Introduction to Soils
By Rick Bandy, Resource Soil Scientist
Natural Resources Conservation Service
Soil Forming Factors
• Parent Material
• Climate
• Living Organisms
• Topography
• Time
Physical Properties of Soils
• Soil Solids
1. Mineral Soil
2. Organic Matter
• Soil Pore Space
1. Air
2. Water
Mineral Constituents In Soils
Physical Properties of Soils
Soil Air
20-30%
Soil Air
Soil
1 Solids
Pore Space
Soil Water
Organic
Matter
Mineral
Soil
Soil Water
20-30%
Organic Matter
Soil Solids
Organic Matter
<5%
<10%
33-50%
33-50%
Living Organisms
Fresh Residue
Decomposing OM
Stabilized OM
Soil Minerals
• Sand
• Silt
• Clay
Soil Minerals
• Sand
- 2.0 to 0.05 mm diameter
- Individual particles visible to the eye
- Rounded or irregular shape
- Gives texture gritty feel
- Influences drainage and air movement
Soil Minerals
• Silt
- 0.05 to .002 mm diameter
- Magnify 10 to 100 times to see particles
- Irregular shape
- Gives texture soft, slippery, floury feel
- Largely influences water holding capacity
•Soil Minerals
• Clay
- Less than .002 mm diameter
- Magnify 2,000 times to see particles
- Gives texture sticky, greasy feel
- Largely influences fertility, permeability,
shrinking and swelling
Soil Textural Triangle
A. Miller
Soil Mineral Properties
Properties Sand
Silt
Clay
Water
Holding
Capacity
Aeration
Low
Medium
to High
High
Good
Medium
Poor
Drainage
Rate
High
Slow to
Medium
Very Slow
Soil Mineral Properties
Properties Sand
Silt
Clay
Soil O.M.
Level
Low
Medium
to High
High to
Medium
Decomp.
Of O.M.
Rapid
Medium
Slow
Warm-Up
in Spring
Rapid
Moderate Slow
Soil Mineral Properties
Properties Sand
Silt
Clay
Shrink
Very Low Low
Swell Pot.
Moderate
to V.High
Sealing of Poor
Ponds etc.
Poor
Good
Tillage
Good
after Rain
Medium
Poor
Bulk Density of Mineral Soils
In the field one cubic centimeter of a
certain soil appears as below
If all the solids were compressed to
the bottom, the cube would look
like this:
1/2 PORE SPACES
SOILDS AND PORE
SPACES
1.33 grams
1/2 SOLIDS
1.33 grams
To calculate bulk density of the soil:
To calculate solid particle density:
Volume = 1 cubic centimeter
Volume = 0.5 cc
Weight = 1.33 grams
Weight = 1.33 grams
Bulk Density = Weight of soil/Volume of soil
Solid particle density = Weight of solids/Volume of solids
Therefore
Therefore
Bulk Density = 1.33/1 = 1.33 g/cc
Solid particle density = 1.33/0.5 = 2.66g/cc
Soil Structure
Granular Soil Structure
Platy Soil Structure
Prismatic Soil Structure
Blocky Soil Structure
Columnar Soil Structure
Forms of water that are held
in soils
Capillary Water
Soil Particle
Adsorbed Water
SOIL WATER DIAGRAM
AIR SPACES
Water Holding Capacity of Soils
SATURATION
FIELD CAPACITY
Solid Space
Pore Space
100 g
SATURATED SOIL
40 g
100 g
100 g
WILTING POINT
20 g
10 g
Air
FIELD CAPACITY
Air
WILTING POINT
Soil Moisture Characteristics
of Individual Soil Particles
0 Atm.
1/3 Atm.
Soil Particle
15 Atm
31 Atm
Available
Water
Capacity
(AWC)
Bound Water
Permanent Wilting Point
Field Capacity
Saturated Soil
SOIL MOISTURE CHARACTERISTICS
AND TEXTURE
PERCENT SOIL WATER
30
25
20
FIELD CAPACITY
AVAILABLE WATER
15
10
UNAVAILABLE WATER
5
0
SAND
WILTING POINT
SANDY LOAM
LOAM
SILT LOAM
TEXTURAL CLASS
CLAY LOAM
CLAY
AVAILABLE WATER CAPACITIES
FOR SOIL TEXTURAL CLASSES
SOIL TEXTURAL
CLASS
ESTIMATED AVER
PLANT AWC (IN/FT
Sandy Soils
Coarse textured
Sands
Loamy sands
Loamy fine sands
Loamy V. fine sands
Fine sands
V. fine sands
0.5
1.0
1.25
1.25
1.25
1.25
Loamy Soils
Moderate Coarse
texture
Sandy loam
Fine sandy loam
1.5
1.5
Medium texture
V. fine sandy loam
Loam
Silt loam
Silt
2.0
2.0
2.0
2.0
Clay loam
Sandy clay loam
Silty clay loam
2.2
2.2
2.2
Sandy clay
Silty clay
Clay
2.0
2.0
2.0
Moderately fine
Texture
Clayey Soils
Fine texture
AWC ADJUSTED FOR ROCKS AND
SALT CONTENT IN SOILS
30
.22
25
.20
.18
.16
.14
.12
.10
.02
.04
.06
70
60
20
50
40
15
30
2010
0
0
4
12
8
.08
.10
.12
.14
.08
.16
.06
.18
.04
.20
.02
.22
0
WATER TABLE
Capillary
Fringe
Water
Table
STRATIFIED SOILS
Clay
Sand
RESTRICTIVE LAYER
Sandy Loam
Clay
SOIL REACTION
The degree of acidity or alkalinity of a soil,
usually expressed as a pH value or by terms
ranging from extremely acid to very strongly
alkaline
pH scale
Ranges found
in soils
Bleach
12
Milk of
Magnesia
Antacid
tablets
Sea Water
Pure water
Natural rain
11
10
Sodic soils
9
8
7
6
Calcareous
soils
Humid region
soils
5
Beer, coffee
4
Vinegar
3
Lemon Juice
2
1
Battery acid
0
Forest soils
Acid sulfate
soils
PLANT NUTRIENT
AVAILABILITY
AFFECTS OF pH ON SOILS
• Plant nutrient availability
• Microbial populations
• Types of Vegetation
• Movement of nutrients and pollutants
Where to get soil information
• Each County has a published soil survey
that provides detailed soil maps.
• The local NRCS office has copies of the
published soil survey for that county.
• Web Soil Survey is a web based tool
where soil maps can be generated online. http://soils.usda.gov/survey/
Soil Food Web
Ag Land
Prairie
Forest
Organisms per gram (teaspoon) of soil
Bacteria 100 mil. -1
bil.
100 mil. -1 bil.
100 mil. -1
bil.
Fungi Several
yards
10s – 100’s of
yds
1-40 miles
1000’s
100,000’s
10’s – 100’s
100’s
Protozoa 1000’s
Nematodes 10-20
(in conifers)
Organisms per square foot
Arthropods < 100
500-2000
10,00025,000
Earthworm 5-30
s
10-50
10-50
(0 in
conifers)
Soil Forming Factors
• Parent Material
• Climate
• Living Organisms
• Topography
• Time