The Origins of Soil
Soil Development
Soil forms through accumulation and decay of organic matter and through the physical and chemical weathering processes described above. The factors that affect the nature of soil and the rate of its formation include parent material, climate, topography, soil biota, and time. Parent material, climate, topography, soil biota, and time are referred to as factors of soil formation. The parent material from which soils form consists of unconsolidated and more or less chemically changed (weathered) mineral or accumulated organic particles.
Parent Material
Parent materials provide the main raw materials for soil formation. The initial stage of soil formation is the accumulation of the parent materials—the sediments or rocks in which the soils will form. It can consist of consolidated rocks, and it can also include unconsolidated deposits such as river alluvium, lake or marine sediments, glacial tills, loess (silt-sized, wind-deposited particles), volcanic ash, and organic matter (such as accumulations in swamps or bogs).
Climate
Climate is perhaps the most influential of the four factors acting on parent material because it determines the nature and intensity of the weathering that occurs over large geographic areas. Climatic conditions at a place depend on geographic location; elevation; velocity and direction of prevalent wind; position of land in relation to sea, ocean, and mountains; characteristics of oceanic current; etc. Thus, climate is a complex phenomenon and exhibit considerable variation within a particular geographic region. For example, there are wet, dry, and desert climates within the tropics.
Weathering Process Effect on Soil Properties
The weathering process that forms a soil also affects its properties. For example, less chemical weathering occurs in arid (low rainfall) regions than in humid (higher rainfall) regions. This results in the formation of less clay particles and nutrients in arid zones.
Time
Time is the last of the five soil forming factors to consider. However, this does not mean that it is not important. Time is just as important as each of the other soil forming factors. In geologic time, many of the soil-forming processes and landscapes that result from weathering are relatively temporary. Geologically speaking, landscapes are continually building and degrading throughout time. Soils have been referred to as young, mature, and old, depending on the degree of weathering. Young soils are usually easy to recognize because they have little or weak soil horizon development and the horizons commonly are indistinct.
Topography
The term topography refers to the configuration of the land’s surface. The processes that create topography are usually geologic in nature; e.g., tectonic uplift, fluvial erosion and deposition, mass wasting, volcanic activity, and glaciation. A landscape produced by these processes is the blank canvas upon which soil patterns are painted by processes that are linked to topography. As time passes, these processes leave characteristic pedogenic imprints on different parts of the landscape, altering the original parent materials and differentiating the physical, chemical, and biological nature of the soils by topographic position.
Slope and Aspect
Aspect and slope are two features of topography that affect soil formation.
Soil Biota
Soil biota, including plants, animals, and micro-organisms, contribute to soil formation by adding organic matter; transforming organic and mineral matter; protecting soil; and removing some substances during soil formation. Plants can have a strong influence on soil properties. Plant roots increase infiltration, break up dense soil layers, and can pull nutrients and moisture from deep within the soil profile. Vegetative cover reduces natural soil erosion rates, thereby slowing the rate of mineral surface soil removal.
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