Today prairie preserves are managed with controlled burns. Their roots may extend to depths of 9 feet or more, binding the soil and enriching it with organic matter as roots die back at the end of the growing season. The above-ground parts of the plants also die down in winter and are converted to abundant humus when bacterial action begins in the spring. Three herb layers are apparent in the tall grass prairie, each characterized by relatively high species diversity. Both sod-forming and bunch grasses are present.
Bluestems comprise the uppermost herb layer.
Other, shorter upright grasses and forbs form an intermediate layer. It is this formation that is associated with the darkest of the mollisols, the chernozems. Much of it today has been converted to corn. The mixed prairie. Centrally positioned between the tall grass and short grass prairies, the mixed prairie is intermediate.