Look below your feet. Chances are you are standing on flooring built on concrete. And what is under the concrete? Just mud. Earth. The ground.
Soil is such a mundane feature of our world that, unless we are selecting footwear for a rainy outing in the countryside, we seldom give it thought. But it is running out. Every year, 75 billion tonnes of soil – covering more than 38,610 sq miles (100,000 sq km) of arable land – is lost. Around 80% of global farmland is now moderately or severely degraded and, in the past 40 years alone, one-third of cropland has had to be abandoned because of severe soil erosion.
And this is at a time when farmers are struggling to feed a global population of seven billion and growing, which currently gets more than 95% of food calories from the land. Over the next 50 years, more food will have to be produced than over the last 10,000 years combined.
Mud, in other words, is the vital thin brown line between us and starvation.
Soil erosion, which is driven by humans through farming or livestock grazing, has played a big role in the decline of entire civilisations, including in Mesopotamia, Greece, Rome and Central America, according to University of Washington soil scientist David Montgomery. He points out that civilisations only last as long as it takes to plough through the depth of their regional topsoil. Having seen firsthand the dire consequences of severely degraded soils in East Africa, where literally "dirt-poor" people are entirely dependent on whatever food aid they receive, I have developed a new respect for the brown stuff.
Even if we could produce enough food without soils – and I have seen some great vegetable gardens growing on floating lily beds in Bangladesh, for example – it is doubtful we could replace as cheaply the many ecosystem functions that soils provide, from water management to pollution, remediation to supporting structures, and from trees to houses.
The biggest problem is that it takes such a long time for soil to form – some 10-12,000 years to build up to depths we might describe as productive land. First, the rocks that have made it from the Earth's interior to the surface must be "weathered" by wind and rain, a disintegration that is assisted by microorganisms, insects and lichen. This organic matter decays, feeding more organisms, including, in time, plants. It is the accumulation of hundreds of years of this organic matter, living organisms and minerals that we call soil. It takes a few hundred years to produce each centimetre of soil (although it is a little faster in the tropics), but it can be lost in a matter of hours.
One of the culprits contributing to loss of farming land is "sealing" and it is likely that the rooms you and I are in now are at least partly to blame. Whenever a new road or building is constructed, soil is effectively sealed underneath it. Because most cities and towns around the world have grown from small settlements where hundreds of years ago people discovered the most fertile land, vast urban centres are now effectively squatting on some of our best food sources. By developing on so-called brownfield sites of contaminated land and building upward rather than out into fields, some of this sealing can be limited. In many cities, urban agriculture is making use of some of this blocked-in soil.
If we are not sealing the soil down with concrete, we are allowing the wind and rain to wash it into the oceans. Soil erosion is such a serious problem that some scientists believe European soil could last less than a century. And Europe is one of the better places. In China, soil is being lost 57 times faster than it can be replenished naturally, while in Europe it is 17 times, in America 10 times and in Australia five times, according to a team at the University of Sydney led by John Crawford.