(WES) Soil Lesson

Soil

Soil is one of the three major natural resources, alongside air and water. It is one of the marvelous products of nature. If we had no soil we would have no life.

Soil is made up of three main components – minerals that come from rocks below or nearby, organic matter which is the remains of plants and animals that use the soil, and the living organisms that reside in the soil.

The proportion of each of these is important in determining the type of soil that is present. But other factors such as climate, vegetation, time, the surrounding terrain, and even human activities (eg. farming, grazing, gardening etc.), are also important in influencing how soil is formed and the types of soil that occur in a particular landscape.

Formation of Soil

Before soil can be formed, the landscape must first become covered with regolith. Over time, maybe taking millions of years, weathering on stone will break the rock down or the land may become covered by broken down pieces carried in by wind or water and deposited on the barren land. The dirt will resemble the rock from which it was broken down

Factors that Affect Soil

The factors that affect soil formation are climate, the type of rock, slope, time, and biological activity. Differences in these factors will produce different types of soil.

Climate

Climate is one of the most important factors affecting the formation of soil. Warmer temperatures and an abundance of water have a tendency to speed up the formation of soil, in some cases rather dramatically. Whereas cooler temperatures and less precipitation slow down soil formation. As a result, warm, humid locations tend to have a richer and deeper soil than do cooler more arid landscapes. Even the same rock type in different climates will not produce the same type of soil. This is true because most rocks on Earth are made of the same eight elements and when the rock breaks down to become soil, those elements dominate.

The same factors that lead to increased weathering also lead to greater soil formation.

• • More rain equals more chemical reactions to weather minerals and rocks. Those reactions are most efficient in the top layers of the soil, where the water is fresh and has not yet reacted with other materials.
  • Increased rainfall increases the amount of rock that is dissolved as well as the amount of material that is carried away by moving water. As materials are carried away, new surfaces are exposed, which also increases the rate of weathering.
  • Increased temperature increases the rate of chemical reactions, which also increases soil formation.
  • In warmer regions, plants and bacteria grow faster, which helps to weather material and produce soils. In tropical regions, where temperature and precipitation are consistently high, thick soils form. Arid regions have thin soils.

Soil type also influences the type of vegetation that can grow in the region. We can identify climate types by the types of plants that grow there.

Types of Rock

The original rock is the source of the inorganic portion of the soil. The minerals that are present in the rock determine the composition of the material that is available to make soil. Soils may form in place or from material that has been moved.

Types of Soil

Residual Soil	Transported Soil
Residual soils form in place. The underlying rock breaks down to form the layers of soil that reside above it. Only about one-third of the soils in the United States are residual.	Transported soils have been transported in from somewhere else. Sediments can be transported into an area by glaciers, wind, water, or gravity. Soils form from the loose particles that have been transported to a new location and deposited.

Slope/Topography

The steeper the slope, the less likely material will be able to stay in place to form soil. Material on a steep slope is likely to go downhill. Materials will accumulate and soil will form where land areas are flat or gently undulating. In areas which are flat, the soil typically tends to get deeper quicker than the surface erodes away. On steep slopes, erosion usually takes place quicker than the formation of new soil beneath.

The result is that flat locations typically have a deeper, more mature soil layer than do locations with a steep slope.

The topography will further determine how well the land will drain water. Water that does not drain but rather pools will lack oxygen and poor soil will develop. Soil must drain, allowing excess water to leave the area.

Time

Soils thicken as the amount of time available for weathering increases. The longer the amount of time that soil remains in a particular area, the greater the degree of alteration. Soil development takes a very long time. It may take hundreds or even thousands of years to form the fertile upper layer of soil. Soil scientists estimate that in the very best soil forming conditions, soil forms at a rate of about 1mm/year. In poor conditions, it may take thousands of years!

Biological Activity

The partial decay of plant material and animal remains produces the organic material and nutrients in soil. In soil, decomposing organisms breakdown the complex organic molecules of plant matter and animal remains to form simpler inorganic molecules that are soluble in water. Decomposing organisms also create organic acids that increase the rate of weathering and soil formation. Bacteria in the soil change atmospheric nitrogen into nitrates.

The decayed remains of plant and animal life are called humus, which is an extremely important part of the soil. Humus coats the mineral grains. It binds them together into clumps that then hold the soil together, creating its structure. Humus increases the soil's porosity and water-holding capacity and helps to buffer rapid changes in soil acidity. Humus also helps the soil to hold its nutrients, increasing its fertility. Fertile soils are rich in nitrogen, contain a high percentage of organic materials, and are usually black or dark brown in color. Soils that are nitrogen poor and low in organic material might be gray or yellow or even red in color. Fertile soils are more easily cultivated. A rich soil that has a lot of humus in it is called loam.

The air in soil is different from the air we breathe. Oxygen is removed from the soil by living organisms leaving carbon dioxide behind. Slowly, the carbon dioxide will seep out of the soil and replenish the atmosphere's carbon dioxide.

Soil can form from the rocks below, or from rocks a very long distance away - perhaps being carried by wind or water. The glaciers of the last ice age acted as giant bulldozers pushing truly huge amounts of soil along as they grew and dropping the soil as they melted.

Soil Profile

There are different types of soil, each with its own set of characteristics. Dig down deep into any soil, and you'll see that it is made of layers, or horizons (O, A, E, B, C, R). Put the horizons together, and they form a soil profile. Like a biography, each profile tells a story about the life of a soil. Most soils have three major horizons (A, B, C) and some have an organic horizon (O).

The horizons are:

O – (humus or organic) Mostly organic matter such as decomposing leaves. The O horizon is thin in some soils, thick in others, and not present at all in others.
A - (topsoil) Mostly minerals from parent material with organic matter incorporated. A good material for plants and other organisms to live.
E – (eluviated) Leached of clay, minerals, and organic matter, leaving a concentration of sand and silt particles of quartz or other resistant materials – missing in some soils but often found in older soils and forest soils.
B – (subsoil) Rich in minerals that leached (moved down) from the A or E horizons and accumulated here.
C – (parent material) The deposit at Earth's surface from which the soil developed.
R – (bedrock) A mass of rock such as granite, basalt, quartzite, limestone or sandstone that forms the parent material for some soils – if the bedrock is close enough to the surface to weather. This is not soil and is located under the C horizon.

Soil is formed slowly through the weathering of geologic materials called parent material, and parent material is defined as bedrock or unconsolidated mineral and organic matter from which soils develop. As bedrock erodes into smaller particles near the earth's surface, organic matter decays and mixes with inorganic material (rock fragments, soil minerals, water, and gases) to form soil.

The processes of weathering include both physical and chemical weathering reactions.

The Five Functions of Soil

See description of diagram Links to an external site.

Soil Properties

Color

The most obvious property when looking at soil is its color. The color of soil can tell scientists a lot about it. Geologist officially recognize over 170 different soil colors. Most of these are shades of black, brown, red, gray, and white.

Generally speaking, the darker a soil is, the more nutrient rich it is. The darker color often indicates an increase in decomposed organic matter known as humus.

Gray soils often indicate poor drainage, while red soils can indicate very poor soils.

These general rules about soil colors can, however, be misleading. Under certain conditions, a very poor soil can appear as dark black, while a rich healthy soil can appear as red.

Texture

The type of rock that makes up the soil and how that rock was broken down will determine the texture of the soil. Soils that have a good balance of small and large pieces of particles making up the soil tend to be the most healthy type of soil.

Structure 

Soil formed in moist climate have the ability to bond fragments together forming clumps of soil, called peds. These peds affect how easily water is able to move through the soil, as well as plant roots and other organisms. Drier climates tend to form fewer peds while deserts and arid climates will form no peds at all.

pH Level

Soil pH is the measure of acidity or alkalinity in soils.

Soil pH directly affects the life and growth of plants because it affects the availability of all plant nutrients.

Between pH 6.0 and 6.5, most plant nutrients are in their most available state. Soil acidity or alkalinity (pH) is extremely important because it has an effect on the decomposition of mineral rock into essential elements that plants can use. The ideal range of pH in soil for plants is 6.0 to 6.5.

 

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