Nose Dive by Harold Mcgee

Author:Harold Mcgee [McGee, Harold]
Language: eng
Format: epub
Publisher: Penguin Publishing Group
Published: 2020-10-20T00:00:00+00:00

Soil smells: generic life, enigmatic geosmin

Eventually plant litter and compost end up belowground as part of the soil proper, where they contribute to the background of its distinctive smell. Organic remains account for just a few percent of the soil’s volume, and most of that portion is humus, a mix of persistent molecular remnants that absorb and hold some water and minerals, help keep the soil porous and moist and nourishing, and give it its characteristic brown-black color.

Undisturbed soil teems with life of all kinds, much of it as yet unidentified. It’s estimated that there are tens of billions of bacteria in every gram of soil, and tens of thousands of different species of microbes, fungi included—along with minute worms and insects. The bulk of most soils is roughly equal volumes of rock particles and the spaces in between them, either empty or filled with water. The small wet pores are usually occupied by bacteria, and fungal hyphae thread their way through the larger ones to find rare oases of plant or animal remains, or living plant roots.

With such a diverse cast of creatures in the soil, it’s no surprise that its volatiles are also legion. They’re dominated by several odorless gases: methane, carbon dioxide, and nitrogen oxides, the first the product of microbial metabolism in the oxygen-poor regions of the soil, the others from aerobic breakdown of plant and animal remains. The remaining emissions, a tenth to a third of the total, are largely the usual starter-set volatiles (see this page), along with terpenoids that originated in needles and leaves. And a small fraction consists of volatiles constructed by the soil denizens as signals, defenses, hormones, and other aids for growing and reproducing.

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