(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 ...

This soil from Iowa is dark due to high organic matter content; combined with high plant-available, water-holding capacity due to a high silt concentration. Soils with these characteristics are ...