Cochise College        Student Papers in Geology

Geology Home Page              physical geology  historical geology  planetary  gems

Roger Weller, geology instructor

Marti Stoner
Physical Geology
Spring 2006

   Soils are one of our most important natural resources.  Soil is a thin layer of material on the Earth's surface. Soil is a porous natural material made up of many materials including rocks, weathered minerals, and dead decaying plants and animals.  The ratio of these materials vary from place to place depending on how it was formed. Soils are very old -- as old as 200,000 years.  Some soil profiles are formed in as little as 2000 years. There are 5 soil forming factors; parent material, climate, organisms, location, and time.  There are four main mineral types found in soil; sand, silt, clay, and loam.    

Soil Formation

The formation of soils is a process that takes a very long time creating a thin layer of soil. Physical and chemical weathering cause the rocks on the surface of the Earth to break down into smaller pieces.

            Soil and topsoil are produced at about an average of 1 ton/year, and lost at a rate of 10-40 times faster than it is produced. Worldwide there is a total of 1200 million hectares of agricultural land of which we are losing at the rate 6 million hectares per year.

                                  soil formation


            The soil is formed from a "parent material".  This material could be several different types; bedrock, organic material, old soil surface, or a deposit from water, wind, glaciers, volcanoes, or material moving down a slope.   

The climate is another factor that helps in forming soil.  Environmental forces such as sunshine, heat, rain, ice, snow, wind, weather and breakdown the material, and influence how fast or slow the process of soil formation takes.  When weathering takes place, four components are released from the rock; minerals in solution (the basis of plant nutrition), oxides of iron and alumina, various forms of silica, and stable wastes as very fine silt (mostly fine quartz) and coarser quartz (sand). These have no nutritious value for plants.  If the climate is favorable, the oxides of iron and alumina combine with silica to form clay, a new mineral.

Decaying plants and animals add organic matter. In addition to adding organic matter to the topsoil, plants add hydrogen helping the water percolate through, moving some parts of the minerals in the soil vertically. Microorganisms (bacteria, fungi, algae, protozoa and more) help breakdown plant matter producing carbon dioxide.  Carbon dioxide mixed with moisture in the soil will dissolve mineral materials.  Some bacteria also extract nitrogen from the air, to assist in plant growth.  Insects, earthworms roundworms, millipedes, centipedes help decompose plant material by feeding upon it and loosening the soil by burrowing through it.  Mice, squirrels, hares, caribou, moose, bison, camels, horses etc all contribute to the enrichment of the soil also.

The location of where the soil is can determines how the climate affects the breakdown of the parent material.  The drier soils are the ones that are facing the sun on slopes. At the bottom of a hill the soil is moister than the soils on the slopes.

Soil Profile

Every soil profile is made up of layers called soil horizons. The main horizons are A-horizon, B horizon, and C-horizon. Horizon A is the upper layer of soil at the surface of the earth's crust, more commonly known as topsoil. On top of the A-horizon is the O horizon made mostly of organic matter from the vegetation, which helps prevent erosion, and adds nutrients to the soil once it is broken down.



The A-horizon is made of mineral matter mixed with some humified (decomposed) organic matter. You will find most plant roots and soil organisms in the A-horizon. It has very little nutrients because the nutrients have been used by plants or leached downward into the B Horizon.

            B-horizon, also called the zone of accumulation because it is the area where nutrients are leached from above, and new material from below accumulate.  In the B-horizon concentrations of clay, iron or organic matter can be found. Some lime may be present also.

            C-horizon is found below the B-horizon and is mainly made up of big rocks (parent material) that have been through some weathering process as it is breaking down to form new soil.

Soil Types

Soils are porous bodies that retain water and are made of a silicate group of minerals called silicate clays, mainly consisting of the chemical elements oxygen, silicon, aluminum, and iron.  Potassium, calcium, magnesium, sodium, carbonates, oxides, phosphates and sulfates are other minerals that are in the soils.

There are four main mineral types found in soil: sand, silt, clay, and loam. The large pieces in the soil are sand.  It feels rough when you rub it because it has sharp edges, and it doesn't have many nutrients in it. In between the sand and clay particle size there is silt. Silt feels smooth and powdery and it has a smooth non-sticky feel when it is wet.  The smallest particle in soil is known as clay. Clay is smooth when dry and unlike silt, it is sticky when wet. Soils high in clay content are called heavy soils. Clay is not very porous but it can have a lot nutrients.  Particle size has a lot to do with a soil's drainage and nutrient holding.



The colors of soils are caused by what elements the soil is made of.   Darker soils have a high moisture and organic content.  Surface soils are usually darker than subsoils.  Red, yellow and gray colors of subsoils tell us about the oxidation and moisture states or iron oxide content of the soil.  Good drainage can be found in soils that have red and yellow hues.  Soils that have very little aeration are usually grey hued. 

Types of soil vary around the world depending on different conditions of temperature and humidity.  Tundra soil is mostly made up of organic matter, it is boggy and unable to thaw completely, and has a short growing season.

Grey podsols soils are in areas made mostly of clay from the B-horizon with a sandy A-horizon because the soils are acidified by the resin from the trees, causing the nutrients and clays to be leached downward. The growing season is during cold summers.

Deeper soils (grey/brown) are formed where evaporation is basically the same as the rainfall. Deciduous forests hibernate in the winter and have a long growing season during the summer months.

Fertile black chernozem soils, rich in humus, are formed in places here the water table is deeper.  These dryer conditions promote deep-rooting grasses that produce enough acid to keep a layer of clay, which in turn does not allow the soil nutrients to be leached.        

Chestnut soils are formed in areas where the water table becomes deeper, and where rainfall is less, than the evaporation rate, causing soils to form that have very little clay, humus and organic material, and high concentrations of iron oxides, making it hard to grow plants.

You can find rigorous erosion and slow weathering in desert soils.  It is difficult to grow most plants in desert soils because they lack many nutrients.  Desert soil is easily disturbed by wind and occasional rain. These are the areas where you can find gypsum in the horizons.

Life on Earth would not be possible without healthy vital soils because they are necessary for plant food and the formation of the oxygen we breathe. The way humans make use of soils has a huge impact on soil formation. The quality of the soil ecosystem is important to growing plants and the formation of more soil.


URL Addresses