Physically, soils are composed of mineral and organic particles of varying sizes. The particles are arranged in a matrix that results in about 50 percent pore space, which is occupied by water and air. This produces a three-phase system of solids, liquids, and gases.
Essentially, all uses of soils are greatly affected by certain physical properties. The physical properties considered in this article include texture, structure, consistence, porosity, density, color, and temperature.
The physical and chemical weathering of rocks and minerals results in a wide range in size of particles from stones, to gravel, to sand, to silt, and very small clay particles. The particle-size distribution determines the soil’s coarseness or fineness, or soil texture. Specifically, the texture is the relative proportions of sand, silt, and clay in the soil.
Soil Texture Classes
Soil texture is the comparative proportions of sand, clay, and silt in soil. Once the percentages of sand, silt, and clay have been determined, the soil can be placed in one of 12 major textural classes.
For soil that contains 15 percent clay, 65 percent sand, and 20 percent silt, the logical question is: What is the texture or textural class of the soil?
The texture of the soil is expressed with the use of class names, as shown in the image above. The sum of the percentages of sand, silt, and clay at any point in the triangle is 100.
Point A represents soil containing 15 percent clay, 65 percent sand, and 20 percent silt, resulting in a textural class name of sandy loam.
A soil containing equal amounts of sand, silt, and clay is a clay loam. The area outlined by the bold lines in the triangle defines a given class. For example, loam soil contains 7 to 27 percent clay, 28 to 50 percent silt, and between 22 and 52 percent sand.
Soils in the loam class are influenced almost equally by all three separates-sand, silt, and clay. For sandy soils (sand and loamy sand), the properties and use of the soil are influenced mainly by the sand content of the soil. For clays (sandy clay, clay, silty clay), the properties and use of the soil are influenced mainly by the high clay content.
Texture is used in reference to the size of soil particles, whereas structure is used in reference to the arrangement of the soil particles. Sand, silt, and clay particles are typically arranged into secondary particles called peds, or aggregates. The shape and size of the peds determine the soil’s
Types of Soil Structure
Soil peds are classified based on shape, and the four basic soil structural types are spheroid, platelike, blocklike, and prismlike. These shapes give rise to granular, platy, blocky, and prismatic types of structure.
The columnar structure is prismatic-shaped peds with rounded caps. Soil structure generally develops from material that is without structure.
There are two structureless conditions, the first of which is sands that remain loose and incoherent. They are referred to as single-grained. Second, materials with a significant clay content tend to be massive if they do not have a developed structure.
Massive soil has no observable aggregation or no definite and orderly arrangement of natural lines of weakness.
Consistence is the resistance of the soil to deformation or rupture. It is determined by the cohesive and adhesive properties of the entire soil mass. Whereas structure deals with the shape, size, and distinctiveness of natural soil aggregates, consistence deals with the strength and nature of the forces between the sand, silt, and clay particles.
Consistence is important for tillage
and traffic considerations. Dune sand exhibits minimal cohesive and adhesive properties, and because sand is easily deformed, vehicles can easily get stuck in it. Clay soils can become sticky when wet, and thus make hoeing or plowing difficult.
Soil Consistence Terms
Consistence is described for three moisture levels: wet, moist, and dry. A given soil may be sticky when wet, firm when moist, and hard when dry. A partial list of terms used to describe consistence includes:
1. Wet soil-nonsticky, sticky, nonplastic,
2. Moist soil-loose, friable, firm
3. Dry soil-loose, soft, hard
Plastic soil is capable of being molded or deformed continuously and permanently, by relatively moderate pressure, into various shapes when wet. Friable soils readily break apart and are not sticky when moist.
Two additional consistence terms for special situations are cemented and indurated. Cementation is caused by cementing agents such as calcium carbonate, silica, and oxides of iron and aluminum. Cemented horizons are not affected by water content and limit root penetration.
When a cemented horizon is so hard that a sharp blow of a hammer is required to break the soil apart, the soil is considered to be indurated. A silica-cemented horizon is called a duripan.
Large rippers pulled by tractors are used to break up duripans to increase rooting depth in soils. Sometimes a layer with an accumulation of carbonates (a Ck horizon, for example) accumulates so much calcium carbonate as to become cemented or indurated and is transformed into a Petrocalcic horizon. Petrocalcic horizons are also called caliche.
Cemented and indurated horizons tend to occur in soils of great age.
DENSITY AND WEIGHT RELATIONSHIPS
Two terms are used to express soil density. Particle density is the average density of the soil particles, and bulk density is the density of the bulk of the soil in its natural state, including both the particles and pore space.
Particle Density and Bulk Density
Soil is composed of mineral and organic particles of varying composition and density. Feldspar minerals (including orthoclase) are the most common minerals in rocks of the earth’s crust and are very common in soils.
They have densities ranging from 2.56 g/cm3 to 2.76 g/cm3. Quartz is also a common soil mineral; it has a density of 2.65 g/cm3. Most mineral soils consist mainly of a wide variety of minerals and a small amount of organic matter.
The average particle density for mineral soils is usually given as 2.65 g/cm3 and must be exercised in the collection of soil cores so that the natural structure is preserved. Any change in structure, or compaction of soil, will alter the amount of pore space and, therefore, will alter the bulk density. The bulk density is the mass per unit volume of oven-dry soil, calculated as follows:
bulk density = mass oven-dry volume
PROBLEM: If the soil in the core of Figure 3.9 weighs 600 grams over dry and the core has a volume of 400 cm3, calculate the bulk density.
bulk density = 400 0
cm 3 = 1.5 g/cm3
The bulk density of soil is inversely related to porosity. Soils without structure, such as single-grained and massive soil, have a bulk density of about 1.6 to 1.7 g/cm3.
Development of structure results in the formation of pore spaces between
peds (inter-ped spaces), resulting in an increase in porosity and a decrease in bulk density. A value of 1.3 g/cm3
is considered typical of loam surface soils that have a granular structure. As the clay content of surface soils increases, there tends to be an increase in structural development and a decrease in bulk density.
As clay accumulates in B horizons, however, the clay fills existing pore space, resulting in a decrease in pore space volume. As a result, the formation of Bt horizons is associated with an increase in bulk density.
The bulk densities of the various horizons of soil are shown in the image below;
In this soil, the C horizon has the greatest bulk density,-1.7 g/cm3. The C horizon is massive, or structureless, and this is associated with high bulk density and low porosity. Note that the Bt horizon is more dense than the A horizon and there is an inverse relationship between bulk density and total porosity (macropore space plus micropore space).
Organic soils have very low bulk density
compared with mineral soils. Considerable variation exists, depending on the nature of the organic matter and the moisture content at the time of sampling to determine bulk density.
Bulk densities for organic soils commonly range from 0.1 to 0.6 g/cm3, and the changes in soil porosity due to compaction are commonly evaluated in terms of changes in bulk density.
SOIL PORE SPACE AND POROSITY
The fact that mineral soils have a particle density and bulk density of about 2.65 and 1.3 g/cm3, respectively, means that soils have about 50 percent total pore space or porosity. In simple terms, rocks without pore space are broken down by weathering to form mineral soils that have about 50 percent porosity. The pore spaces vary in size, and the size of the pore spaces themselves can be as important as the total amount of pore space.
Determination of Porosity
Oven-dry soil cores, which have been used to determine bulk density, can be placed in a pan of water, allowed to saturate or satiate, and then reweighed to obtain the data needed to calculate soil porosity. When the soil is satiated, the pore space is filled with water, except for a small amount of entrapped air.
The volume of water in a satiated soil core approximates the amount of pore space and is used to calculate the soil’s approximate porosity. For example, if the soil in a 400 cm2 core weighed 600 grams at oven dry, and 800 grams at water satiation, the satiated soil would contain 200 grams of water that occupies 200 cubic centimeters of space (1 g of water has a volume of 1 cm3).
Porosity is calculated as follows:
cm3 pore space
porosity = 3 X 100
cm soil volume
x 100 = 50%
PROBLEM: A 500 cm3 oven dry core has a bulk density of 1.1 g/cm3. The soil core is placed in a pan of water and becomes water saturated. The oven-dry soil and water at saturation weigh 825 grams. Calculate the total soil porosity.
Effects of Texture and Structure
Spheres in closest packing result in a porosity of 26 percent, and spheres in open packing have a porosity of 48 percent. As stated in another way, a ball that just fits in a box occupies 52 percent of the volume of the box, whereas 48 percent of the volume is empty space.
These facts are true regardless of the size of the spheres or balls. Single-grained sands have a porosity of about 40 percent, and this suggests that the sand particles are not perfect spheres and the sand particles are not in a perfect close packing arrangement.
The low porosity of single-grained sands is related to the absence of structure (peds) and, therefore, an absence of inter-ped spaces. Fine-textured A horizons, or surface soils, have a wide range of particle sizes and shapes, and the particles are usually arranged into peds. This results in pore spaces within and between peds.
These A horizons with well-developed granular structures may have as much as 60 percent porosity and bulk density values as low as 1.0 g/cm3. Fine-textured Bt horizons have a different structural condition and tend to have less porosity and, consequently, a greater bulk density than fine-textured A horizons.
This is consistent with the filling of pore space by translocated clay and the effects of the weight of the overlying soil, which applies pressure on the Bt horizon. The pore spaces within the peds will generally be smaller than the pore spaces between the peds, resulting in a wide range of pore sizes.
It has been pointed out that sand surface soils have less porosity than clayey surface soils. Yet, our everyday experiences tell us that water moves much more rapidly through the sandy soil.
The explanation for this apparent paradox lies in the pore size differences in the two soils. Sands contain mostly macropores that normally cannot retain water against gravity and are usually filled with air.
As a consequence, macropores have also been called aeration pores. Since the porosity of sands is composed mainly of macropores, sands transmit water rapidly. Pores that are small enough to retain water against gravity will remain water-filled after soil wetting by rain or irrigation and are called capillary or micropores. Because sands have little micropore space they
are unable to retain much water.
Fine-textured soils tend to contain mainly micropores and thus can retain a lot of water but have little ability to transmit water rapidly. An example of the distribution of micropore and macropore space in the various horizons of a soil profile is given in the image above.
Note that the amounts of both total porosity and macropore space are inversely related to the bulk density.
Porosity and Soil Aeration
The atmosphere contains by volume nearly 79 percent nitrogen, 21 percent oxygen, and 0.03 percent carbon dioxide. Respiration of roots, and other organisms, consumes oxygen and produces carbon dioxide. As a result, soil air commonly contains 10 to 100 times more carbon dioxide and slightly less oxygen than the atmosphere (nitrogen remains about constant).
Differences in the pressures of the two gases are created between the soil and the atmosphere. This causes carbon dioxide to diffuse out of the soil and oxygen to diffuse into the soil. Normally, this diffusion is sufficiently rapid to prevent oxygen deficiency or carbon dioxide toxicity for roots.
Although water movement through a uniformly porous medium is greatly dependent on pore size, the movement and diffusion of gas are closely correlated with total porosity. Gaseous diffusion in soil, however, is also dependent on pore space continuity.
When oxygen is diffusing through a macropore and encounters a micropore that is filled with water, the water-filled micropore acts as a barrier to further gas movement.
The diffusion of oxygen through the water barrier is essentially zero because oxygen diffusion through water is about 10,000 times slower than through air.
Clayey soils are particularly susceptible to poor soil aeration when wet because most of the pore space consists of micropores that may be filled
with water. Sands tend to have good aeration or gaseous diffusion because most of the porosity is composed of macropores.
In general, a desirable soil for plant growth has a total porosity of 50 percent, which is one-half macropore porosity and one-half micropore porosity. Such soil has a good balance between the retention of water for plant use and an oxygen supply for root respiration.
Oxygen deficiencies are created when soils become water saturated or satiated. The wilting of tomatoes, due to water-saturated soil, is shown in the image below
Plants vary in their susceptibility to oxygen deficiency. Tomatoes and peas are very susceptible to oxygen deficiency and may be killed when soils are water-saturated for a few hours (less than a day). Two yew plants that were planted along the side of a house at the same time are shown in Image 3 above.
The plant on the right side died from oxygen stress caused by water-saturated soil as a result of flooding from the downspout water. The soil had a high clay content, and the use of pipes to carry water away from building foundations is also important in preventing building foundation damage, where soils with high clay content expand and contract because of cycles of wetting and drying.
When changes in soil aeration occur slowly, plants can make some adjustments. All plant roots must have oxygen for respiration, and plants growing on flooded soil or in swamps have special mechanisms for obtaining oxygen.
Color is the most obvious and easily determined soil property. Soil color is important because it is an indirect measure of other important characteristics such as water drainage, aeration, and organic matter content.
Thus, color is used with other characteristics to make many important inferences regarding soil formation and land use.
In this section, we will work you through some of the important factors of soil color which are determined by the factors affecting them, how to determine the soil color, and their significance.
Determination of Soil Color
Soil colors are determined by matching the color of a soil sample with color chips in a Munsell soil-color book. The book consists of pages, each having color chips arranged systematically according to their hue, value, and chroma, and the three variables that combine to give colors.
Hue refers to the dominant wavelength or color of light. Value, sometimes color brilliance, refers to the quantity of light. It increases from dark to light colors. Chroma is the relative purity of the dominant wavelength of light.
The three properties are always given in the order of hue, value, and chroma. In the notation, 10YR 6/4, 10YR is the hue, 6 is the value, and 4 is the chroma. This color is a light-yellowish brown. This color system enables a person to communicate accurately the color of soil to anyone in the world.
Factors Affecting Soil Color
Organic matter is a major coloring agent that affects soil color, depending on its nature, amount, and distribution in the soil profile. Raw peat is usually brown; well-decomposed organic matter, such as humus, is black or nearly so. Many organic soils have a black color.
In most mineral soils, the organic matter content is usually greatest in the surface soil horizons and the color becomes darker as the organic matter content increases. Even so, many A horizon does not have a black color.
Black-colored A horizons, however, are common in soils that developed under tall grass on the pampa of Argentina and the prairies of the United States.
An interesting color phenomenon occurs in the clayey soils of the Texas Blacklands. The A horizon has a black color; however, the black color may extend to the depth of a meter even though there is a considerable decrease in organic matter content with increasing soil depth.
In some soils, finely divided manganese oxides contribute to black color, and the major coloring agents of most subsoil horizons are iron compounds in various states of oxidation and hydration.
The rusting of iron is an oxidation process that produces rusty or reddish-colored iron oxide. The bright red color of many tropical soils is due to dehydration and
oxidized iron oxide, hematite (Fe2O3).
Organic matter accumulation in the A horizons of these soils results in a brownish-red or mahogany color.
Hydrated and oxidized iron, goethite (FeOOH), has a yellow or yellowish-brown color and reduced and hydrated iron oxide has a gray color. Various combinations of iron oxides result in brown and yellow-brown colors.
Thus, the color of the iron oxides is related to aeration and hydration conditions as controlled by the absence or presence of water. Soils on slopes that never saturate with water have subsoils indicative of well-drained and aerated soil subsoils with reddish and brownish colors.
Soils in depressions that collect water, and poorly drained locations in which soils are water-saturated much of the time, will tend to have gray-colored B horizons. Soils in intermediate situations will tend to have yellowish-colored B horizons. If the soil has a gray-colored B horizon, the soil is likely to be water saturated at least part of the time unless it is artificially drained see image below.
The light and grayish colors of E horizons are related to the illuviation of iron oxides and low organic matter content. Some soil horizons may have a white color because of soluble salt accumulation at the soil surface or calcium carbonate accumulation in subsoils. Horizons in young soils may be strongly influenced by the color of the soil parent material.
Significance of Soil Color
Many people tend to equate black-colored soils with fertile and productive soils. Broad generalizations between soil color and soil fertility are not always valid. Within a local region, increases in the organic matter content of surface soils may be related to increases in soil fertility, because organic matter is an important reservoir of nitrogen.
Subsoil colors are very useful in predicting the likelihood of subsoil saturation with water and poor aeration. Gray subsoil color indicates a fairly constant water-saturated condition. Such soils are poor building sites because basements tend to be wet and septic tank filter fields do not operate properly in water-saturated soil.
Installation of a drainage system is necessary to use the soil as a building site successfully. Subsoils that have bright brown and red colors are indicative of good aeration and drainage. These sites are good locations for buildings and the production of tree fruits.
Many landscape plants have specific needs and tolerances for water, aeration, and soil color that are useful guides in the selection of plant species.
Alternating water saturation and drying of the subsoil, which may occur because of alternating wet and dry seasons, may produce an intermediate color situation.
During the wet season, iron is hydrated and reduced, and during the dry season, iron is dehydrated and oxidized. This causes a mixed pattern of soil colors called mottling.
Mottled-colored B horizons are indicative of soils that are intermediate between frequent water saturation and soils with continuous well-drained conditions.
Below freezing, there is extremely limited biological activity. Water does not move through the soil as a liquid and, unless there is frost heaving, time stands still for the soil.
A soil horizon as cold as 5° C acts as a deterrent to the elongation of roots. The chemical processes and activities of microorganisms are temperature-dependent. The alternate freezing and thawing of soils results in the alternate expansion and contraction of soils.
This affects rock weathering, structure formation, and the heaving of plant roots. Thus, the temperature is an important soil property.
Here we would discuss some of the factors that contribute to the temperature of the soil.
Heat Balance of Soils
The heat balance of soil consists of the gains and losses of heat energy. Solar radiation received at the soil surface is partly reflected into the atmosphere and partly absorbed by the soil surface.
Dark-colored soil and light-colored quartz sand may absorb about 80 and 30 percent of the incoming solar radiation, respectively.
Of the total solar radiation available for the earth, about 34 percent is reflected in space, 19 percent is absorbed by the atmosphere, and 47 percent is absorbed by the land.
Heat is lost from the soil by:
(1) evaporation of water,
(2) radiation back into the atmosphere,
(3) heating of the air above the soil,
(4) heating of the soil.
For the most part, the gains and losses balance each other. But during the daytime or in the summer, the gains exceed the losses, whereas the reverse is true for nights and winters.
The amount of heat needed to increase the the temperature of the soil is strongly related to water content, and it takes only 0.2 calories of heat energy to increase the temperature of 1 gram of dry soil to 1°C compared with 1.0 calories per gram per degree for water.
This is important in the temperate regions where soils become very cold in winter and planting dates in the spring depend on a large rise in soil temperature.
In general, sandy soils warm more quickly and allow earlier planting than fine-textured soils, because sands retain less water and heat up faster.
Control of Soil Temperature
Two practical steps can be taken to change soil temperature. Wet soils can be drained to remove water, and mulches can be applied on the surface of the soil to alter the energy relationships.
When used for agriculture, wet soils are drained to create an aerated root zone. The removal of water also causes the soil to warm more quickly in the spring.
A light-colored organic matter mulch, such as straw, will tend to lower soil temperature because;
(1) more solar radiation will be reflected and less absorbed,
(2) the water content of the soil will
tend to be greater because more water will infiltrate.
The mulch will, however, tend to increase soil temperature because the mulch tends to;
(1) reduce loss of heat by radiation,
(2) reduce water evaporation from the soil surface, which requires energy. The net effect, however, is to reduce soil temperature in the spring.
In the northern corn belt, crop residues left on the soil surface promote cooler soils in the spring and tend to result in slightly lower corn yields. A black plastic mulch increases soil temperature because it increases the absorption of solar radiation, reduces heat loss by radiation, and reduces evaporation of water from the soil surface.
Black mulches have been used to increase soil temperatures to produce vegetables and melons for an earlier
Soil’s physical properties affect virtually every use made of the soil. Texture relates to the amount of sand, silt, and clay in the soil, and structure relates to the arrangement of the sand, silt, and clay into peds.
Texture and structure greatly affect plant growth by influencing water and air relationships. Soils that expand and shrink with wetting and drying affect the stability of building foundations.
About one-half of the volume of mineral soils is pore space. Such soils have a bulk density of about 1.3 g/cm3 and 50 percent porosity. In soils with favorable conditions for water retention and aeration, about one-half of the porosity is macropore space and one-half is micropore space.
Soil color is used as an indicator of organic matter content, drainage, and aeration.
Soil temperature affects plant growth. Soil temperature is greatly affected by soil color, water content, and the presence or absence of surface materials, such as mulches. Permafrost occurs in soils with an average temperature below freezing.
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