Soil Science: Understanding Earth’s Foundation

Posted on Aug 24, 2024 in Geology

Soil: The Foundation of Life

It allows us to grow food and the materials we use to make everything from the shirt you have on to the medicine you took this morning.

Soil is made up of small pieces of rock that have broken down over thousands of years.

Soil is also partly made up of the remains of plants and animals, and is home to many organisms, from earthworms to ants.

In this Unit, you will learn how soil forms, what it contains, and how to protect it.

The Lithosphere

Soil is found on the uppermost layer of the Earth, the Lithosphere.

This nutrient-rich layer typically only extends downward a few centimeters, about as deep as plant roots extend.

Essentially, all life depends upon the soil… There can be no life without soil and no soil without life; they have evolved together.

Soil Composition

Organic Matter

A small fraction of the total materials in soil yet extremely important.

Made up of living organisms such as plants and animals, dead plants and animals, and nutrients that have come from decomposed plants and animals.

Solid Fraction: Mineral Matter

It forms as rocks are eroded by the forces of weather: ice, wind, rain.

About half of the total mass of most soils.

Made up of particles of various sizes: Sand, silt, clay, and loam.

Sandy Soils

Sandy soils have big spaces between the particles. They don’t hold water or nutrients, don’t stick together very well.

Plant roots can’t hold onto this soil. The big spaces allow air into the soil.

Silty Soils

Finer than sand, but still feels gritty. It’s commonly found in floodplains and known as mud. Silty soils make excellent farmland, but erode easily (blown away in dust storms and carried downstream in floods).

Clay Soils

Clay soil is heavy and dense. The spaces between soil particles are very tiny. When clay soil is dry, it’s almost as hard as concrete and plant roots can’t push through it. No air can get in from the surface and most soil organisms that need oxygen can’t breathe. But clay is important because it gives off minerals and absorbs acids and changes the soil chemistry.

Loam Soils

This soil has enough large and small spaces for air and water to flow in.

It also has enough clay to let it stick together and hold humus. Plant roots can easily grow through these spaces.

Soil Air

Very different from the air found in the atmosphere.

Not exposed to moving air currents and is much more moist, or humid.

Oxygen is removed by soil organisms, and carbon dioxide is left behind, leaking out of the soil, slowly replenishing the atmosphere’s carbon dioxide supplies.

Soil Water

One of the most important ingredients.

Without water, soil formation would not be possible.

The presence of plants and soil organisms depends on the amount of water in the soil.

Some of the water falling on the soil surface runs off into drains or streams, but a lot of it enters the soil through precipitation, such as rain and snow, draining down into it (infiltration).

Water may reach a barrier that it can’t get through. This barrier may be clay or some dense rock or some other impermeable substance.

A layer of water can collect in the ground above this barrier, forming a waterlogged layer. This is called groundwater. The top of the groundwater is the water table, or limit between the saturated and the unsaturated zones.

The depth of the water table varies with the season.

It is higher in wet weather and lower in dry weather.

Many substances dissolve into water, and are carried from one portion of the soil to another.

Water makes chemical reactions in the soil possible, and allows micro-organisms to survive.

Water leaves soil via evaporation, as well as through drainage.

Excessive water can rob soils of their nutrients by carrying them away to other locations.

This process is known as mineral leaching.

Tropical rain forests have poor soils.

This is because most of the nutrients have been drained out by frequent rainfall.

Soil Properties

Soil Color

The most obvious property.

Can tell scientists a lot about it.

Over 170 different soil colors.

Indicates its fertility: Black or dark brown soils are rich in nitrogen and contain a high percentage of organic materials.

Soils that are nitrogen-poor and low in organic material might be gray, yellow, or red.

Soil Texture

Soil is made up of many different sized particles. These particles give the soil its texture.

Soil texture determines the porosity or the water holding capacity of the soil.

It also determines the soil permeability, or rate at which water moves through it.

• Without proper pore space, water and air movement through the soil is restricted.
• Plants can’t thrive and run-off increases, causing greater soil erosion.
• Often occurs under the weight of heavy equipment, intense foot traffic, over-saturation of soils, and light foot traffic on moist soils.
• Plants absorb nutrients through their roots that have been dissolved into the ground water.
• ACID SOIL: (pH below 7) the nutrients in the soil dissolve too quickly, and leach away as the water drains.
• ALKALINE SOIL, (pH above 7) the nutrients don’t dissolve quickly enough, and plants can’t absorb them well.
• NEUTRAL SOIL, pH=7, the preferred type of soil for plant growth.
• TEMPERATURE: Influenced by the climate of the area and the season of the year.
• Affects the speed of chemical reactions and determines how fast plants can grow and take up water and nutrients.
• Warm temperatures speed up reactions and colder ones slow them down.
• The deeper you go the less the soil temperature will fluctuate.

Organic Content (OC)

Determined by five main factors: climate, water content, pH, and the oxygen levels. For example, organic matter is broken down slowly in a cold climate or during the winter, because micro-organisms are not very active.

The more oxygen in soil, the faster bacteria turn organic matter into humus.

Soil Horizons

If you dig a deep hole in the ground, you may see each of the different layers or soil horizons.

All together, the layers are a soil profile. Each horizon has its own set of characteristics.

In the simplest soil profile, a soil has three basic horizons on top of the parent material or bedrock:

• Topsoil or A horizon
• Subsoil or B horizon
• C-Horizon.

Topsoil (A Horizon)

• Darkest layer of the soil. It is the layer with the most organic material, where humus forms.
• Most biological activity. Where most organisms live and plant roots develop and help to hold the topsoil in place.
• Many minerals and clay material dissolve in the fresh water that moves through the topsoil. These minerals and clays are carried down to the lower layers of soil.

Subsoil (B Horizon)

• Soluble minerals and clays accumulate in the subsoil.
• Less organic material, lighter brown in color.
• Holds more water due to the presence of iron and clay.

C-Horizon

• Made of partially altered bedrock.
• It is possible to identify the original rock type from which this soil formed.
• Not all regions develop the same soil horizons, or even soil at all.
• Arid regions are poor at soil development.

Soil Formation and Conservation

Soil as a Resource

• The original rock is the source of the inorganic portion of the soil, resulting from weathering processes.
• Soil is a renewable resource. But it is only renewable if we take care of it.
• Natural events can degrade soil. These events include droughts, floods, insect plagues, or diseases that damage soil ecosystems. Human activities can also degrade soil.
• Deforestation or even just walking or riding your bike over the same place can kill the grass.
• Plants help to hold the soil in place. Without plants to protect it, soil may be carried away by wind or running water.
• In many areas, soil is eroding faster than it is forming. In these locations, soil is a non-renewable resource.
• Soil is contaminated if too much salt accumulates. Soil can also be contaminated by pollutants (industry, agriculture).

Soil Conservation

• There are many ways to protect soil.
• We can add organic material like manure or compost. This increases the soil’s fertility.
• Increased fertility improves the soil’s ability to hold water and nutrients.
• Organic farming is a good way of protecting soils.
• Planting trees as windbreaks, building terraces into steeper slopes, or no-till or low-till farming are also examples of good practice.

Factors Affecting Soil Formation

• How well soil forms and what type of soil forms depends on many factors. These include climate (rain and temperature), the original rock type, the slope, the amount of time, and biological activity.
• 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!

Climate

• Influences the rate of weathering.
• More rain means that more rainwater passes through the soil reacting chemically with the particles.
• Mild and rainy climates produce thick and rich soils.
• Dry and hot climates produce thin, poor soils.
• Given enough time, a climate will produce a particular type of soil. The original rock type does not matter. The same rock type will form a different soil type in each different climate.
• The rate of chemical reactions increases with higher temperatures.
• Plants and bacteria grow and multiply faster in warmer areas.

Time

• Soil formation increases with time.
• The warmer the temperatures, the more rainfall, and the greater the amount of time, the thicker the soils will become.
• It takes a long time for a rock or mountain to weather.
• Weathering breaks down solid rock into sediments or rock particles of different sizes.
• Two types of weathering: Mechanical and Chemical.

Mechanical Weathering

It’s a physical process that breaks big rocks into smaller pieces without changing its composition or chemical structure.

Temperature changes can have this effect. Cool nights and hot days always cause rocks to expand and contract. That movement can cause rocks to crack and break apart.

Roots and plants also push into the rocks and break them apart.

Chemical Weathering

It changes the rock’s minerals and chemical composition, becoming a different type of rock.

For these chemical reactions to happen in nature, moisture, and heat must be present.

Water, carbon dioxide, and oxygen are important agents of chemical weathering.

Oxidation and acidification.

When you see rocks next to each other that are different colors (often shades of red), or that look as if they have been eaten away (often shades of gray) then you know chemical reactions have taken place.

Rock Type and Resistance

Certain types of rock are very resistant to weathering.

Igneous rocks such as granite tend to weather slowly because they are hard. Water cannot easily penetrate them.

Limestone is a sedimentary rock that dissolves easily. When softer rocks wear away, the more resistant rocks form ridges or hills.

A soil is a residual soil when it forms in place. Only about one third of soils form this way. The material comes from the underlying bedrock.