Our Indispensable Soil - Soil: Reflections on the Basis of our Existence - page 5

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Underground transport routes. Using computer tomography, it is possible to generate three-
dimensional X-ray images of undisturbed soil ( upper image ) that reveal what soil aggregates and
cavities look like. The image shows a cubic centimetre of soil from a field, with all the white
areas representing cavities. More than half of the soil volume can consist of pores, but this pro-
portion decreases when soil is compacted by heavy machinery or careless feet in a cultivation
bed. Plants find it harder to grow in soil that has been compacted as the air penetrates less easily
to oxygen-requiring organisms
Soil that dries out forms small lumps or variously sized aggregates. Organic matter in the
lumps absorbs moisture, keeping the soil humid. The cavities between the lumps are full of air,
and rainwater moves through these spaces on its way down through the soil. The soil is in con-
stant flux, with transport channels for water and air coming and going as the weather changes

Light, air, water and nutrition. A delicate mycorrhizal fungal mycelium grows from the roots of
a pine seedling. The plant is cultivated on a Plexiglas plate with a thin layer of soil to reveal how
the mycelium forms the tree's contact interface with the soil. The fungus absorbs the water and
nutrition the tree needs and transfers it to the roots. In exchange, the fungus gains access to car-
bohydrates that the pine stores in its needles with the help of sunlight. All the white areas on the
image are fungal mycelium, while individual roots can be distinguished in the centre of the image.
In forest soil, a single teaspoon of humus can contain several hundred metres of fungal hyphae. A
tree's mycorrhizal mycelium is easy to glimpse if you lift up the moss in the forest, but you need a
microscope to identify the type of fungal mycelia that grow from the roots of herbs and grasses
Boundary-spanning collaboration. “Nothing in evolution makes sense except in the light of sym-
biosis,” says symbiosis researcher Lynn Margulis, paraphrasing the classic words of the geneti-
cist Theodosius Dobzhansky, who said, “Nothing in biology makes sense except in the light of
evolution.” Symbiosis—cohabitation between organisms of different kinds—is common in nature
and was a prerequisite for the explosion of biodiversity on Earth. When algae and fungi met and
formed lichens, we gained a new group of organisms that, in time, spread to almost all corners of
the planet. Growing on the carved handshake in the photograph are crustose lichens