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Smell of earth

QUESTION: What is the peculiar smell of the earth after the first shower?

T.S. Chellamal Anni, Chennai

ANSWER 1: The characteristic earthy odour of soil is caused by the production of a series of streptomycete metabolites called geosmins.

These substances are sesquiterpenoid compounds and unsaturated compound of carbon, oxygen and hydrogen. The geosmins first discovered has the chemical name trans-1, 10-dimethyl-trans-9-decalol; however, other volatile products produced by certain species of Streptomyces may also be responsible for the characteristic smell.

An unforgettable attribute of the streptomycetes is the musty odour they emit, an odour reminiscent of freshly turned soil.

Streptomyces are primarily soil micro-organisms requiring a lower potential for growth. The most significant environmental adaptation of the Streptomyces group is their ability to withstand dessication. Geosmins are also produced by some cyanobacteria.

Dr. I. M. Sarawad

Regional Agricultural Research Station

Bijapur, Rajasthan

ANSWER 2: The piquant, musky odour that hangs in the air emanates from an odorous chemical buried in the soil called `geosmin' (literally, earth smell).

The smell is given off by Streptomyces bacteria, a genus belonging to the Actinomycetales order of Gram-positive eubacteria, also called actinomycetes. The soil normally contains a multitude of environmental saprophytic fungi.

Actinomycetes, a type of filamentous bacteria grow in soil when conditions are damp and warm. When the soil dries out, the bacteria produces the geosmin spores in the soil. Rain hitting the ground kicks up an aerosol of water and soil and spores into the air, where they are easier to smell. (just like an aerosol air freshener).

We breathe in fine particles of soil containing the bacteria.

K. Kamalakkannan, Doha, Qatar

ANSWER 3: A pleasant smell after the first shower is because of a group of filamentous bacteria Actinomycetes found in the soil. They grow well in soil when the conditions are damp and warm.

When the soil is too hot, the bacteria are not able to tolerate the dessication, so it produces spores as survival strategies. The spores remain invulnerable for years and are resistant to dessication and heat. During the rainfall, the spores are taken up in the air by the force of wind and suspended in the air as aerosol. When we breathe the air, which contains spores, we are able to feel the earthy "after the rain smell". Geosmine (dimethyl-9-decalols) is the microbial product found in the spores is responsible for the pleasant smell.

Dr. P. Mariappan

JJ College of Arts and Science

Pudukkottai, Tamil Nadu

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