Introduction to Soil Science

This is an introductory soil science presentation that I give to Master Gardeners, agribusiness personnel, farmers, and soil science students. Please feel free to contact me at [email protected] with any comments regarding the presentation.

1. Andy Kleinschmidt Don McClure
Extension Educator Soil Scientist
Van Wert County NRCS-USDA

2. What is Soil?What is Soil?
A logical place to begin
today is with a definition of
soil.
Soil: Unconsolidated mineral or Soil: Unconsolidated mineral or
material on the surface of the earth material on the surface of the earth
resulting from and influenced by resulting from and influenced by
time, parent material, climate, time, parent material, climate,
organisms, and topography.organisms, and topography.
Not all soil is created equal, Not all soil is created equal,
‘the soil’ vs. a soil. ‘the soil’ vs. a soil.

3. Why are soils important?
Great integrator
Producer and
absorber of
gases
Medium for
plant
growth
Medium of crop
production
Home to organisms
(plants, animals and
others)
Waste decomposer
Snapshot of
geologic,
climatic,
biological, and
human history
Source material
for construction,
medicine, art, etc.
Filter of
water and
wastesEssential natural resource

4. ObjectivesObjectives
‣Soil color
‣Soil texture
‣Soil structure
‣Soil pH
‣CEC
‣Microorganisms
‣Nutrient movement

5. What ‘stands out’ about the landscape?

6. Color is the most obvious characteristic of soil.
What are some colors encouraged by well aerated
conditions?
REDRED YELLOWYELLOWBROWNBROWN
What are some colors encouraged by poorly aerated
conditions?
GRAYGRAYBLUEBLUE
Soil color is influenced by the oxidation state of iron
and manganese.
Soil ColorSoil Color

9. Soil Color, Soil Aeration or Drainage,
and the Oxidation State of Iron
1. Iron is reduced1. Iron is reduced
2. Fe2. Fe
++++
3. dull colors (grays, 3. dull colors (grays,
blueblue))
4. poorly drained4. poorly drained
1. Iron is oxidized1. Iron is oxidized
2. 2. FeFe
++++++
3. bright colors 3. bright colors
((yellowsyellows, browns), browns)
4. well drained4. well drained
POOR AERATION GOOD AERATION

11. Soil Color Tells A StorySoil Color Tells A Story
Well
Drained
Poorly
Drained
Drainage on this farm?

12. Soil HorizonsSoil Horizons
B
C
Ap
Zone of highest organic matter content.
The ‘p’ denotes that this soil has been
plowed.
A layer of accumulation of iron and
clays. Blocky structure is readily seen
in this layer.
Unconsolidated material. Outside the
zone of major biological activity and is
not affected by soil forming processes.

13. Soil ProfileSoil Profile
What do we see?
•organic matter - surface soil is organic matter - surface soil is
darker due to organic matterdarker due to organic matter
•iron oxides - subsoil has iron oxides - subsoil has
brighter browns and tans due brighter browns and tans due
to iron oxidesto iron oxides
•drainagedrainage
•horizons - layers of different horizons - layers of different
color or texture; formed from color or texture; formed from
the top downthe top down

15. . . . more on Soil Horizons. . . more on Soil Horizons
Mollisol Alfisol
B
C
Ap
A

16. USDA-NRCS National Soil Survey Center

17. Average Soil Composition
{ }
Pore
space
50%
Solids
50%
25% Water
25% Air 5% Organic Matter
45% Inorganic
(mineral materials)

18. Soil TextureSoil Texture
•Determined by the relative
proportion of sand, silt and clay
Surface AreaSurface Area ChargeCharge
SandSand 50 cm50 cm
22
/g/g nonenone
SiltSilt 500 cm500 cm
22
/g/g nonenone
ClayClay 5,000,000 cm5,000,000 cm
22
/g/g negativenegative

19. Relative Size Comparison Relative Size Comparison
of Soil Particlesof Soil Particles
Sand
- feels gritty
Silt
- feels floury
(2.00 - 0.05 mm)
(0.05 - 0.002 mm)
(< 0.002 mm)
barrel
plate
coin
Clay
- feels sticky
USDA system for determining soil separates

20. Clay
Silty
Clay
Silty Clay
Loam
Clay
Loam
Sandy
Clay
Sandy Clay
Loam
Loam
Silt Loam
Silt
Sandy
Loam
Sand
Percent
Clay
Percent
Silt
Percent Sand
Fine
Medium
Coarse
Loamy
Sand

21. SandSandy
loam
Silt
loam
Clay
loam
Clay
1
2
3
4
Inches water/ft soil
Plant Available Water

22. Rhoads and Yonts, 1984.
Available Water Holding CapacityAvailable Water Holding Capacity
Storage capacity
Silty clay loam 1.8
Clay loam 1.8
Silty clay 1.6
Silt loam 2.0
Sandy loam 1.4
Texture (in./ft.)

24. Comparison of Coarse Textured and Fine Textured Soils
Coarse Textured Soil
Less porespace but
more macropores
Fine Textured Soil
More total porespace
Texture and Pore SpaceTexture and Pore Space

25. Soil StructureSoil Structure
•Soil structure is the combination or
arrangement of primary soil particles
into secondary units
•The way soil particles are arranged to
form stable aggregates
•Compare this to clods, which are
caused by disturbance (plowing or
digging)
•Compaction – results from implement
traffic, stable soil aggregates are
broken down

26. Common Types of Soil Common Types of Soil
StructureStructure
Common to Ohio soils just
below Ap (> 8”)
Plow layer
Sand
Deeper in profile (>3-4’)
Granular
Platy
Prismatic
Columnar
Blocky
Single Grain
Massive

27. GRANULAR

28. BLOCKY

29. C
B
E
A
Granular
Platy
Blocky
Massive

30. Bulk Density DeterminationBulk Density Determination
For our example, let’s
assume we have 1 cubic
centimeter of soil that
weighs 1.33 grams
Soil is made
of solids and
pore spaces
1.33
grams
To calculate Bulk Density:
Volume = 1 cm
3
Weight = 1.33 grams
Bulk Density =
Weight of Soil
Volume of Soil
Bulk Density =
1.33
1
Bulk Density = 1.33 grams/cm
3
{}

31. Bulk density (g/cm
3
)
Soil Cropped Uncropped
Hagerstown loam (PA) 1.25
Marshall silt loam (IA) 1.13
Nappanese silt loam (OH) 1.31
(50%)
(56%)
(51%)
(57%)
(63%)
(60%)
1.07
0.93
1.05
What impact does this have on pore space?
Bulk Density (con’t.)
Data from Lyon et al.

32. Some Common Bulk Some Common Bulk
DensitiesDensities
•Uncultivated/undisturbed woodlots
–1.0 to 1.2 g/cm
3
•Cultivated clay and silt loams
–1.1 to 1.5 g/cm
3
•Cultivated sandy loams
–1.3 to 1.7 g/cm
3
•Compacted glacial till
–1.9 to 2.2 g/cm
3
•Concrete
–2.4 g/cm
3

33. Bulk Density and CompactionBulk Density and Compaction
8 inches
1.43
0 inches
7 inches
9 inches
10 inches
Bulk Density (g/cm
3
)
1.90
1.87
1.84
1.80
1.60
Plow layer
Compacted
zone
Uncompacted
subsoil
Depth
Data from Camp and Lund
Till2.20

34. Influences of Soil Texture, Influences of Soil Texture,
Soil Structure and DensitySoil Structure and Density
1)Water movement
2)Water retention
3)Soil temperature
4)Gas exchange
5)Erosion potential
6)Fertility

35. Hydrologic Cycle and SoilHydrologic Cycle and Soil
Color
Structure
Bulk
Density
Texture
pH
Temperature
Moisture
Horizon
Depths
Soil properties that are part of the hydrologic cycle.

36. Soil pH - a “master variable”Soil pH - a “master variable”
Acid
(pH=1.0)
Neutral
(pH=7.0)
Alkaline
(pH=14.0)
•A measure of the hydrogen (H
+
)
ion activity
•One pH unit change = a ten fold
change in acidity or alkalinity

37. Why called “master variable”?Why called “master variable”?
•soil pH controls:
1)soil microbe activity
2)nutrient exchanges
3)nutrient availability
4)gaseous exchanges
5)chemical degradation
6)CEC

38. 4.0
5.0
6.0
7.0
8.0
9.0
10.0
pH value
{
{
Too alkaline
for most plants
Too acidic for
most plants
Range of alkalinity
soils of arid and
semiarid regions
have pH greater
than 8.0
Range of acidity
weathered soils of
the southeastern US
coastal plains
typically have pH
less than 5.0

39. Soil pHSoil pH
•Methods for soil pH determination can
vary widely
•Measure of the direct concentration of H
+

ions in the soil solution
•Buffer pH measures both H
+
ions in the
soil solution and the reserve H
+
ions
bound on cation exchange sites
–It is used to express the acidity or
alkalinity of the soil solution, not lime
requirement
•pH represents the equation -log[H
+
]

40. Factors Affecting Soil pHFactors Affecting Soil pH
Parent Soil Material
Precipitation
Nitrogen Applications
Cropping Sequence
Organic Matter Breakdown

41. Making Acid RainMaking Acid Rain
•acid rain a concern
•air has carbon dioxide
(CO
2
)
•acid produced by nitrogen applications
HH
22O + COO + CO
22 H H
22COCO
33
(carbonic (carbonic
acid)acid)
•atmospheric CO
2

gives rain a pH of 5.6
•fossil fuels add to acid
rain, but requires only
25 lbs. limestone per
year to neutralize

42. Clay MineralsClay Minerals
Clay structure magnified 1,600 times

43. Clays are layered minerals made of . . .
. . . together they form . . .

44. Organic MatterOrganic Matter
OROR

45. Factors Influencing Organic Factors Influencing Organic
Matter AccumulationMatter Accumulation
1)1)TopographyTopography
2)2)Native VegetationNative Vegetation
3)3)ClimateClimate
4)4)TimeTime
5)5)OrganismsOrganisms

46. Forest SoilForest Soil
0
4
8
12
16
20
24
28
32
02468
0
4
8
12
16
20
24
28
32
02468101214
Percent Organic Matter in Soil
Well Drained Poorly Drained
Soil
Depth
in
Inches
Soil
Depth
in
Inches

47. Prairie vs. Forest SoilPrairie vs. Forest Soil
Prairie Forest
Effect of Native Vegetation
A horizon = 14 inches
A horizon = 4 inches

48. Importance of Soil Organic Matter Importance of Soil Organic Matter
Physical and Chemical PropertiesPhysical and Chemical Properties
1)Improves physical condition
•Allows for good aggregation of soil particles
because of the plant and animal residues in
the surface
2)Increased water infiltration
•Allows water saturation by acting as an
absorbent
3)Improves Soil Tilth
•Allows for more uniformity of the soil
aggregates in proportion to the plant, animal
and mineral residues present

49. Cation Exchange Capacity (CEC)Cation Exchange Capacity (CEC)
Cations
Ca
2+
, Mg
2+
Na
+
, K
+
Anions
SO
4
2-
, PO
4
3-
Cl
-
, NO
3
-
Ability of a soil to hold and exchange cations
–Ions are atoms with an electrical charge
Negatively charged colloids (organic matter
and clay) attract and hold cations

50. CEC of a soil is due to:CEC of a soil is due to:
1)Organic Matter Content
2)Clay Content
3)Type of Clay
1)Montmorillonite high CEC
2)Illite mod. CEC
3)Kaolinite low CEC

51. NRCS STATSGO Database

52. CEC (con’t.)CEC (con’t.)
•Most soils are negatively charged and
hold cations.
•Cations held on exchange sites may
move into the soil solution & be taken
up by roots.
•Anions are not held on soil and are
subject to leaching (P is exception).

53. SOIL
COLLOID
N S N SS N N S
+ -+ -- +- + + -+ -+ -+ -
Like poles
(charges) repel
Opposite poles
(charges) attract
Ca
2+
K
+
Na
+
Mg
2+
SO
4
2-
NO
3
-
Cl
-
NH
4
+

54. Common CEC
RangeHeavy
Clay
50 CEC50 CEC
Sand
2 CEC2 CEC
CEC
25
More Clay, More
Positions to Hold Cations
CEC
5
Less Clay, Fewer
Positions to Hold
Cations
K
+
Ca
2+
Mg
2+
NH
4
+
Na
+
K
+
Ca
2+K
+
Sand
Clay
Another Schematic Look at CEC

55. Some practical applicationsSome practical applications
Soil CEC 11-50 Soil CEC 1-10
Clay content
Nutrient
relationship
Water Holding
Capacity
Lime relationship
Higher clay content Lower clay content
Requires more lime
to correct a given
pH
Requires less lime
to correct a given
pH
Greater capacity to
hold nutrients
Leaching more
likely
Higher water
holding capacity
Lower water
holding capacity

56. MicroorganismsMicroorganisms
**
in the Soil in the Soil
1)Microbes live in small clumps
•In fact, less than 1% of the soil surface will
support the growth of microbes
2)Do not migrate much
3)Their goal: maintain species
•7,000 different species in one gram of soil!
•4.5 x 10
16
bacteria/acre, 3 inches deep
*
Excludes nonarthropod and arthropod animals, as well as vertebrates
Microbes are hungry, Microbes are hungry,
immobile and ready immobile and ready
to reproduceto reproduce

57. One final thought . . . One final thought . . .
What do you notice about this soil core?What do you notice about this soil core?
macropores

58. Preferential Flow Preferential Flow
Example of
pesticide leaching
through
preferential flow.
Atrazine applied.
Initial storm of
season.
Notice
preferential flow.
A
B
C
Soil
Horizon
Calculated from Kladivco, et al. (1999); models from Cornell
68% of leachable
atrazine was lost
to preferential
flow during the
first storm.
What are the
implications from
a soil fertility
standpoint?

59. SoilsSoils
ENJOY THE
REMAINDER OF THE
TRAINING.
WE’RE GLAD YOU
ARE HERE.