Deschutes & Ochoco National Forests
Crooked River National Grassland

Soils

General Soils Information

Soils are a combination of sand, silt and clay particles which comprise the growing medium in which forest vegetation, biological organisms and nutrients complete their life cycles. The source material from which a soil is derived is called the parent material and may be underlying bedrock or material transported and deposited by environmental agents such as water, ice, wind, or gravity. Soils developed in place from underlying bedrock are considered to be residual, while those formed from water deposited sediments in lake bottoms, stream flood plains or oceans are called lacustrine, alluvial and marine soils, respectively. Soils developed from material deposited from receding glaciers are called glacial till or outwash, while those derived of sediments transported and deposited by wind are called loess. Sediment can also be transported by gravity, which forms soils on unconsolidated material called colluvium.

Soil formation, a process called weathering, occurs from the breakdown of the parent materials from which they are derived. Climatic factors such as air and water continue to change the composition and mineralogy of rock fragments and sediments through physical and chemical processes. Plants and animals contribute additional nutrients and chemicals from a variety of metabolic pathways that influence the rate of the weathering process and the characteristics of the weathering byproducts. As soils weather, they develop distinct physical and chemical properties that determine the type and amount of vegetative species that inhabit the landscape naturally. Physical properties of a soil include texture, which is determined by the percent composition of mineral sand, silt and clay within a given soil, and structure, which is determined by the physical distribution and conglomeration of these mineral particles. Chemical properties of a soil include pH and nutrient levels such as nitrogen, phosphorous and sulfur. All of these characteristics influence the interaction of plant roots, soil organisms, air, water and nutrients in solution with each other, in turn favoring certain types of vegetative growth in each soil.