What is soil?

O = organic, A = surface soil, B = subsoil, C = parent rock

Soil is commonly referred to as earth or dirt. Technically speaking, only displaced soil is properly called dirt.

The USDA Natural Resources Conservation Services describes soil as: (i) The unconsolidated mineral or organic material on the immediate surface of the Earth that serves as a natural medium for the growth of land plants. (ii) The unconsolidated mineral or organic matter on the surface of the Earth that has been subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time. A product-soil differs from the material from which it is derived in many physical, chemical, biological, and morphological properties and characteristics.

Wikipedia describes soil as a natural body made up of layers (soil horizons) that have different morphological, physical, chemical, and mineralogical characteristics than their parent materials. Soil is composed of particles of broken rock that have been altered by chemical and mechanical processes that include weathering and erosion. Soil differs from its parent rock due to interactions between the lithosphere, hydrosphere, atmosphere, and the biosphere. How is soil formed?

The three functions of soil

Soils are living systems vital in producing the food and fiber we need to sustain life. Soils also maintain the ecosystems on which all life ultimately depends. Soils affect global climate by functioning as a medium for plant growth, a partitioner of water flow, and an environmental buffer.

Soils make it possible for plants to grow

The biological, chemical, and physical processes that supply nutrients, water, and other elements to growing plants occur through soils. Microorganisms in soils transform nutrients into forms that can be used by growing plants. Soils are the water and nutrient storehouses on which plants draw when they need nutrients to produce roots, stems, and leaves. Eventually, these become food and fiber for human consumption. Soils — and the biological, chemical, and physical processes that they make possible — are a fundamental resource on which the productivities of agricultural and natural ecosystems depend.

Soils regulate and partition water flow through the environment

Rainfall in terrestrial ecosystems falls on the soil surface, infiltrating into the soil or moving across the soil surface into streams or lakes. The condition of the soil surface determines whether rainfall infiltrates or runs off. If it infiltrates the soil, it may be stored and later taken up by plants, move into groundwaters, or move laterally through the earth, appearing later in springs or seeps. This partitioning of rainfall between infiltration and runoff determines whether a storm results in a replenishing rain or a damaging flood. The movement of water through soils to streams, lakes, and groundwater is an essential component of recharge and base flow in the hydrological cycle.

Soils buffer environmental change

The biological, chemical, and physical processes that occur in soils buffer environmental changes in air quality, water quality, and global climate. The soil matrix is the major incubation chamber for the decomposition of organic wastes including pesticides, sewage, solid wastes, and a variety of other wastes. The accumulation of pesticide residues, heavy metals, pathogens, or other potentially toxic materials in the soil may affect the safety and quality of food produced on those soils. Depending on how they are managed, soils can be important sources or sinks for carbon dioxide and other gases that contribute to the greenhouse effect (greenhouse gases). Soils store, degrade, or immobilize nitrates, phosphorus, pesticides, and other substances that can become pollutants in air or water.

This “three functions” section was adapted from Soil and Water Quality: An Agenda for Agriculture, Committee on Long-Range Soil and Water Conservation, Board on Agriculture, National Academy Press, Washington, DC, 1993.

How invasive plants impact soils

Garlic Mustard (Alliaria petiolata)

Garlic Mustard (Alliaria petiolata)

Soils are the foundation for the productivity of land. They define the fertility of your land, and shape the vegetation that grows there. One inch of top soil may take 500 years or more to develop. Protecting this natural resource is critically important to preserving the productivity of your land.

Some invasive weed species alter the chemistry of soils and reduce their productivity. These plants release chemicals from their roots system or through a build up of plant material at the soil surface. These chemicals directly inhibit the growth and reproduction of other plants by altering the chemical composition of soils or by adversely affecting soil biota. This process is known as allelopathy, and serves to protect the weed from animals and from competition with other plants.

A species like Garlic mustard (Alliaria petiolata) is well known as an alellopathic competitor. Its roots release chemicals that alter soils microbes which can permanently diminish the productivity of a site. The diminished soils prevent tree seedlings from becoming established, and ultimately lead to soils that will no longer support our native or economically important vegetation.

Non-native invasive plants often provide little or no value to native creatures. By controlling invasives and restoring native plants where possible, you can help support the rich diversity of native life that makes Oregon such a special place.

More soil resources

Soil slideshow from Oregon Ag In The Classroom

Clackamas SWCD