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
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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
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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?
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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
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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