Mount Rushmore National Memorial - Soils

Mount Rushmore is located in the higher elevation of the central Black Hills. This area is where granite outcroppings form the high peaks of the Black Hills. Below the high granite domes ponderosa pines dominate the landscape. The appearance of the landscape is influenced by many factors including the soil. The high elevation areas have little or no soil and therefore have little vegetation. Where the Ponderosa pines grow, there is moderately deep soil. This is enough for the Ponderosas to take root and sink in a taproot to obtain moisture when there is little precipitation. Soil influences the plants that grow in a particular area. However, Ponderosa pines have a great effect on the soil too.

Soils vary based on attributes of the parent material or bedrock, elevation and climate. The parent material for soils in the central Black Hills is mostly granite or mica schist. The granite is very hard and slow to break down. The mica schist is a metamorphic rock that breaksdown more readily. The mica schist develops into a well-drained soil. Except where parent materials are of recent igneous origin, deep zones of fractured bedrock usually underlie soils. Joints and fissures in this rock admit and store soil water that has percolated down through, and they are often penetrated deeply by roots-particularly those of ponderosa pine. A well drained, moderately deep soil is ideal for Ponderosa seedlings to start. The parent material first influenced the soil condition but climate and trees also influence the soil. The climate of the Black Hills Ponderosa pine forest is cold and dry. The cold, dry climate means that dead plant material, or detritus, is slow to break down and return nutrients to the soil. Pine needles with their thick, waxy coat form a thick mat on the surface of the soil. When rain and snow melt percolate through this layer of detritus it forms humic acid. Humus and minerals, particularly aluminum and iron are transported from the top, or A horizon, into the B-horizon. Humus is the dark colored organic portion of the soil, between the detritus and upper soil horizon. The results is an acidic, gray colored soil. The acidic soil is inhospitable and unfavorable for other understory plants to start in. Young Ponderosa seedlings are more tolerate of the soil and able to out compete other plant life.

Growing in the acidic soil is a vast fungal network of branching hyphae. Hyphae are hairlike filaments. The hyphae release enzymes that breakdown detritus and return some nutrients to the soil. In the dry ponderosa forest soils fungi parasitize the tree roots to increase their nutrient intake. The trees in return receive additional moisture from the extended "reach" of the fungi"s hyphae. This is a symbiotic relationship where both members of the relationship benefit.

The acidic soils of the Black Hills prove to be an ideal foundation for the plant and animal communities that live in and above its surface.