Here's why:

* Mollisols are characterized by a thick, dark, fertile A horizon (topsoil) rich in organic matter. This is due to the accumulation of decaying grasses over thousands of years.

* Prairies are grasslands with deep roots that bring nutrients to the surface. This contributes to the rich topsoil.

* Other soil characteristics of Mollisols:

* They have a high base saturation (meaning they are rich in calcium, magnesium, and potassium).

* They have a good water-holding capacity.

* They are well-drained, but not excessively so.

While Mollisols are the dominant soil type in North American prairies, there can be variations depending on specific location and climate. For instance, you might find some Aridisols in drier areas or Alfisols in areas with more rainfall.