“The soil is a heritage, our duty is to pass it on”

Professor at the National Museum of Natural History in Paris, Marc-André Selosse is a specialist in the links between soil fungi and plant roots, a symbiosis essential to both partners. In his latest provocatively titled book, The origin of the world (Actes Sud, 480 p., 24 €), he invites us on an underground journey through the soils and their overflowing life, while pointing out the damage we inflict on them. A vibrant plea to invent gestures respectful of these ecosystems of tremendous biodiversity, largely ignored, because invisible.

Is the soil “the origin of the world”, really?

He is indeed at the origin of the current world. It influences the climate sometimes negatively, when it is a source of greenhouse gases, sometimes positively, when it stores organic matter. In addition, the waters of rivers and streams come out of the soil where they are loaded with nutrients and mineral salts. Soils “cry” in the oceans, fueling their fertility. This is why most fish are caught along the continents, where algae thrives – and why Breton fishermen want to fish off the coast of Great Britain … And then, the soil plays a central role in the cycle of water. It retains it like a sponge and will stop flooding, support low water levels and create water reserves from which humans and, above all, plants draw.

Before the soil was put in place, what did the surface of the earth look like?

It presented a dynamic of wadi, between sudden floods and total droughts. There were no plants, but just small crusts of microbes, a bit like those that adorn the facades of our buildings. And the oceans near the coast were less fertile.

“The agricultural soils of our regions are very poor in organic matter because they are plowed, which favors the respiration of the microbes which consume it”

So what is the alchemy of these ecosystems?

Soil, to put it simply, is dead organic matter in the process of becoming, under the effect of living organisms which thus recycle mineral salts. They are also mineral fragments colonized by microbes which dissolve them, in particular by local acidification processes. The fertility thus released is used to nourish the plants. And the ground contains gases which penetrate there, they also in becoming under the effect of the alive one. Oxygen is thus consumed for respiration. Here and there, in oxygen-free (anoxic) soils, bacteria produce methane. Elsewhere still, some bacteria use atmospheric nitrogen to produce proteins: it is the source of the nitrogen stock in the soil.

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