Thank goodness for mechanical and chemical weathering, because without these forces working to breakdown rock we would not have any soil on Earth. It is unlikely that humans would have been able to live on Earth without soil! Your life and the lives of many organisms depend on soil. We get wood, paper, cotton, medicines and even pure water from soil. So soil is a very important resource. Even though it is actually only a very thin layer on Earth’s surface over the solid rocks below, it is the place where our atmosphere, hydrosphere, biosphere and the rocks of the Earth meet.
Within our soil layer, reactions between solid rock, liquid water and air take place. It is a mistake on our part to disregard this important resource, yet we say things like “soiled” or “dirty” when we talk about ruining something. Our precious soil resource needs to be carefully managed and cared for. If we neglect or abuse the soil we have, it will not remain the renewable resource that we have relied on throughout human existence.
We can think about soil as a living resource or an ecosystem all by itself. Within soil, there are many elements. It is a complex mixture of different materials. Some of them are inorganic, like the products of weathered rock, including pebbles, sand, silt and clay particles. There are also bits of organic materials, formed from the partial breakdown and decomposition of plants and animals. In general, the pieces of rock and minerals make up about half of the soil, with the other half made of organic materials.
In between, in the spaces of soil, there are thousands or even millions of living organisms. Those organisms could be earthworms, ants, bacteria, or fungi, as well as many other types of organisms. In between the solid pieces, there are tiny spaces filled with air and water. In some soils, the organic portion could be entirely missing, such as desert sand. At the other extreme, a soil could be completely organic, such as the materials that make up peat in a bog or swamp. The organic materials are necessary for a soil to be fertile. The organic portion provides the nutrients, like nitrogen, needed for strong plant growth. We will learn about that organic portion in just a bit.
Soil Formation
How well soil forms and what type of soil forms depends on several different factors. Some of these factors are the climate, the original rock the soil formed from, the slope, the amount of time and biological activity. Climate is ultimately the most important factor and will determine the type of soil that forms in a particular region. The climate of a region is principally determined by temperature and the amount of precipitation. This also influences the type of vegetation that grows in the region. We can identify different climates by the types of plants that grow there. Depending on how closely you look, we can divide land areas into many different climate regions.
Given enough time, even different rock types will produce a similar climate related soil. Climate is such an important factor that even the same type of rock in different climates will produce a different climate related soil. This is true because the rocks on Earth are predominantly composed of eight elements and as rock breaks down, there will mostly be just these eight elements. Surely, if an element is not present in the original rock, then it will also not be present in the soil that forms from it. The amount of precipitation in an area is important because it influences the rate of weathering.
The more it rains, the more rainwater passes through the soil and the more it reacts chemically with the particles. Those reactions are most efficient in the top layers of the soil and become less effective as the water continues to percolate through lower layers of soil. The top layers of soil in contact with the freshest water react most. Increased rainfall in a region increases the amount of rock that is dissolved as well as the amount of material carried away by moving water. As materials are carried away, new surfaces are exposed and this also increases the rate of weathering.
The temperature for a region is important too. The rate of chemical reactions increases with higher temperatures. For every 10o C increase in temperature, the rate of chemical reactions doubles. Warmer regions also have more vegetation because plants and bacteria grow and multiply faster. This means that in tropical regions, where temperatures and amounts of rainfall are consistently high, thick soils form with no unstable minerals and therefore no nutrients. Conversely, arid regions produce thin soils, rich in unstable minerals. The rate of soil formation increases with greater amounts of time. The longer the amount of time that soil remains in a particular area, the greater the degree of alteration.
The original rock is the source of the inorganic portion of the soil. Chemical reactions from weathering break down the rock’s original minerals into sand, silt and clays. A soil is called a residual soil when it forms in place, with the underlying rock breaking down to form the layers of soil that reside above it. Only about one third of the soils in the United States form this way. The rest of the soils form from materials that have been transported in from somewhere else. These soils are called transported soils.
Glaciers bring bits of rock from far away, depositing the materials they carried as the ice of the glacier melts. Wind and rivers also transport materials from their places of origin. These soils form from the loose particles that have been transported in to a new location and deposited. For transported materials, the rate of soil formation is faster because the transported materials have already been weathered. The closer the materials are to their place of origin, the greater the influence of the original materials. The further those materials move from their origin, the greater the degree of weathering and the influence of the original materials becomes obscured.
Soils thicken as the amount of time available for weathering increases. The warmer the temperatures, the more rainfall and greater amount of time, the thicker the soils will become. Biological activity produces the organic material and nutrients in soil. The partial decay of plant material and animal remains also forms organic acids which in turn increase the rate of soil formation and the rate of weathering. The organic material increases the ability of the soil to hold water, create a soil’s structure and enhance its fertility and ability to be cultivated.
The decayed remains of plant and animal life are called humus. Humus is an extremely important part of the soil. It coats the mineral grains, binding them together into clumps that then hold the soil together. The humus in soil also increases the porosity and water holding capacity of a soil. Humus helps to buffer rapid changes in soil acidity and helps the soil to hold its nutrients. Decomposing organisms in the soil breakdown the complex organic molecules of plant matter and animal remains to form simpler inorganic molecules that are soluble in water. Bacteria in the soil change atmospheric nitrogen into nitrates. One indicator of a soil’s fertility is its color. Soils that are rich in nitrogen and contain a high percentage of organic materials 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.
Soil Texture and Composition
The inorganic portion of soil is made of many different size particles. In addition to many particle sizes, there can be different proportions of each particle size. The combination of these two factors determines some of the properties of the soil. A soil will be very permeable, which means that water will flow through it easily, if the spaces between the inorganic particles are large enough and are well connected. Soils that have lots of very small spaces tend to be water holding soils. Clays are an example of a type of soil that holds water. When clay is present in a soil, the soil is heavier and holds together more tightly. Sandy or silty soils are considered ‘light’ soils because they are permeable, water draining types of soils. When a soil contains a mixture of grain sizes, the soil is called a loam. When soil scientists want to precisely determine the soil type, they measure the percentage of sand, silt and clay and plot this information on a triangular diagram, with each type of soil at one corner.
Soil scientists use a diagram like this to plot the percentages of sand, silt & clay in a soil. The soil type can then be determined from the location on the diagram. At the top, a soil would be a clay; at the left corner, it would be a sand and at the right corner it would be a silt. Regions in the lower middle with less than 50% clay are called loams.
Soil Horizons and Profiles
A residual soil forms from the underlying bedrock. This happens over many years, as mechanical and chemical weathering slowly change solid rock into soil. The more time available, the greater the degree of alteration that will occur. Perhaps the first changes to bare rock would be cracks or fractures due to mechanical weathering from ice wedging. Then plants like lichens or grasses become established. As more and more layers of material weather, the soil develops soil horizons, as each layer becomes progressively altered.
The place where the greatest degree of weathering occurs is the top layer. Each successive, lower layer is altered just a little bit less. This is because the first place where water and air come in contact with the soil is at the top. As water moves down through the layers, it is able to do less work to change the soil. If you were able to dig a deep hole into the ground, you could see each of the different layers of soil. All together, these are called a soil profile. Each horizon has its own particular set of characteristics.
In the simplest soil profile, a soil is considered to have three horizons. The first horizon is the top soil, which is called the A horizon. The topsoil will usually be the darkest layer of the soil, because this is the layer with the highest proportion of organic material. Remember that humus forms from all the plant and animal debris that falls to the ground. This includes branches and twigs, acorns and pine needles as well as waste from animals and fungi. The top soil is the region of most intense biological activity.
Many living organisms live within this layer and plants stretch their roots down into this layer. In fact, plant roots are very important to this layer because vegetation helps to hold this layer of soil in place. The top soil layer is usually a layer where minerals that can dissolve and very small particles like clay are absent. This is because clay sized particles get carried to lower layers as water seeps down into the ground. Soluble minerals are missing because they readily dissolve in the fresh water that moves through this layer and are carried down to lower layers of the soil.
The next soil horizon is the subsoil, which is called the B horizon. This is the region where soluble minerals and clays accumulate. This layer will be lighter brown in color and more water holding than the top soil, due to the presence of iron and clay minerals. There will be less organic material in this layer. The next layer down, called the C horizon will be a layer of partially altered bedrock. There will be some evidence of weathering in this layer, but pieces of the original rock can still be seen and it would be possible to identify the original type of rock from which this soil formed.
Not all climate regions develop soils, and not all regions develop the same horizons. Some areas develop as many as five or six distinct layers, while others develop only very thin soils or perhaps soil doesn’t form well at all.
EmoticonEmoticon