25 Physical Properties of Soil

Meenakshi Nandal

epgp books

 

 

 

Objectives:

 

To study the Various Physical properties of Soil

 

To understand the soil behavior under different temperature and pressure conditions. To study the arrangement and organization of soil particles in the soil.

 

 

26.1 Introduction

 

 

The soils consists of mineral particles, organic matter, water and air. The associations of these conclude the various soils’ properties such as its texture, porosity, structure, chemistry as well as color (Fig 26.1).

 

 

Soil health as well as its biological, physical, and chemical processes is deeply interrelated with the plant growth quality. Fundamental knowledge of various soil properties is required for the successful reforestation management programs.

26.2 Physical Properties of Soil

 

 

Soil consists of minerals, soil organic matter (SOM), water, and air (Fig 26.2). The constitution and combination of these elements greatly changes the soil physical properties, such as structure, texture, porosity as well as the portion of pore space in a soil. Consecutively, these soil properties change the air and water movement within the soil, thereby affecting the soil’s ability to perform. Soil also possesses various other properties, such as water holding capacity and pH (whether the soils are acid or alkaline) also. The combination of these properties makes soils beneficial for a wide range of purposes. These soil characteristics decide what kind of plants can be grown in that soil in a particular region.

26.3 Soil Texture

Soil texture is considered among the most important physical property that can have a profound effect on various other properties. Texture is the combination of three mineral particles including sand, silt and clay, in a soil. These particles vary in size and constitute the fine mineral fraction (Table 1).

 

Particles with size higher than 2 mm in diameter are not considered in texture, although in certain cases they may affect water retention and other properties. The comparative amount of various sizes of particles in a soil describes its texture such as clay, loam, sandy loam or other textural category as a result of ‘weathering,’ by physical and chemical breakdown of rocks as well as minerals. Because of these differences in the structure and composition, materials will weather at varying rates, disturbing the texture of soil. For example, granite is a slow weathering rock, which forms sandy, coarse soils whereas shale which is an easily weathered rock, forms clay-rich soils. As weathering is a comparatively slow procedure, texture remains relatively constant and is not changed by management practices. The quantity of silt, clay, sand, and organic matter in a particular type of soil predicts how it can be managed as well as can be used to grow? Sandy soils have low water holding capacity as well as droughty but they are easy for cultivation of plants in comparison to clay soils which are more difficult to cultivate but has high water holding capacity and ii winters they can become waterlogged.

 

According to USDA textural classification triangle (Fig 26.3) the only soil which neither consists of sand, silt nor clay is called “loams”. Whereas even pure sand, silt or clay can also be considered as soil, from the perception of food production a loam soil with little amount of organic material is designed as ideal. The mineral content of loamy soil consists of 40% sand, 40% silt and 20% clay by weight. Texture of soil influences the soil behavior, particularly retention capacity for nutrients and water. Sand and silt are formed by physical and chemical weathering of the parent rock; whereas clay is formed by the precipitation of the dissolved parent rock as a secondary mineral. The surface area to volume ratio of soil particles and the unbalanced ionic charges determine their role in the fertility of soil, which is estimated by its cation exchange capacity. Clay being the most active followed by silt and sand being least active.

 

Sand’s ultimate benefit to soil is that it hinders compaction thereby enhances the porosity of soil. Silt is mineralogically similar to sand but due to its higher specific surface area it is chemically more active in comparison to sand. Whereas the clay content of soil, with very high specific surface area as well as large number of negative charges, which gives soil its high retention capacity for water and nutrients. Clay soils also help in hindering wind and water erosion better than silts and sandy soils, since the particles are adhered tightly to each other. Sand being the steadiest among the mineral components of soil; consists of rock fragments, primarily quartz particles, varying in size from 2.0 to 0.05 mm (0.0787 to 0.0020 in) in diameter. Silt differs in size from 0.05 – 0.002 mm (0.002 to 0.00008 in). Clay cannot be examined by optical microscopes as its particles range from 0.002 mm (7.9×10−5 in) or less in diameter and with thickness of only 10 angstroms (10−10 m). In medium-textured soils, clay is often washed downward through the soil profile and accumulates in the subsoil. Soil particles larger than 2.0 mm (0.079 in) are classified as rock and gravel and are removed before selecting the percentages of the remaining components and the texture class of the soil, but are included in the name. For example, a sandy loam soil with 20% gravel would be called gravelly sandy loam.

 

When the organic component of a soil is considerable, the soil is called organic soil rather than mineral soil. A soil is called organic if:

 

  1. Mineral composition is 0% clay and organic matter is 20% or more.
  2. Mineral composition is 0% to 50% clay and organic matter is between 20% and 30%.
  3. Mineral composition is 50% or more clay and organic matter 30% or more.

26.4 Soil Structure

 

Soil structure is the arrangement and compaction of soil particles into larger clusters, known as aggregates or ‘peds.’ (Fig 26.4). Aggregation is essential for enhancing the stability against erosion, for maintaining porosity as well as soil water movement, and for improving the fertility and carbon sequestration in the soil. ‘Granular’ structure comprises of loosely packed spheroidal peds that are bonded together mostly by organic substances.

 

Granular structure is characteristic of many A-horizons, with high SOM content and biological activities whereas large peds, in the form of plates, blocks, or prisms are characteristics associated with the B horizon and are formed by shrinks well processes and adhesive substances. As soil swells (wets or freezes) and then shrinks (dries or thaws), cracks are formed around soil masses, creating peds which are held together in place by the adhesion of organic substances, iron oxides, clays or carbonates. Channels and cracks between these peds are important for air, water, and solute transport as well as deep water drainage. Fine textured soils usually have a stronger, more defined structure in comparison to coarser soils formed as a result of shrink swell processes predominating in clay-rich soils and more cohesive strength between particles. The sand and clay particles that constitute the soil rarely occur as separate particles but are more or less loosely combined to form aggregates. The type of structure in soil depends to a large extent on the texture and the amount of organic matter in the soil and the way the land is managed. The aggregates that make up the structure may be as small in millimeters, such as granules and crumbs, or larger in centimeters, such as columns and prisms. The granular or crumb structure is the one favoured by farmers and gardeners as it makes a better bed for the seeds to grow. Soil structure determines its texture, biological activity, organic matter content, past soil evolution, human use, and the chemical and mineralogical conditions under which the soil formed. Whereas texture is determined by the mineral composition of the soil and is an inherent property of the soil that does not change with agricultural events, soil structure can be enhanced or damaged by the preference and timing of farming applications.

 

Soil structural classes

 

  1. Based on the shape and arrangement of peds
  • Ø Platy: Peds are flattened on the top the other 1–10 mm thick.

They are found in the A-horizon of forest soils and lake sedimentation.

 

  • Ø Prismatic and Columnar: Prism like peds are long in the vertical dimension, 10–100 mm wide. Prismatic peds have flat tops, columnar peds have rounded tops. Tend to form in the B-horizon in high sodium soil where clay has accumulated.
  • Ø Angular and subangular: Blocky peds are imperfect cubes, 5–50 mm, angular have sharp edges, subangular have rounded edges. Tend to form in the B-horizon where clay has accumulated and indicate poor water penetration.

 

 

Angular   Sub Angular

 

 

  • Ø Granular and Crumb: Spheroid peds of polyhedrons, 1–10 mm, most often found in the A-horizon in the presence of organic material. Crumb peds are more porous and are considered ideal.
  1. Based on the Size of peds whose ranges depend upon the above type
    • Ø Very fine or very thin: <1 mm platy and spherical; <5 mm blocky; <10 mm prism like.
  • Ø Fine or thin: 1–2 mm platy, and spherical; 5–10 mm blocky; 10–20 mm prism like.
  • Ø Medium: 2–5 mm platy, granular; 10–20 mm blocky; 20-50 prism like.
  • Ø Coarse or thick: 5–10 mm platy, granular; 20–50 mm blocky; 50–100 mm prism like.
  • Ø Very coarse or very thick: >10 mm platy, granular; >50 mm blocky; >100 mm prism like.
  1. Based on the Grades: Grade is a measure of the degree of development or cementation within the peds that results in their strength and stability.
  • Ø Weak: Weak cementation allows peds to fall apart into the three textural constituents, sand, silt and clay.
  • Ø Moderate: Peds are not distinct in undisturbed soil but when removed they break into aggregates, some broken aggregates and some unaggregated material. This is considered ideal.
  • Ø Strong: Peds are distinct before removed from the profile and do not break apart easily.
  • Ø Structure less: Soil is entirely cemented together in one great mass such as slabs of clay or no cementation at all such as with sand.

 

At the bulkiest scale, the forces that constitute the soil’s structure leads to swelling and shrinkage that primarily tends to act horizontally, causing vertically oriented prismatic peds. Whereas clayey soil, as a result of its differential drying rate with respect to the surface, will produce horizontal cracks, shrinking columns to blocky peds. Rodents, Roots, worms, and freezing-thawing cycles contribute into breaking the peds into a spherical shape. At a lower scale, plant roots expand into voids thereby removing water and increasing the open space, thus decreasing aggregate size. Simultaneously roots, fungal hyphae, and earthworms results into microscopic tunnels that break up peds. Soil aggregation continues as bacteria and fungi ooze out sticky polysaccharides which bind soil into smaller peds. The augmentation of the raw organic matter which bacteria and fungi feed upon stimulates the formation of desirable soil structure. The soil chemistry influences the aggregation as well as dispersal of soil particles. The clay particles contain polyvalent cations which give negative charges. At the same time, the edges of the clay plates have slightly positive charge, thus allowing the edges to stick to the negative charges on the faces of other clay particles or to flocculate (form clumps). On the other hand, when monovalent ions, such as sodium, replace the polyvalent cations, weaken the positive charges on the edges, whereas the negative surface charges are relatively strengthened. This leaves negative charge on the clay faces that repel other clay, causing the particles to push apart, and by doing, the flocculation of clay particles into larger, open assemblages. As a result, the clay particles disperse and settle into spaces between peds. In this way the open structure of the soil is destroyed and the soil is made porous to air and water. Such sodic soil tends to form columnar structures near the surface.

 

26.5 Density

 

Soil particle density ranges from 2.60 to 2.75 grams per cm3 and is usually constant for a particular soil. Soils with high organic matter content have lower value of soil particle density and are higher for soils with high iron-oxides content. Soil bulk density is equal to the dry mass of the soil divided by the volume of the soil; i.e. It includes air space and organic materials of the soil volume. The soil bulk density of cultivated loam is approximately 1.1 to 1.4 g/cm3 (for comparison water is 1.0 g/cm3). Soil bulk density is extremely variable for a given type of soil. A lower bulk density by itself does not indicate suitability for plant growth due to the impact of soil texture and structure. A high bulk density is indicative of either soil compaction or high sand content. Soil bulk density is inherently always less than the soil particle density.

 

26.6 Porosity

 

Porosity also called pore space is that part of the bulk volume of soil which is not occupied by either mineral or organic matter but is open space occupied by either gases or water. In a constructive, medium-textured soil the total pore space is about 50% of the soil volume. Pore size changes considerably; the smallest pores (cryptopores; <0.1 µm) hold water too tightly for use by plant roots; plant available water is held in ultra microspores and mesopores (0.1-75 µm) and micropores (>75 µm) are generally filled with air when the soil is at field capacity. Soil texture demonstrates total volume of the smallest pores; clay soils have smaller pores, but more total pore space than sands. Soil structure has a strong impact on the larger pores that affect soil aeration, water infiltration and drainage. Tillage has the short-term facilitation of temporarily enhancing the number of pores of largest size, but these can be rapidly breakdown by the devastation of soil aggregation. The pore size distribution improves the ability of plants and other organisms to access water and oxygen; large, continuous pores allow rapid transmission of air, water and dissolved nutrients through soil, and small pores store water between rainfall or irrigation events. Pore size variation also divides the soil pore space such that many microorganisms are not in direct competition with one another, which may explain not only the large number of species present, but the fact that functionally unwanted microorganisms can co-exist within the same soil.

 

26.7 Color

 

Soil color is the first impression one has while viewing the soil. Striking colors and contrasting patterns are particularly noticeable. The Red River of the South supports sediment eroded from extensive reddish soils like Port Silt Loam in Oklahoma. The Yellow River in China holds yellow sediment from eroding loess soils. Mollisols in the Great Plains of North America are darkened and supplemented by organic matter. Podsols in boreal forests have variable contrasting layers as a result of acidity and leaching. In general, color is determined by the content of organic matter, drainage conditions and degree of oxidation. Whereas it has little benefit in predicting soil characteristics. It is used in differentiating boundaries within a soil profile, defining the origin of a soil’s parent material, as an indication of wetness and waterlogged conditions, as well as a qualitative means of measuring organic, salt and carbonates contents of soils. Color is recorded in the Munsell color system as for instance 10YR3/4 Dusky Red. Soil color is primarily influenced by soil mineralogy. Variations in soil colors are due to various iron minerals. The development and distribution of color in a soil profile result from chemical and biological weathering, especially redox reactions. As the primary minerals in soil parent material weather, the elements combine into new and colorful compounds. Iron forms secondary minerals of a yellow or red color, organic matter decomposes into black and brown compounds, and manganese, sulfur and nitrogen can form black mineral deposits. These pigments can produce different color patterns within a soil. Aerobic conditions results in uniform or gradual color changes, while reducing environments (anaerobic) result in rapid color flow with complex, mottled patterns and points of color concentration.

26.8 Water Holding Capacity

All soils have the tendency to hold water in their pores as well as on the surfaces of mineral grains and structural aggregates. This ability differs from soil to soil and closely relates to the texture of the soil. Sandy soils cannot hold onto much water and have a poor water holding capacity. They are often known as thirsty soils. Clay soils by contrast have many small pores which can be used to store water. This means that they always have some water for the plants that grow in them and thus have a good water holding capacity.

 

Table 2 The water holding capacity of different soil types

 

Textural soil Water holding capacity, inches/foot of soil
Coarse sand 0.25-.75
Fine sand 0.75- 1
Loamy sand 1.10-1.20
Sandy sand 1.25-1.40
Fine sandy loam 1.50-2.0
Silt loam 2.0-2.50
Silt clay loam 1.80-2.0
Silt clay 1.50-1.70

 

 

Soils that can hold a large amount of water support more plant growth and are less susceptible to leaching losses of nutrients and pesticides. All the water sustained by soil is not obtainable for plant growth. Two different laboratory tests are conducted to determine how much plant available water a soil can hold. The first test run on the soil determines the amount of water the soil can hold at field capacity. The second test determines how much water the soil holds when plant roots can no longer extract water (wilting point). The water available for plant growth is the difference between field capacity water content and wilting point water content.

 

Field capacity water content – wilting point water content = Plant available water Field Capacity Water Content (1/3 Bar Water Content)

 

The first step in selecting the field capacity water content of a soil is to place a dry pulverized soil sample on a ceramic plate. The sample is then saturated with water and left to equilibrate overnight. The next day, the porous ceramic plate is placed into a container that is pressurized with 1/3 atmosphere of pressure (about 5 psi). The slight pressure in the container pushes excess water out of the soil sample through the ceramic plate. After 24 hours the moisture content in the soil sample is said to be at field capacity. The soil samples are then weighed, placed in an oven at 105oC for two hours and then weighed again.

Wilting Point Water Content (15 Bar Water Content)

The next step is to determine how much water the soil holds when it is so dry that plant roots can no longer remove water (wilting point). First a dry pulverized soil sample is placed on a ceramic plate and saturated with water overnight. The next day the ceramic plate is placed into a container that is  pressurized with 15 atmospheres of pressure (about 225 psi). This pressure pushes most of the water out of the soil sample and through the ceramic plate. The samples must be left in this pressurized container for 48 hours. The samples are then weighed before they are placed in an oven at 105oC for two hours to remove the remaining water. The amount of water that is left in the soil is held too tightly for plants to extract (hygroscopic water). Once this step is completed the amount of plant available water in the soil can be calculated as shown in an example below. Field Capacity Water Content (1/3 Bar Water Content) = 14.9%

 

Wilting Point Water Content (15 Bar Water Content) = 4. 1 % Available water= 14.9% – 4. 1 % = 10.8%

 

26.9 Resistivity

 

Soil resistivity is a measurement of soil’s ability to hinder the conduction of an electric current. The electrical resistivity of soil can affect the rate of galvanic corrosion of metallic structures in contact with the soil. Higher moisture content or increased electrolyte concentration can reduce resistivity and enhance conductivity, thereby increasing the corrosion rate. Soil resistivity values typically vary between 2 to 1000 Ω·m, but more extreme values are not unusual.

 

26.10 Consistency

 

Consistency is the tendency of soil to adhere to itself or to other objects (cohesion and adhesion respectively) as well as its ability to abstain from deformation and rupture. It is of approximate use in determining cultivation problems and the engineering of foundations. Consistency is measured at three moisture conditions: air-dry, moist, and wet. Under such conditions the consistency quality depends upon the content of clay. In the wet state, the two qualities of adhesiveness and plasticity are estimated. A soil’s resistance to disintegration and crumbling is determined in the dry state by rubbing the sample. Its resistance to shearing forces is examined in the moist state by thumb and finger pressure. Additionally, the cemented consistency depends on cementation by substances other than clay, such as calcium carbonate, silica, oxides and salts; moisture content has little effect on its assessment. The measures of consistency border on subjective compared to other measures such as pH, since they employ the apparent feel of the soil in those states.

The terms used to describe the soil consistency in three moisture states and a last not affected by the amount of moisture are as follows:

 

Consistency of Dry Soil: loose, soft, slightly hard, hard, very hard, extremely hard Consistency of Moist Soil: loose, very friable, friable, firm, very firm, extremely firm Consistency of Wet Soil: non sticky, slightly sticky, sticky, very sticky; non plastic, slightly plastic, plastic, very plastic Consistency of Cemented Soil: weakly cemented, strongly cemented, indurate (requires hammer blows to break up).

Soil consistency is useful in determining the ability of soil to support buildings and roads. More precise measures of soil strength are often made prior to construction.

 

26.11 Temperature

 

Soil temperature is governed by the ratio of the energy absorbed to that lost. Soil temperature ranges between -20 to 60 °C. It regulates seed germination, root growth and the availability of nutrients. Below 50 cm (20 in), soil temperature sometimes changes and can be approximated by adding 1.8 °C (2 °F) to the mean annual air temperature. Soil temperature has significant seasonal, monthly and daily variations. Fluctuations in soil temperature are much lower with increasing soil depth. Heavy mulching lowers the warming of soil, and at the same time, decreases the fluctuations in surface temperature. Soil temperatures affect the anatomical and morphological characteristics of root systems. All physical, chemical, and biological processes in soil and roots are affected as a result of the increased viscosities of water and protoplasm at low temperatures. In general, climates that do not prevent survival and growth of white spruce above ground are sufficiently benign to provide soil temperatures able to maintain white spruce root systems.

26.12 Water flow in soils

Water moves through soil due to the force of gravity, osmosis and capillarity. At zero to one-third bar suction, water is pushed through soil from the point of its application under the force of gravity and the pressure gradient created by the pressure of the water; called as saturated flow. At higher suction, water movement is pulled by capillarity from wetter to dryer soil. This is caused by water’s adhesion to soil solids, and is called unsaturated flow.

 

Water infiltration and movement in soil is controlled by six factors:

 

  1. Soil texture
  2. Soil structure. Fine-textured soils with granular structure are most favorable to infiltration of water.
  3. Coarse matter is best as it helps in preventing the destruction of soil structure and the creation of crusts.
  4. Depth of soil to impervious layers such as hardpans or bedrock
  5. The amount of water already in the soil
  6. Soil temperature. Warm soils take in water faster while frozen soils may not be able to absorb depending on the type of freezing.

 

Water infiltration rates range from 0.25 cm (0.098 in) per hour for high clay soils to 2.5 cm (0.98 in) per hour for sand and well stabilized and aggregated soil structures. Water flows through the ground unevenly, called “gravity fingers”, because of the surface tension between water particles. Tree roots create paths for rainwater flow through soil by breaking though soil including clay layers and the flooding temporarily increases soil permeability in river beds, helping to recharge aquifers.

 

Summary:

 

The soil texture, structure and porosity influence the movement and retention of water air and solutes in the soil which subsequently effect the plant growth and activity of organisms. Soil consists of minerals, soil organic matter (SOM), water, and air. The combination of these elements greatly changes the soil physical properties, such as structure, texture, porosity as well as the pore space in a soil. Soil physical properties have detrimental effect on many soil processes. Due to excess use of chemicals soil quality decreases and therefore it becomes essential for analysis of soil parameter. Above information regarding the soil help the farmers for use of management practice to maintain soil status like practices such as tillage can have effect on soil structure rapidly.

Some of the properties like the movement and storage of soil water, the aeration and the mechanical resistance of a soil governs the plant root penetration and establishment in the soil which is very important for the farmers to have good crops in hand. Therefore proper land use and management should be adopted so that it may not have constraints on productivity of our agricultural lands by having profound impact on soil properties and indirectly affecting soil quality. In this way we can have sustainable agricultural in the long run and it is vital for producing the food and fiber that humans need. Hence maintaining the ecosystems on which ultimately all life depends.

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