(a) Soil in central Iowa with dark, organic-matter-rich topsoil. Iowa's average topsoil depth decreased from 35-45 cm to 15-20 cm during the 20th century due to farming. (b) Severe gully erosion ...

In Iowa they call it “black gold” – a fertile blanket covering ... mulch with sand and silt to form a nutrient-rich type of soil called loam. It gave Iowa one of the most fertile soils ...