1. Physical Properties of Soil
Chapter 4
Physical Properties of Soil

2. Objectives After completing this topic, you should be able to:
Describe the concept of soil texture and its importance
Identify the texture of a sample of soil
Describe soil permeability and related properties
Describe structure and its formation and importance
Explain other physical properties
Discuss soil compaction and tilth

3. Physical Properties of Soil
Soil characteristics: grower can see or feel
Neither chemical nor biological, but both affect them
Greatly affect how soils are used to grow plants or other activities

4. Soil Texture Most fundamental soil property
Determined by the proportion of soil particles
Sand (large)
Silt (medium)
Clay (small)
The size of soil particles, in turn, affects such soil traits as water-holding capacity and aeration

5. Effects of Particle Size
Soil particle size affects two important features:
Specific surface area
Soil pores: number and size
Specific surface area is defined as the amount of surface area exposed by all the particles in a certain weight of soil. 
The smaller the soil particles, the greater the specific surface area
Soil surface area is important because reactions occur on the surface of soil particles, and because water is held as a film around soil particles
Thus the smaller the particles in a soil, the more water and nutrients the soil can retain

6. Soil Pores Soil pore number and size depends on particle size
Macropores (aeration pores): large
Micropores: small
(A) small particles create many small pores.
(B) Pores are larger but fewer in number between large particles. Micropores usually hold water, macropores air.
Sometimes the larger micropores are distinguished as mesopores, medium-sized pores that hold readily plant-available water

7. Soil Separates
Soil scientists divide mineral particles into size groups called soil separates
This consists of three broad classes
Sand (divided into four subcategories)
Silt
Clay
These three together make up the fine earth fraction of soil used to determine texture
Larger particles, such as gravel, are considered to be coarse fragments, and are not considered in texture

8. The USDA system of soil separates
The USDA system of soil separates. The comparison shows the difference by setting a very coarse sand grain equal to 3 feet in diameter. Engineers use a different system
Separate
Diameter (mm)
Comparison
Feel
Very coarse sand
2.00–1.00
36″
Grains easily seen, sharp, gritty
Coarse sand
1.00–0.50
18″
Medium sand
0.50–0.25 9″
Fine sand
0.25–0.10
4½″
Gritty, each grain barely visible
Very fine sand
0.10–0.05
1¾″
Silt
0.05–0.002
7/16″
Grains invisible to eye, silky to touch
Clay
<0.002
1/32″
Sticky when wet, dry pellets hard, harsh

9. Comparing the size of soil separates
Comparing the size of soil separates. On this scale, very coarse sand would be 3 feet across

10. Sand Characteristics Largest of the soil separates
Composed mainly of weathered grains of quartz or other minerals
Particles range in size
Enough sand in a soil creates large pores, so sand improves water infiltration (rate at which water enters the soil) and aeration
Large amounts of sand lower the ability of the soil to retain water and nutrients

11. Silt Characteristics Medium-sized soil separate
Silt particles are silky or powdery to the touch, like talc
Best ability to hold large amounts of water in a form plants can use
Erodes readily in moving water and wind

12. Clay Characteristics Smallest of the soil separates
Consists of tiny, sheet-like crystals
Results from chemical reactions between weathered minerals to form tiny particles of new minerals
USDA size classification for stones in the soil.
Class
Diameter Range (mm)
Diameter Range (in.)
Gravel
2–75
1/12–3
Cobbles
75–250
3–10
Stones
250–600
10–24
Boulders
>600
>24

13. Textural Classification
Soil usually consists of more than one soil separate
All three separates are found in most soils
Actual percentages are called soil texture
12 textural classes are shown in the soil triangle
Another important textural name is loam, a soil in which sand, silt, and clay contribute equally to the soil’s properties.
Determining soil texture
Amount of sand, silt, and clay in a soil can be measured by mechanical analysis
Mechanical analysis is based on the fact that the larger a soil particle, the faster it sinks in water
E.g. it takes only 45 seconds for very fine sand to settle through 4 inches of water, but it takes about 8 hours for large clay particles

14. Question
Identify a soil that is 10% cay, 10% silt and 80% sand

15. Soil Triangle Each side of the triangle is a soil separate
Soil Triangle Each side of the triangle is a soil separate. The numbers are percentage of soil particles of that type.

16. Soil triangle redrawn to show fine-, medium-, and coarse-textured soils.

17. Characteristics of Textural Classes
Soils can generally be classified as fine, medium, or coarse
Indicative of a number of soil properties
Infiltration –water entering the soil
Percolation –water draining through soil
Water-holding capacity
Fine soils retain plant nutrients better than coarse soils.
This is true partly because the rapid percolation of water through coarse soil leaches out nutrients.
Also, clay particles have the best ability to retain nutrient chemicals.

18. Modifying soil texture
Impractical, except in small areas e.g. golf greens or potting soils
Very large quantities of sand are needed to loosen clay soils—enough that sand grains touch and there isn’t enough clay to fill all the gaps.

19. Soil Density and Permeability
Particle density (PD)
This is the density of solid particles only
Average 2.65 grams per cubic centimeter
PD varies according to the type of minerals in the parent material and the amount of organic matter in the soil.
Mineral
Density (grams/cm3)
Density (lbs/ft3)
Water
1.0
62.5
Quartz
2.65
166
Feldspars
2.5–2.7
156–169
Micas
2.7–3.0
169–188

20. BD = Weight dry soil/Volume dry soil = g/cm3
Bulk density
Actual density of a soil is less than the PD
 It is the mass of a volume of undisturbed oven-dry soil.
To measure BD, a core of soil of known volume is carefully removed from the field.
The soil core is then dried in an oven at 105°C until it reaches a constant weight
The example that follows is for a core of 500 cubic centimeters (cm3) that weighs 650 grams (g):
BD = Weight dry soil/Volume dry soil = g/cm3
Determine the bulk density of the soil given in the above example
Determine the volume of the particles in the soil given in the above example (Hint: use the average particle density of soils to solve this problem)

21. Questions
A oven dry soil weighs 800 grams and has a bulk density of 1.3 g/cm3
What is the volume of the undisturbed soil?
What is the volume of the soil particles?

22. Porosity = wet weight (g) − dry weight (g) x 100 soil volume (cm3)
Soil porosity
Measure of the soil volume that holds air and water
 usually expressed as a percentage
Thus, a soil with a 50 percent porosity is half solid particles and half pore space.
Porosity can be measured by placing an oven-dry soil core in a pan of water until all of the empty pore space is filled with water.
The water volume, which fills the pore space, divided by the total core volume, is porosity.
Example:
The soil core used as an example before had a volume of 500 cubic centimeters and weighed 650 grams when dry. When wet, the same core weighed 900 grams. Porosity is thus calculated as follows:
Porosity = wet weight (g) − dry weight (g) x 100
soil volume (cm3)

23. Porosity = 900 − 650 × 100 = 50%
500 
Porosity can also be calculated from BD and PD.
If there were no pore space, then BD would be the same as PD
The ratio BD/PD would be equal to 1
The more the pore space, the smaller the BD and smaller the ratio BD/PD
The ratio BD/PD is simply the percentage of the soil that is solid matter
If one subtracts that percentage from 100 percent, the difference is the percentage of pore space
 The following equation can be used to calculate porosity:
Porosity = 100% − BD × 100
PD 
If we substitute the values for the BD calculated earlier:

24. Porosity = 100% − 1.3  × 100 = 50%
2.65
The porosity of sand (about 30 percent) is lower than that of clay (about 50 percent)

25. Permeability
Ease with which air, water, and roots move through the soil
In highly permeable soil, water infiltrates soil rapidly, and aeration keeps roots well supplied with oxygen.
Permeability depends partially on the number of soil pores, but it depends more on the size and continuity of the pores. 
Permeability is simply a descriptive term with no numerical value; it cannot be measured directly
Hydraulic conductivity is a measure of the rate of water movement through a soil.
Coarse soils might have conductivities of 1 and 1.5 inches per hour or more, while fine-textured soils might measure a hundredth of that

26. Soil Structure
Structure refers to the way soil particles clump together into large units
These large units are called soil aggregates
Peds: naturally occurring aggregates
Clods: clumps of soil caused by tillage
Well-aggregated soils contain large, continuous pores that promote good air and water movement and that provide easy pathways for root growth.
Classification traits
Type: shape
Class: ped size
Grade: ped distinctiveness and strength

27. Types of soil structure

29. Structureless Soil Single-grain Massive soils
Sandy soils and soils without structure
Massive soils
Fine textured soils that lack structure or function as a soil mass
Lack permeability

30. Types of Soil Structure
Granular structure
Commonly found in A horizons
Platy structure
Usually found in E horizons
Blocky structure
Typical of many B horizons
Prismatic structures
Tend to occupy lower B and C horizons

31. Formation of Soil Structure
Two-step process
Soil creates a loose ped and the second cements it
Weak aggregates are cemented to make them distinct and strong
Structure is not a permanent soil feature
Can be degraded by mismanagement

32. Soil Consistence Behavior of soil when pressure is applied
Relates to the degree that soil particles stick to one another or other objects
Soils types and characteristics
Wet soil: stickiness and plasticity
Moist soil: loose, friable, and firm
Dry soil: determined by trying to crush an air-dried mass of soil in the hand

33. Soil Tilth Physical condition of tilled soil Tillage
Suggests how easy the soil is to till, a good seedbed can be made, how easily seedlings can come up, and the ease of root growth
Tillage
Improves soil tilth for a time, improving soil air–water relations for new seedling

34. Compaction Results when pressure is applied to the soil surface
Primarily alters soil traits related to pores and soil strength
Can create:
Reduced porosity and permeability
Reduced air exchange and potassium uptake
Decreased infiltration rates
Increased erosion and evolution of nitrous oxide
Reduced percolation and oxygen availability

35. Physical Property Management
Considerations
Avoid aggregate destruction
Avoid puddling and clods
Minimize surface crusting
Improve tilth

36. Soil Channels and Pans Channels Any hardened layer of soil is a pan
Large, continuous pores extending from the surface and leading deeper into the soil
Any hardened layer of soil is a pan
Claypans
Fragipans
Plinthite
Caliche
Duripans

37. Soil Temperature Critical for plant growth and development
Important factors
Sunlight and air energy inputs
Absorption and conductance of heat
Loss of heat at the surface
Temperature effects
Affects seed germination, root growth, as well as water and nutrient availability and biological activity

38. Soil Temperature (cont’d.)
Managing soil temperature
Natural conditions
Watering and soil manipulations
Mulching
Frost heaving
Inorganic materials and plastic sheeting
Fire and soil temperature
Soil temperature affects soil and plants in natural ecosystems
Fire is present or necessary in many natural ecosystems

39. Soil Color Indicates: Color as a guide to soil use
Nutrient composition
Organic matter
Drainage in soils
Color as a guide to soil use
Used to classify soils according to color

40. Summary This chapter reviewed several topics
Texture and textural classes
Permeability
Consistence
Tilth
Compaction
Physical property management
Temperature and color