1.3 Soil Formation
1.4 Soil Profile and Horizons
1.5 Soil Structure and Porosity
1_Soil-Features
1_Soil-Features
Soil Features
Overview
Title Image "Figure 1" by the United States Department of Agriculture, Natural Resources Conservation Service, Soil Survey Staff is in the Public Domain.
The Soil Features section gives a basic introduction to general concepts of soil including soil properties, profiles, and levels of saturation and how those features allow soil to interact with plants.
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Introduction
Lesson Objectives
Examine the physical and hydrological features of the soil.
Explain vertical section of soil.
Describe the three horizons of soil
Distinguish between various soil types, clay, silt, loam & sandy.
Explain how pore size dictate field capacity, PWP and SWC.
Key Terms
A horizon - consists of a mixture of organic material with inorganic products of weathering
adhesion - attraction between water molecules and other molecules
available water capacity - the water available for plant growth held between field capacity and permanent wilting point
B horizon - soil layer that is an accumulation of mostly fine material that has moved downward
bedrock - solid rock that lies beneath the soil, known as R horizon
C horizon - layer of soil that contains the parent material, and the organic and inorganic material that is broken down to form soil; also known as the soil base
capillary rise - the upward movement of water that is responsible for the loss of water from the soil surface by evaporation
clay - soil particles that are less than 0.002 mm in diameter
cohesion - the force of attraction holding a solid or liquid together, owing to attraction between like molecules
field capacity - the relatively constant soil water content reached after 48 hours drainage of water from a saturated soil
horizon - soil layer with distinct physical and chemical properties, which differs from other layers depending on how and when it was formed
humus - organic material of soil; made up of microorganisms, dead animals, and plants in varying stages of decay
hygroscopic water - water that surrounds and is tightly held by soil particles, making the water unavailable to plants
inorganic compound - chemical compound that lacks carbon
loam - soil that has no dominant particle size
O horizon - layer of soil with humus at the surface and decomposed vegetation at the base
organic compound - chemical compound that contains carbon (foundation of living things)
permanent wilting point - the water content of a soil that has been exhausted of its available water by a crop, such that only non-available water remains
sand - soil particles between 0.1–2 mm in diameter
saturation water content - the maximum amount of water that a soil can store
silt - soil particles between 0.002 and 0.1 mm in diameter
soil - outer loose layer that covers the surface of Earth
soil formation - the chemical changes and mixing of materials that create soil
soil pore - space between soil particles that is filled with air or water
soil profile - vertical section of a soil
soil properties - features of soil including color, texture, structure, bulk density, porosity, consistency, temperature, and horizonation
soil saturation - a soil's water content when practically all pore spaces are filled with water
topsoil - the top layer of soil
Introduction
Plants obtain elements from soil, which serves as a natural medium for land plants. Soil is the outer loose layer that covers the surface of Earth. Along with climate, a major determinant of plant distribution and growth is soil quality. Soil quality depends not only on the chemical composition of the soil but also the topography (regional surface features) and the presence of living organisms. In agriculture, the history of the soil, such as the cultivating practices and previous crops, modify the characteristics and fertility of that soil.
Soil Composition and Types
Soil develops very slowly over long periods of time, and its formation results from natural and environmental forces acting on mineral, rock, and organic compounds. Soils can be divided into two groups: 1) organic soils are those that are formed from sedimentation and are primarily composed of organic matter; 2) mineral soils are those that are formed from the weathering of rocks and are primarily composed of inorganic material. Mineral soils are predominant in terrestrial ecosystems, where soils may be covered by water for part of the year or exposed to the atmosphere.
Soil consists of these four major components (Figure 4.1.1):
- inorganic mineral matter, which constitutes about 40 to 45 percent of the soil volume
- organic matter, which constitutes about 5 percent of the soil volume
- water and air, which constitutes about 50 percent of the soil volume
The amount of each of the four major components of soil depends on the amount of vegetation, soil compaction, and water present in the soil. A good healthy soil has sufficient air, water, minerals, and organic material to promote and sustain plant life. The organic material of soil, called humus, is made up of microorganisms (dead and alive), as well as dead animals and plants in varying stages of decay. Humus improves soil structure and provides plants with water and minerals. The inorganic material of soil consists of rock, slowly broken down into smaller particles that vary in size (Figure 4.1.2).
Soil particles that are 0.1 to 2 mm in diameter are sand. Soil particles between 0.002 and 0.1 mm are called silt, and even smaller particles, less than 0.002 mm in diameter, are called clay. Some soils have no dominant particle size and contain a mixture of sand, silt, and humus; these soils are called loams.
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Soil Formation
Soil formation is the consequence of a combination of biological, physical, and chemical processes. These processes create different soils with unique soil properties. Soil should ideally contain 50 percent solid material and 50 percent pore space. About one-half of the pore space should contain water, and the other half should contain air. The organic component of soil serves as a cementing agent, returns nutrients to the plant, allows soil to store moisture, makes soil tillable for farming, and provides energy for soil microorganisms. Most soil microorganisms—bacteria, algae, or fungi—are dormant in dry soil, but become active once moisture is available.
Five factors account for soil formation: parent material, climate, topography, biological factors, and time. The organic and inorganic material in which soils form is the parent material. Mineral soils form directly from the weathering of bedrock, the solid rock that lies beneath the soil; therefore, they have a similar composition to the original rock. Other soils form in materials that came from elsewhere, such as sand and glacial drift. Materials located in the depth of the soil are relatively unchanged compared with the deposited material. Sediments in rivers may have different characteristics, depending on whether the stream moves quickly or slowly. A fast-moving river could have sediments of rocks and sand; whereas, a slow-moving river could have fine-textured material, such as clay.
Soil formation is a dynamic process, and time is an important factor in soil formation because soils develop over long periods. Temperature, moisture, and wind cause different patterns of weathering and therefore affect soil characteristics. Biological activity is a key component of a quality soil that is promoted by such characteristics as the presence of moisture and nutrients from weathering. Regional surface features (familiarly called “the lay of the land”) can have a major influence on the characteristics and fertility of a soil. Topography affects water runoff, which strips away parent material and affects plant growth. Steeps soils are more prone to erosion and may be thinner than soils that are relatively flat or level. The presence of living organisms greatly affects soil formation and structure. Animals and microorganisms can produce pores and crevices, and plant roots can penetrate crevices to produce more fragmentation. Plant secretions promote the development of microorganisms around the root, in an area known as the rhizosphere. Additionally, leaves and other material that fall from plants decompose and contribute to soil composition. Materials are deposited over time, decompose, and transform into other materials that can be used by living organisms or deposited onto the surface of the soil.
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Soil Profile and Horizons
Soil distribution is not homogenous because its formation results in the production of layers; together, the vertical section of a soil is called the profile. Within the soil profile, soil scientists define zones called horizons. A horizon is a soil layer with distinct physical and chemical soil properties that differ from those of other layers. Soils are named and classified based on their horizons. The soil profile has four main distinct layers, listed in order from top to bottom: 1) O horizon; 2) A horizon; 3) B horizon—or subsoil; and 4) C horizon—or soil base (Figure 4.1.3).
Figure 4.3 shows a cross-section of soil layers, or horizons. The top layer, from zero to two inches, is the O horizon. The O horizon is a rich, deep brown color. From two to ten inches is the A horizon. This layer is slightly lighter in color than the O horizon, and extensive root systems are visible. From ten to thirty inches is the B horizon. The B horizon is reddish brown. Longer roots extend to the bottom of this layer. The C horizon extends from 30 to 48 inches. This layer is rocky and devoid of roots.
The four distinct main layers perform different roles. The O horizon has freshly decomposing organic matter—humus—at its surface, with decomposed vegetation at its base. Humus enriches the soil with nutrients and enhances soil moisture retention. Topsoil—the top layer of soil—is usually two to three inches deep, but this depth can vary considerably. For instance, river deltas like the Mississippi River delta have deep layers of topsoil. Topsoil is rich in organic material; it is considered the “workhorse” of plant production because microbial processes occur there. The A horizon consists of a mixture of organic material with inorganic products of weathering; therefore, it is the beginning of true mineral soil. The A horizon is typically darkly colored because of the presence of organic matter. In this area, rainwater percolates through the soil and carries materials from the surface. The B horizon is an accumulation of mostly fine material that has moved downward, resulting in a dense layer in the soil. In some soils, the B horizon contains nodules or a layer of calcium carbonate. The C horizon, or soil base, includes the parent material, plus the organic and inorganic material that is broken down to form soil. The parent material may be either created in its natural place or transported from elsewhere to its present location. Beneath the C horizon lies bedrock. Bedrock is known as the R horizon. Some soils may have additional layers or lack one of these layers. The thickness of the layers is also variable and depends on the factors that influence soil formation. In general, immature soils may have O, A, and C horizons; whereas, mature soils may display all of these, plus additional layers (Figure 4.1.4).
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Soil Structure and Porosity
Soil structure is the combination or arrangement of primary soil particles into aggregates. Aggregate size, shape, and distinctness are the basis for classes, types, and grades, respectively. Soil structure describes the manner in which soil particles are aggregated. Soil structure affects water and air movement through soil, greatly influencing soil's ability to sustain life and perform other vital soil functions. Soil pores are the spaces between soil particles that are filled with air or water and exist between and within aggregates. Macropores are large soil pores, usually between aggregates, that are generally greater than 0.08 mm in diameter. Macro pores drain freely by gravity and allow easy movement of water and air. They provide habitat for soil organisms and plant roots can grow into them. With diameters less than 0.08 mm, micropores are small soil pores usually found within structural aggregates.
Suction or force is required to remove water from micropores. Capillary rise is the upward movement of water that is responsible for the loss of water from the soil surface by evaporation. Water properties must be examined to better understand this phenomenon. While cohesion is the force of attraction holding water together, adhesion is the force that attracts water molecules to other molecules. Essentially, cohesion and adhesion are the "stickiness" that water molecules have for each other and for other substances. When the adhesive force is greater than the cohesive force, the surface tension forces act against gravity forces to cause water to rise upward.
Available water capacity is an estimate of how much water a soil can hold and release for use by most plants, measured in inches of water per inch of soil. Available water capacity is influenced by soil texture, content of rock fragments, depth to a root-restrictive layer, organic matter, and compaction. It is used in scheduling irrigation and in determining plant populations. The type of soil structure can influence the availability of water to plants and the rate at which water is released to plant roots. A soil with a tillage pan may not allow roots to penetrate and extract the deeper water. Soils with more silt and clay have a greater water holding capacity than sandy soils.
Saturation refers to a soil's water content when air has been displaced and practically all pore spaces are filled with water (Figure 4.5). During this time, no energy is needed to remove water from soil particles. This is a temporary state for well-drained soils, as the excess water quickly drains out of the larger pores under the influence of gravity, to be replaced by air. All soils have a saturation water content that is the maximum amount of water that a soil can store.
Field capacity refers to the relatively constant soil water content reached after 48 hours of free drainage of water by gravity from a saturated soil (Figure 4.5). Drainage occurs through the transmission pores (greater than about 0.05 mm diameter; but note that field capacity can correspond to pores ranging from 0.03 to 0.1 mm diameter). The field capacity concept only applies to well-structured soils where drainage of excess water is relatively rapid. If drainage occurs in poorly structured soils, it will often continue for several weeks; consequently, poorly structured soils seldom possess a clearly defined field capacity. Soil at field capacity feels very moist to the hands. In contrast, the permanent wilting point refers to the water content of a soil that has been exhausted of its available water by a crop, such that only non-available water remains (Figure 4.5).
Hygroscopic water surrounds and is tightly held by soil particles, making the water unavailable to plants (Figure 4.1.5). Water cannot move from the soil to the root of plants. When hygroscopic water is all that remains, the crop becomes permanently wilted and cannot be revived when placed in a water-saturated atmosphere. At this point the soil feels nearly dry or only very slightly moist.
Dig Deeper
USDA: Soil Physical and Chemical Properties
USDA: Technical Reference Examination and Description of Soil Profiles
Water Plant and Soil Relation Under Stress Situations
FAO: Irrigation Water Management: Training Manual No. 1 - Introduction to Irrigation
USGS: The Occurance of Ground Water in the United States
USDE: The Measurement of Water Potential in Low-Level Waste Management
Attributions
"Biology 2e: Chapter 31: Section 2" by Mary Ann Clark, Matthew Douglas, and Jung Choi is licensed under CC BY 4.0.
Food and Agriculture Organization of the United Nations, 2003, Francis Shaxson and Richard Barber, "Optimizing Soil Moisture for Plant Production", https://www.fao.org/3/y4690e/y4690e04.htm. Reproduced with permission.
"From the Surface Down" by the United States Department of Agriculture Natural Resources Conservation Service is in the Public Doman.
"Soil Quality Information Sheet: Available Water Capacity" by the Department of Agriculture Natural Resources Conservation Service is in the Public Domain.