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.

Trapped soil

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.

Agriculture is the main culprit. A raft of practices – from deforestation for land clearance to the types of intensive farming that exhausts nutrients or make the soil too salty for crops – contribute to depleted soils. Farmers are then forced to seek new soil, often by invading forests. If we want to preserve the few remaining forests and other diverse ecosystems around the world, we are going to have to better manage our existing agricultural land.

Deforestation is a major driver of soil loss. Once shrubs and grasses have been cleared from the land, there are no roots to hold soil, nutrients and water in place. A dry spell can turn the ground to dust that is easily blown away, as happened with disastrous consequences in America in the "Dust Bowl" 1930s. This also frequently happens in China, Australia and parts of Africa, where large areas of agricultural land are becoming deserts.

Great green wall

Farmers are fighting back by planting trees and shrubs that help keep the soils moist, buffer the winds and slow rainwater. West African countries, for example, have already achieved remarkable improvements in soil fertility as a result of tree-planting programmes, and there is now an ambitious plan for a 'Great Green Wall of Africa' to cross the continent from Djibouti to Senegal in a tree barrier against the encroaching deserts.

In Indonesia, I found farmers planting vetiver – an Indian grass – to protect their soils from erosion, because its roots grow down deep and hold it in place. Solutions like this are liked because they offer multiple benefits – vetiver reduces weed and pest incursion, produces marketable oil and is also useful for animal feed.

Essential minerals, such as nitrates, that crops suck up from the soil take a particularly long time to be replaced naturally. Although nitrogen is very common in the air, only a few organisms are able to break the tough chemical bond in the gas to create the nitrate form that plants and animals use to develop proteins.

Animal faeces contain many of these chemicals, and recycling essential nitrates from biomatter is the easiest and cheapest way of replenishing the soil. In many places in the world, livestock dung is the only fertilizer available, but it is becoming scarcer because, in trying to preserve the few trees and shrubs around cropland, farmers are burning dung for cooking.

One solution to this problem, which I saw on a farm in India and which is now being trialled in Cameroon, is to use the dung to feed a biodigester – a tank in which bacteria break down the waste. The methane gas produced by the bacteria is then used to fuel cooking stoves, and the decomposed manure can be spread on the fields as fertiliser.

Another way of getting nitrates into the soil is to introduce the few organisms that can create it for you – so-called 'nitrogen-fixers', which includes plants like legumes that contain in their root systems bacteria that can make nitrates from the air. At the Tropical Soil Biology and Fertility Institute in Nairobi, I met soil scientist Peter Okoth, who has developed different types of promiscuous soybean – a nutritious nitrogen fixer – that can be intercropped with cereals and can concentrate an impressive 265 lb (120kg) of nitrate per hectare of soil. Okoth says his soybean variants are, "more effective than industrial fertilisers", which are not only expensive but are also made from oil, meaning they produce greenhouse gas emissions during production.

Give peas a chance

A growing body of scientists is arguing that the common pastoral scene of a man in a hat driving a plough is causing some of the biggest damage to our soils. By churning up the soil, farmers break down its important structure and integrity, reduce its fertility by exposing the minerals to oxygen, release quantities of carbon dioxide and remove the nutrients in the leftover crop stalks.

Instead, they say, farmers should switch to no-tillage agriculture, in which once the grain is harvested, the new seeds are simply sown into holes among the leftover plant matter. Leaving the old crop stalks and roots in place holds retains the soil and its moisture, reducing erosion. The rotting vegetation recycles nutrients and attracts microorganisms, worms and insects, which help maintain soil fertility and provide ecosystem services. The practice is gaining popularity in Brazil and across the Americas, but getting the world's poorest farmers to adopt it will take time, partly because of the prohibitive cost of equipment.

I visited Keith Ashby, a farmer who has been using no-till agriculture for the past six years on his wheat fields in Kent, southeast England. "I get a better yield than when I ploughed, and it's slashed my fuel costs," he said. "Instead of having to drive a tractor up and down the field several times, I only have to do it once to drill and sow." Considering that it costs him 9.9 gallons (45 litres) of fuel an hour for 14-hour days over 2 weeks, while his neighbour who still ploughs uses 19.8 gallons (90 litres) an hour for substantially longer, he is making important cuts in costs and carbon emissions.

"We use the sheep to provide manure for the fields, which halves our fertiliser use," he explained. In the next field, Ashby was "resting" his wheat field by growing turnips for the sheep, and nitrogen-fixing peas and beans to help improve his soil.

Back in my office, I reflect that there is more mud on my boots than on parts of the chalk-exposed Kent Downs, which just a few hundred years ago were as lush and fertile as Ashby's fields. We may have spent the past millennia getting rid of valuable soils, but we are now learning ways of retaining it, from the big transcontinental projects to small-scale grass-planting. I have mentioned some of the methods I have discovered on my travels: let me know on our Facebook page of any methods you have found useful.

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