On the black land, under the blue sky

The picture shows fallow vertisols in an experimental farm near Seville. Vertisols are soils locally common in large flat areas from southern Spain, characterized by the presence of smectites, swelling clays that contract during the dry season, which induces the appearance of visible cracks on the surface, and expand during the wet season, when water molecules enter the interlaminar space. Clay content is greater than 30%, and the soil surface has small mounds, forming a characteristic micro-relief known as "gilgai" (usually not observed if ploughed). Vertisols are known in Spain as "tierras negras" (black land).