Unit 4 - Soil Forming Factors

Chapter 4 - Time as a Soil Forming Factor

The age of a soil is generally considered to be the length of time in years since the land surface became relatively stable, thus enabling soil development to proceed. The length of time required for a soil to form depends on the intensity of the other active soil forming factors of climate and organisms, and how topography and parent material modify their affect.

A given period of time may produce change in one soil and the same time period will have little affect on another soil, due to the other 4 soil forming factors. In Minnesota the soils are between 8 and 10,000 years old, which was when the last glacial ice sheet left the state. Thus the soils are relatively young compared to landscapes that were not glaciated in other parts of the country. The soils in the North Dakota Badlands are old, but the dry climate has limited the amount of weatheringND Badlands

A peneplain is an area of nearly flat, featureless land formed by a long period of erosion. It is used in geomorphology to describe level land surfaces.

In general the age of a soil is not considered in years but in how much development the soil has undergone. Thus young soils have minimal soil development and few horizons while old soils have well developed horizons. Soil forming factors that hasten the rate of soil development are:

Conditions that are prone to retard soil development are:

How many years does it take for a soil to form? Soil scientists have measured the age of many soils and have a wide range of rates of soil formation. A one-meter thick soil developed from loess in southern Wisconsin in 8000 years, while a one-meter thick soil in tropical Africa took 75,000 years to form. An average global soil formation rate has been calculated to be 178 years per cm. Note how this rate compares to the information from Cruse at Iowa State which was 0.003 in / year. Comparison Calculation. A genearl rate of soil formation and time is found in this diagram Time vs Soil Formation

Often it is easier to think about soil formation in terms of how rapid specific horizons develop. In general it takes less time for surface horizons to develop than subsoil horizons. A soil is said to have reached a "steady state" when its individual horizons and properties change little over long periods of time. Often this rate of change is too slow to be seen in our lifetime.

A way to study soils is to look at a chronosequence of soils, or a grouping of soils that have the same soil forming factors except for the time of development. A study in Michigan looked at this kind of sequence (Franzmeier, D.P. and Whiteside, E.P. 1963: A. chronosequence of podsols in northern. Michigan.). Their objective was to determine what changes take place in the soil with time. Here is a look at the age of the landscapes where the soils were located. This area is adjacent to Lake Michigan in northeastern Michigan. As the lake retreated from the land after the last glaciation, the soils farther from the lake had more time to develop and are progressively older the farther from the lake.

chronosequence MI
The sequence of soils to match the age of the lake terraces is the following:

EntisolSpodosolSpodosolAlfisol

With time the sandy parent material developed a thin, dark surface horizon, and a zone of leached soil (white layer), the leached zone thickens with time and the zone of accumulation below this zone begins to develop a redder color and eventually becomes dark red in 10,000 years as both iron and humus accumulate.

In General the following happens with time for a soil:

In the USA the oldest soils occur on the terraces and alluvial fans of the Sierra Nevada Mountains in California and the desert soils of Arizona and New Mexico. In these areas silica or calcium carbonate may accumulate in the soil to the extent that the soil becomes cemented or indurated into a hardpan. This hardpan can not be broken with hand tools and if the upper surface erodes away the cycle of soil development begins again. Soil horizons that are cemented either by silica or calcium carbonate are indicated by the letter 'm". The Californian soil is estimated to be 750,000 years old.

 Durixeralf Petroargid

The oldest soils in the world occur in Australia and Africa where the landscapes have been stable for million of years. See Block Diagram for Age factors Soil Forming Factors

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