Transcript Unit 2
Unit 2: Soil
Physical Properties
Chapter 2
Unit 2 Objectives
Differences in sand, silt, clay & soil
textures
Understand soil structural classes
Importance of soil porosity & aeration
Knowledge of soil color and its
importance
Soil Texture
Soil Separates – particle size groups of
sand, silt, and clay
Proportion of each determines the soil
texture
Texture affects water intake rates, water
storage, soil tilth, aeration, fertility
Soil Texture
Soil Textural Classes
Clay – soils that are more than 60% clay
Silt – soils with high silt content
Sand – soils with highest content of sand
Soils that don’t exhibit a dominant area in
any of the three called loam
Soil Textural Triangle
Organic matter content has no bearing on
these values
Soil Texture
Particle Size Analysis
How to determine soil textural classification
Stoke’s Law
Settling rates of each of the soil separates based
upon its buoyancy, gravity, and resistance to
water friction
Placing a soil sample into proper solution,
then allowing each soil separate to settle will
help determine soil texture
Rock Fragments
Particles >2 mm diameter called rock
fragments & can be classified by shape
Have no bearing on soil texture
Rounded fragments
Gravel, cobble, stone, boulder
Flat fragments
Channer (smallest), flagstone, stone, boulder
Rock Fragments
% of rock fragments in a soil may be
used to help describe a soil texture
<15% by volume: no mention
15 to 35% by volume: name the dominant
kind of rock fragment (ex. Stony loam)
35 to 60% by volume: add very to the
description (ex. Very Stony loam)
>60% by volume: substitute extremely into
description (ex. Extremely Stony loam)
Soil Structure
Soil Structure – arrangement of particles
into aggregates
Aggregates – secondary units composed
of many soil particles held together by
organic matter, iron oxides, carbonates,
clays, etc.
Peds – natural aggregates, vary in water
stability (clod is used if soil is broken by
artificial means)
Soil Structure
Fragment – pieces of broken peds
Concretion/Shot – mass of precipitation of
certain chemical dissolved in percolating
waters
Soil Structural Classes
Peds described by three characteristics
Type (shape)
Class (size)
Grade (strength of cohesion)
Soil Structure
Types
Blocky (angular or subangular)
Columnar
Granular
Platy
Prismatic
Soil Structure
Classes
Very fine, fine, medium, coarse, very coarse
Grades
Evaluated by distinctness, stability, & strength of
the peds
Structureless Soils: no noticeable peds
Noncoherent mass of sand (single grain)
Cohesive mass such as clay soils around here
(massive)
Especially found in lowland wet soils
Soil Structure
Structured soils
Weak: peds can barely be distinguished
Moderate: peds visible, most can be handled
without breaking
Strong: very visible peds, easily handled without
breaking
Structure is very important influence on soil
properties
What affect might different structures have on
soil?
Infiltration of air, fertilizers, & water?
Soil Structure
Genesis of Soil Structure
Peds form due to shrink/swell of soil &
adhesive materials
Mostly 5/6 sided shapes
Prismatic structure tends to develop early in
the genesis of soil w/ vertical cracking
More blocky structure will develop as the soil
matures (especially in clay soils) due to
horizontal cracking
Soil Structure
Granular peds
Tends to be influenced by: tillage, rodents,
worms, frost action
Held together by organic matter
Mostly round shapes
Limited to surface horizon
Platy structure
Requires force: water, equipment, livestock
Soil Structure
Deterioration of Aggregates
Increasing Na+ as exchangeable ions
speeds deterioration of soil structure
Disperses ions in the soil, therefore,
breaking natural soil bonds
Often forms when water has high salt
content, and improper drainage
Soil Porosity &
Permeability
Pore spaces – portion of the soil not
occupied by mineral or organic solids
Often referred to as the soil matrix
Typically occupied by: air, water, living roots
Irregular shape, size, & direction to pores
Which soil has the largest/smallest pores?
How does that affect the soil & crops?
Soil Porosity &
Permeability
Pore sizes are more important than total
pore space
Relative amounts of air & water in pores
fluctuates
Rain
Deep percolation
Transpiration
Evaporation
Soil Air
Free oxygen must be available
Required for root growth (respiration) and by
soil microbes for organic matter
decomposition
Well-aerated soil is best, w/ rapid,
continuous gaseous exchange
Factors affecting gas exchange rates
Pore sizes
Pore continuity
Temperature
Soil Air
Depth in the soil
Wetting/drying
Coverings on the soil surface
Composition of Soil Air
Atmospheric air
N2 = 79%
O2 = 20.9%
CO2 = .038%
Soil Air
Soil air
Some O2 used, much CO2 produced
Soil air CO2 may be 10%
Range of O2 values from 10% to virtually none
What type of soil would be on each end of the range?
Rates of O2 Exchange
Oxygen diffusion rate (ODR) – rate at which
gases in the soil exchange w/ O2 in the
atmosphere
Soil Air
Factors affecting ODR
Pore size
Water filled pores
Diffusion of CO2 gas through water is 10,000x
slower through water than air
Depth in the soil
At ~3’ depth, ODR is ½ to ¼ rate of top few in.
So, how does this affect our high-clay soils?
What does is affect?
What makes the problems worse?
What might improve ODR?
Soil Air
Oxidation-Reduction Potential (Eh or
Redox)
Describes tendency for chemicals in the soil
or water to be oxidized
A measure of the availability of O2 in the soil
High redox = O2 is present, low redox = O2
absent
Most plants must have O2 in the soil at root
growth
Give an example of a plant that doesn’t
Soil Air
Most plants grow best in an oxidized
(aerated) soil
Free oxygen is the primary acceptor of electrons
in the soil
What does this mean?
More soil nutrients stay/converted soil plant available
forms
N is not lost to the atmosphere as much
Plant roots are able to respire
Soil Air
Aeration & Energy for Plant Growth
Energy obtained from sun
Stored in chemical bonds (photosynthesis)
Energy released by breaking the bonds
(respiration)
w/ O2, aerobic glycolysis plus respiration
makes much more energy available to the
plant
~19x more than anaerobic glycolysis
Soil Air
Anaerobic glycolysis
Results in much less energy availability
Decomposition of organic matter is much slower
How do deficient O2 concentrations occur?
Waterlogging
Compaction
High clay soils what pinch pores when wet
O2 consuming organic matter decomposers
What can we do as managers of the soil to
improve O2 concentrations?
Consistence (Strength)
Consistence – soil’s response to
mechanical forces
Resistance to rupture
Soft/hard when dry
Friable (crumbly), firm, rigid when wet
Plasticity
Tolerate considerable deformation w/out breaking
Stickiness
Ease w/ which the soil is manipulated, or even
walked on
Soil Color
Dark soils absorb more heat than light
colored soils
Do you think this helps explain some
planting date differences?
Just because they’re dark doesn’t mean
they’re warmer
Depends on soil moisture as well
Soil Color
Soil Color vs. Soil Properties
White colors – common w/ salts or lime
deposits are present
Mottles (rust colors) – soil may have periods
of inadequate aeration
Gleying (bluish, grayish, greenish) –
subsoils, prolonged periods of waterlogging
Darker colors – higher levels of organic
matter
Soil Color
Munsell Color Charts
Chart used to help ID soil color accurately
Hue: dominant spectral or rainbow color
Value: relative blackness or whiteness
Chroma: purity of the color (as chroma
increases, the color is more brilliant)
Soil Temperature
Relation of Soil & Air Temp
Net heat absorbed by the Earth = heat lost in
form of longwave radiation
Photoperiod – affected by latitude
Soil temp can change by soil depth & time of
day
Takes significant air temp changes to change soil
temp deeper than 12” (& more than just daily
range)
Soil Temperature
Avg. summer & winter soil temps @ 3’ rarely
differ by more than 9° F
Factors Affecting Soil Temp
How much heat reaches the soil surface
Soil coverings
Plastic mulches
Sun angle
Slope face
Soil
Soil Temperature
What happens to the heat in the soil (dissipation)
Amount of heat needed to change soil temp = heat
capacity
Greatly affected by soil water content
How?
Thermal conductivity – increases w/ soil-water
content increasing, decreases as air-filled pores
increase
Moist soils resist temp change, but conduct heat readily
Dry soils change temp faster, but conduct heat poorly
What does this mean for the soil, which is better?
Soil Temperature
Living w/ Existing Temps
Maximizing seed germination & growth
Wheat – 40 to 50° F
Corn – 50 to 85° F
When using anhydrous
Apply when soil temp @ 4” is 50° F or less
Reduces N losses
Freeze/thaw
May cause heaving – resulting in death of
shallow rooted crops
Soil Temperature
Responsible for bringing stones to the surface in fields
Modifying Temp Effects
If you have crops that are feasible/profitable
to do so
Clear plastic surface covers
Increases soil temp faster
Clear plastic mulches
Can speed growth & maturity of sweet corn &
strawberries
Soil Physical Properties &
Engineering
AASHTO & Unified Engineering Soil
Classification System
Used by engineers to classify soils based on
particle size to determine construction
limitations
Atterberg Limits
Liquid limit – relates to the amount of water a soil
can retain & not break
Plastic limit – the water content at which a thread
of soil can no longer hold together
Soil Physical Properties &
Engineering
Plasticity Index – difference between liquid limit &
plastic limit
Important measures for engineers to be able
to understand what the soil will do under
various conditions
Helps then understand what moisture needs
to be present for effective compacting (make
a solid base for roadways, buildings, etc.)
Assignment
Assignment 2.1 on WebCT