Physical Properties of Soil
Physical Properties of Soil
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
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
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
Effects of Particle Size
Soil particle size ffects 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
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, mediumsized pores that hold readily plantavailable water
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
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
Very coarse
sand
2.00–1.00
36″
Coarse sand
1.00–0.50
18″
Medium sand
0.50–0.25
9″
Fine sand
0.25–0.10
4½″
Very fine sand
0.10–0.05
1¾″
Silt
0.05–0.002
7/16″
Grains invisible to eye, silky to
touch
<0.002
1/32″
Sticky when wet, dry pellets
hard, harsh
Clay
Feel
Grains easily seen, sharp, gritty
Gritty, each grain barely visible
Comparing the size of soil separates. On this scale, very coarse sand would be 3
feet across
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
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
Clay
• Characteristics
– Smallest of the soil separates
– Consists of tiny, sheet-like crystals
– Results from chemical reactions between weathered
Class
Diameter
Range
Diameter Range
minerals
to form
tiny
particles
of
new
minerals
(mm)
(in.)
Gravel
2–75
1/12–3
– USDA
size classification
for stones
in the soil.
Cobbles
75–250
3–10
Stones
250–600
10–24
>600
>24
Boulders
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
Question
Identify a soil that is 10% cay, 10% silt and 80% sand
Soil Triangle
Each side of the triangle is a soil
separate.
The numbers are percentage of
soil particles of that type
.
Soil triangle redrawn to show fine-, medium-, and coarse-textured soils.
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.
• 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.
Soil Density and Permeability
• Particle density (PD)
– This is the density of solid particles only
– Average 2.65 grams per cubic centimeter
3 minerals in
3
– PD
varies according
to the
type of
Mineral
Density
(grams/cm
)
Density (lbs/ft )
the parent material and the amount of organic
matter in the soil.
Water
1.0
62.5
Quartz
2.65
166
Feldspars
2.5–2.7
156–169
Micas
2.7–3.0
169–188
• 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)