Soil footprint: a simple indicator of a crop's impact on soil erosion

Agriculture faces a challenge on which the future of the planet depends, to a large extent: that of feeding a growing population through the sustainable use of natural resources, essential for producing food, but also for life on Earth. In this context, concepts such as the 'carbon footprint' and 'water footprint' arise, which refer to the amount of these resources that are invested in the production of a given good, food or service. These concepts are about the cost or impact that producing certain product has on nature, and a way of measuring whether or not their consumption is sustainable. Another concept may be added to these, this one having to do with another limited resource about which there is less awareness, but which is as essential for life as water or energy: soil, the erosion of which means lost nutrients, biodiversity and water retention capacity.

The 'soil footprint' of a food is the amount of soil that is lost during its cultivation process, and it is calculated by dividing the erosion rate by its degree of productivity. This is a term present in the 'A Soil Pact for Europe' community strategy, which has now been formally defined and applied in Spain by researchers Andrés Peñuela, Vanesa García Gamero and Tom Vanwalleghem, with the Hydrology and Agricultural Hydraulics group of the María de Maeztu Unit of Excellence -- Department of Agronomy at the University of Córdoba (DAUCO). The objective, they explain, is to simplify the communication of the serious problem of soil erosion in order to involve consumers, adding them to the action network currently comprised of the scientific community, political and regulatory bodies, and the agricultural sector.

The research team's approach is as follows: if concepts such as 'water footprint' have managed to permeate society, generating awareness of the origin of food and its costs, and promoting changes in consumption habits, introducing the 'soil footprint' could contribute to the transition towards more sustainable production models that are also responsible towards this resource. It is a task in which all the agents involved must do their part: the scientific community, investigating the causes of erosion and proposing solutions; political powers, with regulations that protect the soil; and the agricultural sector, adopting responsible techniques that optimize the use of this resource. The last party that can make a difference is consumers, through their power to influence a market system. The demand for foods that do not erode soil can encourage the productive sector to adopt sustainable production models, as has already happened with food bearing ecological seals, and cruelty-free products.

Olive groves, the crop with the greatest 'soil footprint'

In addition to defining the concept and laying the foundations to measure it, the team calculated the 'soil footprint' of Spain's ten main crops, also analyzing the areas of the Peninsula where they are most problematic. The results show that the crop with the biggest soil footprint (that is, the least food production in proportion to the erosion it generates) is olive trees, followed by cherry trees, and wheat. At the opposite end are onions, potatoes and oranges, these being the crops with the smallest 'soil footprint' of all those analyzed.

The fact that the crop most harmful to Spanish soil is also one of its great economic engines, and a hallmark for the country, could be problematic, but the researchers clarify that erosion does not depend exclusively on the peculiarities of the crop, but on climatic conditions, topography and agricultural management. "The solution is not to reduce the number of olive trees, but rather to incorporate strategies that protect the soil from erosion, such as vegetation cover," said researcher Andrés Peñuela. Several studies have shown how this management is able to significantly curb soil erosion in Mediterranean olive groves. This would contribute to reducing this crop's 'soil footprint,' thus protecting a limited resource whose future should concern society as a whole, like water and energy already do.