Europeans began increasingly to move from villages to cities around 200 years ago with the start of industrialization. They expected the move would bring about an improvement in their quality of life and an end to poverty. The same hopes continue to draw people to cities today, particularly in developing countries, where urbanization began later but quickly acquired greater intensity.

Currently, around half of the world's population live in cities. However, urbanization has had disastrous consequences for soil - and, in turn, for people.

1,000 square kilometers lost per year in Europe

"When urbanization sets in, around 50 percent of the ground is covered with an impenetrable layer, such as concrete or asphalt," said Klaus Lorenz of the Institute for Advanced Sustainability Studies. Each year, Europe loses around 1,000 square kilometers (386 square miles) of soil.

The loss is irreversible. Once infrastructure like concrete or asphalt has been established, it's a hugely labor-intensive task to remove it again, says Lorenz, adding that what's at stake is not just the quantity but also the quality of the soil.

"The problem is that cities are built on fertile land. Those are the areas where people settled, because that's where they could produce food for the inhabitants," Lorenz said.

Building up rather than building out is not an ideal concept for many cities. In the megalopolis Mexico City, for example, Lorenz points out that there's a risk of earthquakes. He believes that one way to address the problem is to use the available surfaces in cities more efficiently. That can include roof-top and other urban farming strategies in which food is grown in the middle of cities.

"However, you have to be very careful about the quality of the soil and whether it contains contaminants," the biologist notes.

Hazardous waste

Soil acidification has been a topic of discussion for some time in Germany, but it's a problem in other parts of the world too, including China.

Guangzhou, China's third largest city, has expanded to encompass a metropolitan area of around 7,000 square kilometers. Industrialization has proceeded there at an aggressive pace. And now attempts are being made to clean hazardous waste from industrial sites that are no longer in use. This is very difficult, though, Lorenz says, because there are no guidelines there for purifying the soil.

Brazil faces similar issues. For agricultural scientist Clistenes Nascimento of the Federal University of Pernambuco, soil pollution represents one of the major problems associated with urbanization in Brazil. Another is erosion. Depleted and dried-out soil is often washed away by rain or floodwater. In 2011, hundreds of people died near Rio de Janeiro during one such landslide.

A forgotten problem

Nascimento believes that many cities in Brazil lack a plan for urbanization, and that a new concept is needed to look at how people occupy and use their land.

His hometown of Recife, whose population has doubled since the 1970s, faces a number of problems related to urbanization. The city lies along the coast, where most Brazilians live. Nascimento argues that incentives need to be created to encourage people to distribute themselves throughout the country, easing up the population pressure on cities.

However, the scientist complains that people in Brazil fail to realize that "the ground is itself a finite resource," as their country covers such a large area.

Klaus Lorenz agrees on the need for action, appealing to consumers be more aware of how they are using resources. "We are simply too far away from the products on our plates, or sitting on our tables, to make the connection with the fact that soil was necessary to produce them," he explains. "We forget that the earth cannot be manufactured. Natural, rich soil: Once it's gone, it's gone."