The Soul of Soil

Enterprising microbes paved the way for large life on land by breaking bedrock into bits.

Soil starts a few meters below the surface with solid bedrock. Above bedrock is a fractured, crumbly layer of subsoil. Sitting atop subsoil is the rich, biologically active layer of soil, where plants can thrive.

“Chemical and physical processes start to crack bedrock, but those processes are not enough to make the minerals that become soil. Once the bedrock cracks sufficiently, microbes enter the cracks and take over. The result is a rapid biological acceleration of weathering.” ~ American biogeochemist Eric Roden

Microbial enjoyment of bedrock begins with oxidation, which moves electrons. Bacteria wiggle their way into the cracks of bedrock, whereupon specialized bacterial proteins stimulate oxidation. The bacteria then sip the freed electrons for energy without supping on indigestible minerals. Many diverse species of bacteria mastered this craft.

“External electron transfer is a way to cope with the difficulty of eating iron. Organisms coupled the oxidation of iron to the generation of ATP, the ‘energy molecule’ in all known life,” said Roden.


Stephanie A. Napieralski et al, “Microbial chemolithotrophy mediates oxidative weathering of granitic bedrock,” PNAS (16 December 2019).

Hard as a rock? Maybe not, say bacteria that help form soil,” ScienceDaily (16 December 2019).