28 29 Beneath our feet is a miracle. It’s a thin cushion between rock and air, on which our lives depend. Few of us think about it. Few of us know anything about it. Try asking someone – anyone – what soil is. I’ve done it a few times, and the results made my head spin. Some people say it’s ground-up rock. That might be true of regolith on the surface of Mars, but not of soil on planet Earth. Others that it’s a mixture of everything that falls to the ground. But the most depressing answer I’ve received is: «It’s just dirt». Dirt is what soil is called in the US, and like dirt is how we treat it. So what is soil? The astonishing truth is that no one really knows. We know that it’s an ecosystem: one of the most diverse and abundant ecosystems on earth. Soil in mid-latitudes can be as diverse as the Amazon rainforest, and as little studied. Beneath one square metre of land, in the right conditions, there may be several hundred thousand tiny animals, ranging across thousands of species. Scientists estimate that only 10% of them have so far been identified1. Wherever you go, you are walking over undiscovered species. We also know that, like a coral reef, it’s a biological structure, built by the creatures that inhabit it. Without them, it would not exist. At the smallest scale, microbes create clusters of tiny particles stuck together with glue made largely from carbon. In fact, much of the organic carbon in the soil is used for this purpose, which is why soil collapses when the carbon content falls too far. Out of the tiny structures built by microbes, little scuttling animals like mites and springtails build bigger clusters. Out of these bigger clusters, the giants of the soil – such as ants and earthworms – create still bigger ones. Soil is fractally scaled. This means it’s organised on the same basic pattern, regardless of magnification: structures, within structures, within structures. This helps to explain its astonishing resilience in the face of droughts and floods: if it were just dirt, it would immediately be swept off the land. In other words, soil is like a wasps’ nest or a beaver dam: a system built by living creatures to secure their survival. But unlike those simpler structures, it becomes an immeasurably intricate, endlessly ramifying catacomb, created by bacteria, plants and soil animals, working unconsciously together. Soil behaves like dust in a Philip Pullman novel: it organises itself spontaneously into coherent worlds. Our ability to feed ourselves is an emergent property of these relationships. In fact, our lives depend on a feature that scarcely anyone has heard about: plants can talk. They speak in a remarkably sophisticated, precise and nuanced chemical language. And they invest vast resources into managing their relationships with the other living beings with which they talk. Of all the sugars that plants make through photosynthesis, they pump between 11% and 40%2 into the soil. Before releasing them, they turn some of these sugars into compounds of tremendous complexity. These complex chemicals are not dumped randomly in the soil, but into the narrow band of soil immediately surrounding the root hairs, a zone we call the rhizosphere. They are released to create and develop the plant’s connections with microbes, especially bacteria. Plants produce such complex chemicals because they want to talk not to bacteria in general, but to the particular species that are most effective3 in promoting their growth. Soil is crammed with bacteria. Its earthy scent is the smell of the chemicals they produce. Petrichor, the smell released by dry ground when it is first touched by rain, is caused in large part by an order of bacteria called the actinomycetes. The reason that no two soils smell the same is that no two soils have the same bacterial community. Each, so to speak, has its own terroir. Biologists call soil microbes eye of the needle, through which the nutrients in decomposing materials must pass, before they can be recycled by the rest of the food web. They live throughout the soil, but in most corners, most of the time, they exist in limbo, waiting for the messages that will wake them up. When a plant root pushes into a lump of soil and pumps out signalling chemicals and sugars, it triggers an explosion of activity. The bacteria responding to its call consume the rich soup the plant feeds them on and proliferate at astonishing speed, to form some of the densest microbial communities on earth. There can be a billion bacteria4 in a single gram of soil in the rhizosphere. The bacteria in the rhizosphere gather and unlock many of the nutrients on which plants survive. They produce growth hormones and other complex chemicals that help plants develop. When plants are starved of certain nutrients, or the soil is too dry or too salty, they will call out to the specific bacteria that can help to overcome these constraints. The microbes favoured by the plant create a defensive ring around the root, fighting off pathogens. They educate and stimulate the plant’s immune system, helping it to withstand attacks by fungi or insects. Beneath our feet George Monbiot
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