http://broome.soil.ncsu.edu/ssc012/Lecture/topic8.htm

Physical Properties

Features of the soil profile and the soil horizons are often described in the field in terms of the soil’s physical properties. Horizons are defined based on difference in the physical properties.

Horizons differ from horizons above and below by some physical property.

Soil physical properties affect the appearance and feel of a soil.

The major soil physical properties are:
o Soil Texture
o Soil Structure
o Soil Consistence/Soil Strength
o Soil Color
o Soil Permeability
o Soil Temperature

1. Soil Texture

1.1. Each soil separate represents a distinct physical size group.

Mineral particles less than 2 millimeters in equivalent diameter and ranging between specified size limits. The names and sizes of the 7 soil separates recognized in the United States are as follows:

Very Coarse Sand  2.0 – 1.0 mm
Coarse Sand  1.0 – 0.5 mm
Medium Sand  0.5 – 0.25 mm
Fine Sand  0.25 – 0.10 mm
Very Fine Sand  0.10 – 0.05 mm
Silt  0.05 – 0.002 mm
Clay  0.002 mm
1.2. We will be using these separates the rest of the semester:
Sand  2.0 – 0.05 mm
Silt  0.05 – 0.002 mm
Clay  <0.002 mm

1.3. Think of this relationship for the three main groups:
1. sand = basketball (sand may be seen with the naked eye)
2. silt = golf ball (silt particles may be seen with a good microscope)
3. clay = pin (clay can only be seen with an electron microscope)

Modifiers that are used for coarse materials that are greater than 2 mm.
gravels  2mm – 3 in
cobbles  3 in – 10 in
stones  10 in – 24 in
boulders  24 in
The term soil separate refers to a specific size of a particle and not the composition of that particle. However, certain minerals will tend to dominate or make up the various separates.
Sands  generally made up of quartz
Silts  commonly composed of quartz and feldspars
Clays  secondary minerals, clay minerals, and Fe oxides

A soil’s texture is then the relative proportion of sand, silt, and clay. Together, the three separates must add up to be 100%. The organic matter is not a part of the soil’s texture. Since there is such a large number number of combinations that could occur, soil scientists group similar amounts of sand, silt and clay into groups called soil textural classes.

1.4. Textural Classes:

The textural class is determined by the combined portion of sand, silt and clay

Textural class affects properties and management of soils.
The smaller the particle size, the greater the total surface area of a given volume of soil.
Surface Area
o 1g of sand = 1.5 ft2
o 1g of silt = 14.9 ft2
o 1g of clay = 262,467 ft2
Surface Area affects the:
adsorption of water
area for chemical reactions
adsorption of nutrients
plasticity
shrink/swell

1.4.1. Texture also influences:

a. porosity (amount and size of pores)
b. plant available water holding capacity

sand  few large pores  low water holding
silt  many medium pores  high plant avl. water holding
clay  many fine pores  high total water holding not all available to plants

Since most reactions and water holding capacity are related to surfaces, you can see that clay will be very reactive and hold much water.

c. Texture is largely determined by parent material and the amount of weathering
d. Texture is modified by soil formation:
 A horizon – coarse textured
 E horizon – coarse textured
 B horizon – fine textured

e. Generally, texture of a field can’t easily be changed
 too expensive to add enough material
 not really practical

1.4.2. Soil Texture and Management 
a. Coarse Textured Soils (sands, loamy sands)
1. Hold low amounts of water and nutrients – may have to irrigate and fertilize frequently
2. Not highly erosize – allows water to infiltrate; less runoff
3. Very permeable – good for waste disposal only if a deep soil
4. May compact to form a hardpan

b. Fine textured soils (clays, sandy clays, silty clays)
1. Hold large amounts of nutrients and water – may hold things too tightly
2. Erosion – slow infiltration; high runoff means much erosion.
3. Slow permeability – often unacceptable for septic tanks
4. May shrink/swell – depends upon the type of clay minerals present

c. Medium Textured Soils (loams, clay loams, sandy loams, silt loams)
1. Properties fall in between the other two.

2. Soil Structure

Structure is the arrangement of primary sand, silt and clay particles into secondary aggregates called peds or structural units which have distinct shapes and are easy to recognize. These differently shaped aggregates are called the structural type.
2.1. There are 5 basic types of structural units:
2.1.1. Platy: Plate-like aggregates that form parallel to the horizons like pages in a book.
a. This type of structure may reduce air, water and root movement.
b. common structure in an E horizon and usually not seen in other horizons.

2.1.2. Blocky: Two types–angular blocky and subangular blocky
a. These types of structures are commonly seen in the B horizon.
b. Angular is cube-like with sharp corners while subangular blocky has rounded corners.

2.1.3. Prismatic: Vertical axis is longer than the horizontal axis. If the top is flat, it is referred to as prismatic.

If the top is rounded, it is called columnar.
2.1.4. Granular: Peds are round and pourous, spheroidal. This is usually the structure of A horizons.
2.1.5. Structureless: No observable aggregation or structural units.
a. Single grain-sand
b. Massive-solid mass without aggregates

2.2. Grade of structure – Describes stability of the aggregates.
2.2.1. structureless
2.2.2. weak
2.2.3. moderate
2.2.4. strong

2.3. Class of structure – Describes size of the aggregates.
2.3.1. very fine
2.3.2. fine
2.3.3. medium
2.3.4. coarse
2.3.5. very coarse

The size of each category varies with the type of structure.
2.4. Formation of soil structure
2.4.1. freeze / thaw
2.4.2. wetting / drying
2.4.3. root pressure
2.4.4. microorganisms
2.4.5. cementing by clay, organic matter, iron and aluminum compounds

2.5. Importance of Soil Structure
2.5.1. Increases infiltration of water, thus reducing runoff and erosion and increases the amount of plant available water.
2.5.2. Improves seedling emergence, root growth and rooting depth.
2.5.3. Large continuous pores increase permeability.

2.6. Maintaining Soil Structure
2.6.1. Till soil only at the proper moisture contents. Never till when the soil is too wet. This will cause the soil to become cloddy. Aggregates are easily destroyed.
2.6.2. Add the proper amounts of lime and fertilizer. Proper plant growth will lead to the development of good soil structure.
2.6.3. Grow grasses and legumes. These plants may help form unstable aggregates and their organic matter will help stablize the aggregate.
2.6.4. Growth of legumes will also give the soil more microorganisms which give certain beneficial fungi which will stabilize peds.

2.6.5. Maintain or increase organic matter contents of Ap horizon.
a. plant cover crops in fall and winter
b. plant more grasses
c. turn under crop residue
d. add manure

3. Soil Color

3.1. It is the most obvious and easily determined soil property

It has little direct effect on the soil, but is an indicator of soil properties. However, there are many things we can tell about the soil by observing the color.
a. Organic matter content; the more organic content the darker the soil color
b. Soil color and soil temperature : dark colored soils absorb more heat so they warm up quicker and have higher soil temperatures.
c. Soil color and parent material : generally dark parent material will develop into dark soils.
d. Soil color and drainage:
– soil drainage refers to the length of time a soil is waterlogged. Not how fast the soil is drained.

3.2. Relationship to Drainage

Soil Color is very important in determining a soils drainage and depth to the water table. It can be very important in predicting land use hazards.
3.2.1. Soil Drainage: refers to the length and duration of saturation.
 it is not a measure of how fast water drains from the soil.

3.2.2. Soil Drainage Classes
a. Well drained – no gray colors throughout the B horizon
b. Mod. Well drained – gray colors or mottles in the lower B horizon
c. Somewhat poorly drained – gray colors in the upper B horizon. Mottles also seen
d. Poorly drained – gray colors throughout the entire B horizon

3.3. Soil Color Measurement

3.3.1. Munsell Color System

Hue: dominant spectral color

Value: lightness – darkness; white to black

Chroma: color purity or intensity
a.
 Fe oxides; red, yellow, and light brown colors
 organic matter; dark brown or black colors

b.
 low color value = high organic matter content
 high chroma = well drained soil, oxidizing conditions

4. Soil Permeability

Permeability – ability of soil to transmit water or air. Expressed as cm of water/hour
4.1. Soil permeability vs Soil drainage
4.1.1. Permeability is the speed of air and water movement in a soil — this is affected by texture and structure
a. if permeability is high : water moves quickly
b. if permeability is low : water moves slowly

4.1.2. drainage is the frequency and duration of saturation. The time that the soil is waterlogged. — this is affected by landscape position and permeability
4.1.3. Another way to view this is; drainage refers to the amount of oxidation which has taken place in the soil and permeability
a. A sand could be permeable, but in a low landscape position and be poorly drained.
b. A clay could be very permeable, but in a low landscape position and be poorly drained.
c. A clay could be impremeable but well drained because it is at the top of a hill.

4.2. Factors affecting permeability

4.2.1. Pore size present in the soil

Water moves faster through larger pores than smaller pores. – the bigger the pore the more material it will move.
4.2.2. Pore Continuity

High permeability – contains pores which are continuous and large enough for air and water to pass through.
a. continuous
b. discontinuous
Just because a soil contains a large amount of pores doesn’t mean it is permeable. The pores could be discontinuous or very small.

Any factor that will affect pore size or continuity of pores will affect permeability.

This means soil permeability is related to soil texture and soil structure.
c. texture

Pores size decreases with smaller particle size. As the size of the separates decrease so does the pore size, but remember that the surface area will increase quickly.
Textural class Permeability
Sand  very high
Sandy loam
silt, loam  high
silt loam  med-low
Clay  low
4.2.3. Soil structure is related to the continuity of the pores
 The more well structured a soil the more permeable it will be. A soil with good structure will be more permeable than a soil of the same texture that has poor structure.

Natural aggregation of the separates will increase the large pore space. When soils with good structure are compacted the large pore space is lost because the big pores are squashed.

This means for a soil manager, the only way to change permeability is to change soil structure. This is why it is important to maintain good soil structure if it already exists.

If changes in pore size occur then changes in the permeability will occur.

Water may back up on the top of the B horizon.

Can be waterlogging for short periods of time. If waterlogging persists, plant roots may die due to lack of oxygen.

Beds of soil or ridges may help keep the roots above the b horizon and out of the waterlogged soil.

Other ways to affect permeability:
a. Root and worm channels:

increase permeability by forming large continuous pores through out the soil. These pores must extend from the soil surface to depth. These allow for rapid water infiltration.
b. organic matter : increases permeability by :

increasing the stability of soil aggregates. Remember that organic matter decomposes to form glues, gums, etc. … that help cement or stick soil particles together.

Organic matter forms large pores when plant residues are added or left on the soil surface.
5. Soil Consistence

Expresses cohesive and adhesive forces holding soil particles together; varies with moisture content. Describes the resistance of a soil at various moisture contents to mechanical stress or manipulation.
5.1. Described at three moisture levels:

5.1.1.

Wet

Stickiness (non-sticky, slightly sticky, sticky,very sticky)

Plasticity (non-plastic, slightly plastic, plastic, very plastic)

5.1.2. Moist

Very friable, friable, firm ,very firm
5.1.3. Dry

Loose.soft. slightly hard,hard, very hard ,extremely hard

5.2. Consistence indicates
a. amount and type of clay material
b. condition for tillage
c. potential for compaction

Consistence is the resistence of the soil to deform or rupture. Soil consistence is the forces of cohesion and adhesion that are holding the peds together. It refers to the degree of plasticity and stickiness of the soil.

Soil consistence is affected by the type and amount of clay that is in the soil.

Soil consistence indicates:
 where are zones that may restrict root growth and seedling emergence.
 whether a soil is likely to develop compacted zones; ruts, crusts, hardpans etc…

5.3. Determining soil consistence
a. Fingers – squeeze aggregates or push fingers into the soil.
b. Penetrometer – measures how hard it is to push into the soil. this would be the same effect as a plant root.
c. Examine roots : J roots or a root mat indicate problems.

Sand has a very weak consistence, there is little force between the particles. This means that a car tire can easily push the sand apart and it is easy to get stuck.

5.4. Factors Affecting Soil Consistence

3 Major Factors
Water Content
Soil Texture
Soil Density

5.4.3. Water Content – this is the most important.
a. soil particles will move more easily when in a wet soil. Water will act as a lubricant
b. soil strength increases as the soils dries out.
c. soils will compact, become densier, if tilled, trampled, driven over, etc….. when wet.
If a soil is worked or tilled when wet:
a. soil has low strength, particles will be easily moved.
b. Soils under wheels will be compacted (bulk density increases).
c. large pores are destroyed; cracks between peds and old root channels are squashed.
d. as soils dries, its strength increases. Roots may not be able to push particles aside.
e. root growth may be restricted
f. these systems are more susceptable to drought and may not get all fertilizer that is present.

5.4.4. Soil Texture
a. soil strength increases with increasing clay content
b. clayey soils are stronger than sandy soils.

5.4.5. Soil Density
As density increases so does soil strength. You have more material in the same area which makes it harder for the plant root to grow.

5.4.6. Measurements of Pore Space

5.4.6.1. Soil Bulk Density
 it is a measure of how compact or dense a soil is.
 it is weight of soil divided by the total volume (lbs/ft3 or g/cm3 or mg/cm3)

Density = mass (weight) / volume

Bulk Density
 relates weight of solids to total volume of soil including solids and pores.
 affected by both the nature of slids and the volume of pores.
 Bulk Density = weight of oven dry soil divided by the soil volume
 If a 100 cubic cm soil sample has an oven-dry weight of 150 grams then
5.4.6.2. Bulk density is affected by the solids and porespace

a high pore space = low bulk density
b low pore space = high bulk density
* fine textures silt loams, clay, clay loam – generally lower bulk density 1-1.6 g/cc
1. sandy soils may range 1.2 – 1.8 g/cc
2. strong structure (well granulated topsoil) – low bulk density
3. high organic matter = low bulk density

5.4.6.3. As bulk density increases:
a. soil strength increases –
b. pore space decreases
c. soils become more compact

Remember that bulk density is related to the amount of pores in the soil.
 the greater the total pore space the lower the bulk density

As bulk density increases the amount of pore space decreases.