Our Indispensable Soil - Soil: Reflections on the Basis of our Existence - page 3

The Constituents of Soil
If you destroy the structure of the soil, with its delicate networks of pores through
which air and water pass, you destroy the soil itself. The best way to preserve the
soil's structure is to let plants grow in it. Plants protect the soil against raindrops,
while dead plant matter in the soil supports fungi and bacteria that help keep the
soil particles together and maintain a stable structure.
Soil is made up of mineral particles, organic substances, air, water and liv-
ing organisms. Pick up a handful of soil and you will notice that it consists of
lumps of varying sizes. These are known as aggregates and can be as small as a
tenth of a millimetre across. Aggregates bond to create larger lumps that together
give the soil its porosity. Without them, the soil would be dense and impermea-
ble. Aggregates are formed by mineral particles that adhere to organic matter via
slime secreted by fungi and bacteria. The excrement of earthworms is beneficial
to soil because the worms' stomachs are filled with slime-creating bacteria. The
aggregates are held together by thread-like fungi hyphae, which are also coated in
water-repellent substances that prevent the soil from collapsing during rainfall.
The soil's network of pores is essential to enable air to reach plant roots and
oxygen-reliant organisms and for water to penetrate the earth. Digging by ter-
restrial animals helps porosity by creating hollows in the ground, but pores are
most concentrated around plant roots, where biological activity is greatest. Roots
secrete carbohydrates and amino acids on which a rich microflora of bacteria and
fungi depends, in turn providing food for amoebae and other small organisms that
live in the soil.
Fungi that exist in a state of symbiosis with plant roots have been my special-
ist area of scientific study for 20 years. They are known as mycorrhizal fungi, a
word that comes from the Greek myko (fungus) and rhiza (root). Ultra-thin fungal
hyphae are adept at penetrating all the nooks and crannies of the soil in their quest
for nutrients. They sprout from the tiniest plant roots and extend in complicated
networks that also play their part in stabilising the soil structure. I like to ask my
students to analyse a handful of soil from a pine forest under the microscope so
they see just how ubiquitous hyphae are: they are everywhere, surrounding every
particle of soil, while roots are much harder to find. Hundreds of metres of hyphae
can grow from a single root tip and it is quickly apparent that the roots' function
is to nourish the fungi so they can locate food sources for the tree. In return, the
fungus gains carbohydrates formed by the pine during photosynthesis. This is
why this symbiosis is so successful in the plant kingdom. When the sun shines,
the plant benefits from carbohydrate formation but struggles to absorb sufficient
quantities of minerals like nitrogen and phosphorus from the ground. By contrast,
soil-dwelling fungi have to compete fiercely for scarce carbohydrates. By tapping
photosynthesising plants they gain a huge advantage when competing with other
organisms in the soil.