Soil forms as a result of physical, chemical, and biological processes of nature involved in complex interaction with the factors of parent material, topography, climate, living organisms, and time.

Soil Forming Factors

Parent Material

The parent material is the material underlying the soil from which, in most cases, the soil develops. It influences both the physical and chemical properties of the soil.

Parent material can come from many different sources. Most of the mineral matter that makes up soil comes from weathered bedrock or from other inorganic materials transported and deposited by wind, water, or glaciers. But soil can also form from sedimentation of organic material, such as peat, in bogs and marshes. These sources result in the two broad groups of soils –mineral soils and organic soils.

Topography

Topography, or relief, relates to the shape and slope of the land, as well as the elevation and landscape position. Topography can slow or hasten weathering by climatic forces. Soils vary with topography because of its influence on soil moisture, rate of weathering and erosion, and soil temperature.

Climate

Climate, the temperature, and rainfall affect the rate of weathering. Weathering is the breakdown or disintegration of rock at or near earth’s surface, by natural processes. Temperature and water are major climatic forces that influence weathering. Weathering may be by physical forces (mechanical) or by chemical processes.

Living Organisms

Plants, animals and other organisms help create and enrich soil. The excrement (waste) from animals adds nutrients and matter to the soil. Burrowing invertebrates, such as earthworms, beetles, ants, and termites, create macropores that allow easier entry and flow of water and air through the soil. They also mix organic matter throughout the soil. When animals or plants die or plants shed leaves, they are broken down by the decomposers, fungi and bacteria, and add organic matter and nutrients to the weathered parent material. The recycling of nutrients is essential to plant growth, which is essential to supplying food and nutrients to other organisms. The organic matter of the soil is largely derived from decomposed plant and animal material (dead bodies, litter, and wastes). Organic matter also includes the billions of organisms that inhabit the soil. While organic matter accounts for only five percent or less of the total volume of soil, it is an important component and important for good soil management. The soil organic matter is also used as a food source by soil organisms from which they receive the energy needed for life.

Time

The length of time that a soil’s parent material is exposed to mechanical and chemical weathering and the other soil forming factors is also a factor and greatly influences the kinds of soils present today. A typical soil’s age must be measured in thousands of years. It may actually take hundreds of years for these factor to form one inch of soil from parent material.

Soil Formation Processes

The soil formation processes are continuous, and over time, soils exhibit features that reflect the other soil forming factors. Recently deposited material, such as the deposition from a flood, exhibits no features from soil development activities. The previous soil surface and underlying horizons become buried. The time clock resets for these soils.  Older soils usually have deeper well-developed soil profiles while young soils still retain many characteristics of their parent material. Older soils are often less fertile than young soils, due to a loss of nutrients by leaching.

The soil forming factors continue to affect soils even on “stable” landscapes. Materials are deposited on their surface and materials are blown or washed away from the surface. Additions, removals, and alterations are slow or rapid, depending on climate, landscape position, and biological activity.