Mollisols

Mollisols are a soil order in the USDA Soil Taxonomy characterized by a thick, dark, organic-rich surface horizon known as the mollic horizon. They form under grassland vegetation in temperate to semiarid climates and are among the most productive agricultural soils due to their high organic matter content and favorable structure. They are widely distributed in grassland regions of North America, Eurasia, South America, and parts of Africa and China.

A defining feature of mollisols is the mollic horizon: a substantial, mineral surface horizon that is dark in color and high in base saturation, typically over 50 percent. The horizon is usually thick (often tens of centimeters) and has low acidity, with pH near neutral. In mollisols, there is little evidence of illuvial clay accumulation, so they generally lack a well-developed argic horizon. The organic matter in the mollic horizon contributes to strong aggregation, good tilth, and high water-holding capacity.

Formation and climate play key roles in mollisol development. They commonly form under long-rooting grasses in climates with moderate to low leaching, where litter from grasses accumulates faster than it decomposes. Parent materials are often quarried by loess, alluvium, or other aeolian deposits, and agricultural practices can influence surface conditions and fertility.

Management and use are central to the value of mollisols. Their inherent fertility supports cereals and other crops, but they are susceptible to erosion if left bare and can experience nutrient imbalances or salinization in arid regions. Sustainable practices—such as residue retention, cropping systems, and appropriate lime or nutrient management—help maintain their productivity.