci thanks for the introduction! I wish I had more courage to ferment things in my kitchen. I’ve not gone much beyond fermenting rice/lentils for south Indian dosas (just 1-2 days at most). I remember the beet kvaas post: I liked the idea of topping it off with more water to keep it going. 

  61. Robert Pence says:


    If you hang sheets outside to dry they smell like that. Also snow has that smell least I think so. Its a very distinct smell.


  62. Thank you Rajini Rao for this wonderful post and the fascinating science behind petrichor. I love the smell of the earth when rain water falls on it after a long spell of drought. We call it “Nawum” smell in Sinhala, meaning “fresh”. I have learned so many new things from this post. My favourite topic of the food chemistry course that I teach is ‘Sensory properties of food’, and the science of smells and flavours that you and Chad Haney have been sharing  give very interesting information which I can use in my lectures! Thanks for the birthday present!

  63. Rajini Rao says:


    I am so glad that you enjoyed our “chemistry of smell posts”, Siromi Samarasinghe 🙂


    P.S. Nawum (or something close to that) means new in most Indian languages too. 

  64. Anne Guillot says:


    Rajini Rao Oh I also make dosas, I love them. It’s been a while since I haven’t made any; back to it!!

  65. Rajini Rao says:


    In that case, I look forward to your future dosa post served with a dose of microbiology Anne Ricci 🙂

  66. Chad Haney says:


    I’m glad you enjoyed your birthday Siromi Samarasinghe. Maybe next year will be a better organized effort. This was ad hoc but fun.

  67. Nick James says:


    A door opens on a whole new world 🙂


    Things you always knew but did not think there were answers for.


  68. Yes, after pregnancy it goes away. Some says it is a way for the body to force us to get mineral elements.

  69. J Stasko says:


    Wow.  10ppt is much more sensitive than that for H2S, which is quoted as minimally 2 ppb ( http://www.arb.ca.gov/research/apr/past/a4-046-33.pdf )   

  70. J Stasko says:


    Olivier Malinur I wonder if she is actually craving the iron….  I do know that one  of the signs of iron deficiency is eating ice (strange!)  http://www.mayoclinic.com/health/chewing-ice/AN01278

  71. Diana Studer says:


    I can smell rain coming, but only when it is minutes away. I’ve seen petrichor translated as the blood of the earth. Wonderful smell when the earth has been hot and dry for what feels like for ever, and the first rain falls!

  72. Rajini Rao says:


    The smell is wonderfully evocative, Diana Studer ! Living in a city suburb, I don’t smell it that often now. Probably too many manicured lawns and concrete, ugh!


    I think blood is a good translation for the fluid running in the veins of Greek gods 🙂

  73. joe breskin says:


    Two thoughts arose from this.


    First, for the past 30 years or so – since I learned about the mycorrhizal relationships that support the Pacific Northwest’s forests, I have referred to the horrific smell that follows softwood forest clear-cutting as the smell of the death of the fungal web, the smell created as an amount of biomass at least comparable to the timber that was removed, now deprived of the pump provided by their symbiants’ year-round evapotranspiration, quietly suffocates.


    Second, back before 9-11 there was quite a bit of research at USDA and elsewhere into training dogs to detect stuff that was a lot more important in the long run than C4 or THC at airports … like the fungi that cause citrus cancer, and heart rot and root rot in trees. 

  74. Rajini Rao says:


    joe breskin , two fascinating facts new to me, thank you. I had not stopped to consider how logging would also destroy the underground fungal hyphae. Chad Haney and I were discussing the use of dogs to sniff out chemicals (tracking versus trailing, in police work). What a great idea to train them to sniff out plant pathogens too. 

  75. Chad Haney says:


    Beagles are used in airports to look for smuggled in food. That’s partly due to concerns about invasive bugs and other harmful things to the US agriculture.

  76. Rajini Rao says:


    The airport dogs are sniffing out more than illegal drugs?…hmmm, better put extra packaging on my spice mixes from India. 

  77. Chad Haney says:


    If I’m not mistaken, beagles are for USDA and labs and German Shepherd dogs are for DEA/TSA. Let me look, so I’m not just making stuff up.

  78. Chad Haney says:


    Sort of confirmed what I was thinking.


    1984


    USDA established a detector dog program at Los Angeles International Airport with one team consisting of a beagle and a canine handler. APHIS worked with the U.S. Customs Service to develop a detector dog program. After selecting beagles as the agency’s detector dogs, APHIS worked with the military at Lackland Air Force Base in Texas to train Beagle Brigade teams.


    http://goo.gl/Lj18E2


    http://goo.gl/1XglMF


    I love how the handler gets choked up when he talks about Rambo retiring.


    Rambo at O’Hare


    http://goo.gl/1OlHYD

  79. Rajini Rao says:


    Oh my, that was such a sweet video, Chad Haney . Impressive too, watching the duo at work. Thanks for finding the links..checking them out. 

  80. Sunil Bajpai says:


    Rajini Rao Thanks for a wonderful post once again! You’re also right about the Hindi word to describe smell of earth after a light, recent shower.


  81. Rajini Rao Hmmmm, this is great but then I have unresolved query…


    Little background-


    When I was in primary school, we used to have a stone slate. We used to write with a stone (soft calcite?) pencil.


    If you use water and try to wash that type of slate, it will smell exactly like earthly smell.


    Where does that come from? What is the overlap?


    Second, from evolutionary perspective, what is the benefit of us having such high sensitivity and liking? Is it due to same as camel? Need to find water source?

  82. TA G says:


    I’ve always commented on how beets smell earthy!….. Great information. Thanks


  83. Wow I enjoyed it a lot thank you andshare more and more posts like this which will help to look this world with consise of science and wonder

  84. Rajini Rao says:


    Sunil Bajpai , TA G and chandrasekhar srinivasa pawan kumar sarma konda glad you enjoyed it. Thanks for letting me know! 

  85. Rajini Rao says:


    mandar khadilkar , hi! I loved your childhood memory of the chalk and slate board. I would have to guess that there was a small amount of Streptomyces spores in the chalk that was released when moistened. Even a small amount should be detectable based on our sensitivity to geosmin and other Streptomyces volatiles. 


    As for the evolutionary benefit, again, one can only speculate that the smell is associated with water, as you guessed. This is hard to test or know for sure, but fun to speculate 🙂


  86. Rajini Rao you are probably right about spores.


    Just one small comment-the pencils we used were really made up of soft sedimentation stone. If you don’t bye the high quality ones, the slate would get scratched.


    The chalk based pencils came later. The chalk based color pencils were particularly interesting to me…..love for art/painting for me started there.

  87. Amir Temoury says:


    hello and thanksfor all your photos

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