Importance & Formation of Soil

Importance of Soil

• Much of the United States’ success is due to the productive capacity of the soil– U. S. produces enough food and fiber for its own use

and exports these commodities to other countries

Importance of Soil

• Habitat for soil organisms• Engineering medium• System for water supply and purification • Original source of most nutrients for plant and

animal life– Provides nutrients for plant growth– Animals get nutrients from plants or from animals

that eat plants

Soil Acts as a Recycling System

Soil Permanence

• Soils appear permanent and unchanging• Actually very dynamic, constantly changing• Changes must balance to maintain

productivity

Soil Formation

• Based on weathering of rocks and minerals• Physical and chemical decomposition break down

rocks and minerals into the mineral matter component of soil

• On average, it takes about 100 years to develop one inch of topsoil

Factors Affecting Soil Formation

• Climate• Living organisms• Parent soil material• Topography• Time

Different combinations of these factors lead to the formation of different soils

Climate

• Refers to the average course of weather at a particular place as exhibited by temperature, wind velocity, and precipitation

• Temperature and precipitation greatly affect the rate of weathering

• Increased temperature causes an increased rate of chemical reaction and increased growth of fungi, soil bacteria and plants

Climate

• Adequate moisture causes an increased rate of chemical reaction and increased growth of fungi, soil bacteria and plants

• Excess moisture retards microbe growth• Water carries minerals and organic matter

downward, forming horizons• High rainfall areas generally have leached,

acidic soils

Average Annual PrecipitationUnited States

Climate

• Low humidity and high winds tend to dry out soils, reducing chemical reaction and microbe growth

Photo courtesy of USDA NRCS

• High winds can cause cause significant erosion on some unprotected soils

Living Organisms

• Bacteria and fungi – break down organic materials

• Insects, “insect-like” creatures, and animals – break down and mix soil materials

• Plants– roots break up mineral matter

• Humans– land clearing and cultivation remove vegetation

and disturb soil structure

Parent Material

• Influences texture, structure,color, and fertility• Clay soils generally form from easily

weathered materials• Sandy soils form from more resistant materials

Topography

• On sloping land, loose material is moved downward by runoff and gravity– breaks up soil materials and deposits them at

lower levels– exposes subsoil materials on upper slopes

• Influences percolation and drainage– sloping land has more runoff and less percolation– low-lying soil may tend to be saturated, leading

to less biologic and chemical activity

Time

• Soils form by chemical and physical weathering of parent material over time– chemical weathering results from chemical

reactions upon soil materials– physical weathering results from mechanical

forces such as freezing and thawing, erosion, and gravitational movements

• Soils age and mature over time

Soil Maturity

• Initially, nutrients are released quickly, plant growth increases, and organic matter accumulates

• Eventually, soluble nutrients are lost through leaching, plant growth decreases, and organic matter is lost

• Fine clay particles move downward from surface• Mature soils gradually decline to lower levels of

production

Variety in Soils

Climate, living organisms, parent material, topography, and time impact soil formation and changes in soil composition.Variations in these factors have resulted in the wide variety of soils that are present today.

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Instructional Materials ServiceTexas A&M University

2588 TAMUSCollege Station, Texas 77843-2588

http://www-ims.tamu.edu 2006