Mr. Gust
Soil Definition
 A layer of natural materials on the earth’s surface
containing both organic and inorganic materials
and capable of supporting plant life.
Why is Soil Important?
 Gives plants something to grow in
 Holds nutrients
 Home to microorganisms
 Holds water
 Provide us with a food source
Soil Formation Factors
Five factors control soil formation:
1.
Parent material-Source of mineral matter
1.
2.
3.
4.
5.
Limestone, sandstone, shale, basalt
Time-longer forming, thicker soil
Climate-greatest effect on rate. Hot wet climate goes
fastest
Vegetation- More vegetation, increased soil forming
Topography-slope. Erosion elevated, less soil formed
Physical properties of soil:
 Soils are made up of four substances:
1. Small mineral particles – from the breakdown of rocks
– about 45%
2. Organic matter – plant and animal material, both
dead and living, microbes – about 5%
3. Water
4. Air
 Percentages of water and air can vary greatly
Ideal soil growing media:
Soil textures:
 Large particles: stones, cobbles, gravel
 Mineral particles in soil:
 Sand (largest particles)
 Silt (smaller)
 Clay (smallest)
 Soil texture refers to the amount of sand, silt and clay
in the soil.
Soil Particle Sizes
Soil Type
Particle Size in Millimeters
Very coarse sand
2.00 - 1.00
Coarse sand
1.00 - 0.50
Medium sand
0.50 - 0.25
Fine sand
0.25 - 0.10
Very fine sand
0.10 - 0.05
Silt
0.05 - 0.002
Clay
less than 0.002
Particle sizes: (diagram)
Sand
Silt
Sand compares to barrel
Silt compares to a plate
Clay
Clay compares to a coin
How can you tell what soil texture
you have?
 Sand can be seen with the naked eye. Feels gritty.
 Silt is intermediate in size. Visible under
microscope. Feels like talcum powder.
 Clay is smallest particle. Feels slick and sticky
when wet, firm when moist and hard when dry.
Size of particles affects moisture-holding
ability, plowing, which crops can be grown.
Hydrometer Method
 Largest particles fall out first
 Measure amount (mm) of each particle size
 Use chart to determine soil type
Pores
 Spaces in soil between the mineral material (sand, silt,
clay), organic matter
 Micro Pores
 Water filled at field capacity
 Macro Pores
 Air filled at field capacity
 Field capacity=amount of moisture in soil after excess
water has drained away
Pores
Soil/water relationships:
 Water drainage depends on soil pore size
 Large pores in the soil can take in water more rapidly
than fine pores.
 Sandy soils drain fastest
 Clay soils retain water longer
Soil/Water Relationships
 Saturation
 Can take no more water, water starts to puddle on top
 Field Capacity
 Moist
 Wilting Point
 Plant can not uptake any more water, starts to wilt
Soil profile (horizon) is …
 The arrangement and properties of the various soil
layers
 Topsoil (A Horizon)

Organic material, dead plant/animals, roots, humus
 Subsoil (B Horizon)

Clay that has moved (leached) down over time
 Parent material (C Horizon)

Rock
More about topsoil:
 Surface, top layer of soil
 From a few inches to several feet thick
 Darker, contains organic material
 Softer, more easily worked than underlying areas.
 Also called the A horizon
 Farmland
More about subsoil:
 Layer just under topsoil
 Lighter in color – may be red, brown, yellow or gray in
color
 Little or no organic material
 Usually higher in clay content than topsoil
 Firmer, more difficult to penetrate
 Also called the B horizon
More about parent material:
 Lower soil layer
 Material from which topsoil and subsoil developed
 Very firm and difficult for roots to penetrate
 Also called the C horizon
What does soil color indicate?
 Drainage
 Brown or dark brown surface soil layers indicate the
presence of organic matter.
 Subsoils are often reddish brown, red, yellowish
brown, yellow or gray, indicating how wet the soil is at
certain times.
 Gray means poorly drained and clay
 Red, brown or yellow colors means well drained
 Light/dark color can affect heat too
Soil depth is:
 The total depth of topsoil, subsoil and parent
material for plants to grow.
 Can cause the yield of a crop to be high or low.
Deep rooted plants, such as alfalfa, will not grow
well when planted on a shallow soil.
 Deep soil= 35 inches plus
 Moderately deep soil=20 to 34 inches deep
 Shallow soil= 10 to 20 inches deep.
 Very shallow soil is 10 inches or less.
Soil pH:
 Water in soil contains dissolved mineral salts – the
“soil solution.”
 Numbers from 1 to 14 measured with a pH meter
Acidic Neutral
Alkaline
1 2 3 4 5 6 7 8 9 10 11 12 13 14
 Some materials added to soil change the pH (peat
moss is acidic, limestone is alkaline)
Cation exchange capacity (CEC):
 The total number of exchangeable cations a soil
can hold (the amount of its negative charge) is
called its cation exchange capacity or CEC.
 The higher a soil's CEC, the more cations it can
retain.
 Cations include Phosphorus, Potassium
 CEC increases as organic matter/clay increases.
Organic matter improves the soil:
 Soil organic matter consists of plant and animal
residues in various stages of decay.
 The 4 benefits of organic matter:
 Improves physical condition and structure
 Increases water infiltration
 Decreases erosion losses
 Supplies plant nutrients (increased CEC)
Soil Erosion
 Erosion is the process of wearing away or removing
 Water
 Splash
 Sheet
 Rill
 Gully
 Stream bank
 Wind
 Surface creep
 Saltation
 Suspension
1965 soil taxonomy system for the
United States:
1.
Gelisols - soils with
permafrost within 6
feet
of the
surface
2. Histosols - organic
soils
NRCS photos
1965 soil taxonomy system for the
United States:
3.
Spodosols - acid soils
with a subsurface
accumulation
of metalhumus
complexes
4. Andisols - soils
formed in
volcanic ash
NRCS photos
1965 soil taxonomy system for the
United States:
5.
Oxisols - intensely
weathered soils of
tropical and
subtropical
environments
6. Vertisols - clayey
soils
NRCS photos
1965 soil taxonomy system for the
United States:
7.
Aridisols - calcium
carbonatecontaining soils
in arid
environments
8. Ultisols - soils with
a subsurface zone of
silicate clay
accumu-
lation
NRCS photos
1965 soil taxonomy system for the
United States:
9.
Mollisols grassland soils
10. Alfisols - another type
of soil with a
subsurface
zone of
silicate
clay
accumulation
NRCS photos
1965 soil taxonomy system for the
United States:
11. Inceptisols - soils
with weakly
developed subsurface
horizons
12. Entisols - soils with
little or no
morphological
develop-
ment
NRCS photos
Wrapping it all up:
 Soil is the soft outer covering of the earth.
 One of our most important
natural resources
 Necessary for plants to grow
 Provides food for plants, which in turn furnish food for
humans and animals
 Stores much of the water that plants use