The dirt beneath our feet is getting poorer and on many farms worldwide, there is less and less of it.Without sufficient soil, our ability to grow food is threatened.In Iowa they call it “black gold” – a fertile blanket covering the landlocked Midwestern state. Thousands of years of prairie grass growth, death and decomposition have left a thick layer of dark, organic matter on the vast plains. When European-American settlers first began ploughing in Iowa, they found the weather and local geology had combined this organic mulch with sand and silt to form a nutrient-rich type of soil called loam.

It gave Iowa one of the most fertile soils on the planet and enabled it to become one of the largest producers of corn, soybeans and oats in the United States over the last 160 or so years. But beneath the feet of Iowa’s farmers, a crisis is unfolding. The average topsoil depth in Iowa decreased from around 14-18 inches (35-45cm) at the start of the 20th Century to 6-8 inches (15-20cm) by its end. Relentless tilling and disturbance from farm vehicles have allowed wind and water to whisk away this priceless resource.

The same picture is seen on farms worldwide. Soils are becoming severely degraded due to a combination of intensive farming practices and natural processes. As the layer of fertile topsoil thins, it gets increasingly difficult to grow crops for food. Without altering agricultural practices and urgently finding ways to preserve soil, the global food supply starts to look precarious. Even in Iowa’s still-fertile fields, the loss of soil is concerning. In just one spring in 2014, Iowa lost nearly 14 million tonnes of soil from its cropland in a series of storms, according to environmental groups.

A study of 82 sites in 21 counties by Iowa State University showed that in the 50 years from 1959, soil structure and levels of organic matter had degraded while acidity had increased. “Erosion from the wind is not as bad as it used to be in the dust bowl era, but in the past 20 years the rainfall pattern has changed,” says Paula Ellis, a farmer in south-east Iowa’s Lee County. “We used to get one to two inches of rain every other week, but now we are getting bigger rain events where six inches fall and that hits the soil on farms.”