9/6/2012
Natural soils
2009 ASLA Honor Award
American Society of
Landscape Architects
vs.
Disturbed urban soils
• Uniform across site
• Natural horizons
• Adequate OM, nutrients,
structure for native plants
Healthy Soils
Part 1:
Physical, Organic
and Chemical
Properties
•
•
•
•
•
Vary across site
Topsoil layer removed
Compaction, low OM
Subsoil (or worse) fill layers
Debris, toxins?
David McDonald
Seattle Public Utilities
[email protected]
James Urban, FASLA, ISA
Urban Tree + Soils
[email protected]
Critical Aspects of Soil
Tree Issues
Tree growth
Structure
Expected canopy size
Clumps / clods peds
Tree stability
Density
weight / volume
pore space
Texture
sand / silt
clay
Soil Biology
Use Issues
Organic matter
Use intensity
Irrigation or rain Storm water?
harvesting?
Lawn?
Maintenance?
Carbon
Nutrients
Food?
N P K +
pH
Acidity
Soil Issues
Space for roots Sufficient soil volume
and trunk flare Imported soil Existing soil Grading
sources
conditions
Soil drainage
Air and water movement / soil profile
Critical Aspects of Soil
Soil Goals and Requirements
Structure
Clumps / clods peds
Density
Texture
weight / volume
pore space
sand / silt
clay
Soil Biology
gy
Organic matter
Carbon
Nutrients
N P K +
pH
Acidity
Air and water movemen
nt / Soil Profile
Physical properties of soil
Igneous
Sedimentary
Glacial
Wind deposited
Alluvial
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Soil formation -Human forces
Percent Sand
Sandy soil - Almost no structure
Silt soil - Weak structure
Clay soil - Strong structure
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Sandy Loam Topsoil
Sandy Clay Loam Topsoil
Beginning of root
Penetrattion limitation
In most s
soil
Density or Compaction
Bulk Density of
Different Soils
Bulk densities of soil mixes are different for similar natural soil textures.
Compost is very light, while sand and lost structure tends to make soil test
heavier.
You have to test bulk density at a stated Proctor percentage.
Units
% maximum bulk density
standard proctor or Bulk
density Lb/CF Dry weight
As compaction increases, pore space for
water and air decreases
Units
PSI LB pressure per Sq In
Units
Bulk density Lb/CF dry weight
Densitometer
Penetrometer
Bulk density cores
Moderately slow 10 minutes
Accurate
Expensive
Must calibrate to soil.
Readings impacted by OM
Soil service only
Fast less than one minute
Not very accurate
Soil moisture limited
Inexpensive
LA can operate
Slow one day
Accurate
Somewhat expensive
LA or soil service
Soil Compaction testing
3
Soil H
Horizons
9/6/2012
There is a decrease in compaction with depth as the compaction
force spread out into the soil in a cone shaped wave.
Mountain soil
A-O
Soil Profile
Coastal soil
Imported topsoil
A-O
B
B
C1
C2
Fill soil
Remnant Soils – Buried layers of
original soil that can support tree
rooting.
C1
R
Remnant topsoil
C2
Remnant subsoil
Structure
Clumps / clods peds
Density
weight / volume
pore space
Texture
sand / silt
clay
Soil Biology
Organic matter
Carbon
Nutrients
N P K +
pH
Acidity
Air and water movem
ment / soil profile
Chan
nges in soil type
Chemical properties of soil
Examining a soil profile with a Dutch soil auger
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9/6/2012
Chemical properties
of soil
- endless cycles
Carbon cycle
Elements Required by Plants
Base elements
Macronutrients
Oxygen (O)
Hydrogen (H)
Carbon (C)
Nitrogen cycle
Sulfur
cycle
Micronutrients
Nitrogen (N)
Boron (B)
Phosphorus (P)
Chlorine (Cl)
Potassium (K)
Cobalt (Co)
Calcium (Ca)
Copper (Cu)
Magnesium (Mg)
Iron (Fe)
Sulfur (S)
Manganese (Mn)
Molybdenum (Mo)
Zinc (Zn)
image: extension.missouri.edu
Trees feed themselves!
Cation Exchange
Capacity (CEC)
Vitamins not food
Food is minerals (made available
by soil organisms) air, water, and
organic matter – leaf fall & mulch
Sand
Silt
Clay
The soil particle is like a bus. The seats are
The soil
the negative charges. All the seats must particle is
always be full with positively charged particles.
the bus
Hydrogen has a weak charge and Ammonium
Lowering pH (increasing acidity)
(NH4 ) a stronger charge, so they “exchange”
increases availability of cations, but
seats if the Ammonium Nitrogen ion wants to
decreases availability of anions.
sit down.
Learn different soil odors:
Sweet smell is good respiration = aerobic
Sour smell is no respiration = anaerobic
The smaller the particle
the greater the CEC.
Humus/clay colloids have the most!
Cation Exchange Capacity (CEC) for
planting soil mixes
Low fertility soil
Less than 5
Medium fertility
5-10
High fertility
10-30
Compost/humus
up to 200!
Relative surface area
Soil Odor
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Organic properties of soil
USDA pH Classification
pH range
1.8 - 3.4
3.5 - 4.4
4.5 - 5.0
5.1 - 5.5
5.6 - 6.0
6 1 - 6.5
6.1
65
6.6 - 7.3
7.4 - 7.8
7.9 - 8.4
8.5 - 9.0
9.1 - 11.0
Structure
Toxic to most plants
Restrictive to most plants
Clumps / clods peds
Density
weight / volume
pore space
Texture
Acid-tolerant plants
sand / silt
clay
Best nutrient availability for most plants
Soil Biology
Organic matter
Alkaline-tolerant plants
Nutrients
Carbon
N P K +
Restrictive to most plants
Toxic to most plants
pH
Acidity
Air and water movem
ment / soil profile
Ultra acid
Extremely acid
Very strong acid
Strongly acid
Moderately acid
Slightly acid
Neutral
Slightly alkaline
Moderately alkaline
Strongly alkaline
Very strongly alkaline
Lower or higher pH decreases availability of different nutrients
Humus (compost) buffers ph towards optimal 6.3 to 6.8
Organic Properties
of Soil
– biology in action!
Soil formation
Soil development processes,
from parent “dirt” & rock
Soil horizons & their evolution
Nutrient cycling & availability
bio-chemical,
bio-physical,
& eco
ecological
og ca
processes
• Substratum (C) or bedrock (R)
weathers physically &
chemically to subsoil (B)
Structure: peds & pores
• Primarily biological processes
create topsoil (A)
and organic (O) horizons
USDA - NRCS
http://soils.usda.gov
Understanding Soil Biology
Soil life provides essential functions
Common organisms in the soil foodweb
Bacteria
Nematodes
Fungi
Arthropods
Soil
is
alive!
Earthworms
Protozoa
S. Rose & E.T. Elliott
USDA-NRCS
“Soil Biology Primer”
http://soils.usda.gov/sqi/
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What Fuels the
Soil Foodweb?
Restoring soil life, to restore soil functions
Soil organisms create:
• soil structure
• fertility = nutrient cycling
• plant disease protection
• Bio-filtration
• erosion control
• stormwater detention &
moisture capacity
Plant photosynthesis:
Sunlight  living and
dead organic matter
Plants exude 20-30% of
their photosynthetic
energy as
carbohydrates
released in the root
zone to feed beneficial
soil organisms
(bacteria & fungi).
Compost kickstarts the soil ecosystem!
(Provides food and home for organisms)
How does soil life create soil structure?
• Bacteria secretions glue clays, silts and sands together
into micro-aggregates.
• Micro-aggregates are bound
together by fungal hyphae,
root hairs and roots.
• Spaces
S
are made
d b
by moving
i
arthropods & earthworms,
and decaying roots.
• Only when all organisms
are present can roots and
water move into the soil
with ease.
How does soil life and increasing organic matter
increase plant-available soil water storage?
Increased structure,
pore space, and
soil colloids.
Organic Matter %
S. Rose & E.T. Elliott
How does soil life provide fertility
(nutrient cycling)?
• Soil foodweb stores nutrients in living & dead organic matter
• Nutrients are released in root zone as organisms eat and
excrete “waste” (nitrogen, etc.)
• Mycorrhizal fungi bring nutrients
and water to roots of plants
How does soil life provide
plant disease protection?
Diversity  predation, parasitization & competition
with the few disease-causing organisms
• Bacteria cover leaf surfaces, block infection
• Ecto- and endomycorrhizae
prevent root
infection
Soil Foodweb Inc.
• Many organisms
prey on the few
disease-causing
organisms
Dr. Michael P. Amaranthus, Mycorrhizal Applications Inc.
SSSA
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9/6/2012
How does soil life filter out
urban pollutants?
How does soil life
control erosion?
• Creates pore spaces,
increases infiltration
• Creates structure
• Breaks down hydrocarbons, pesticides
• Sticks soil particles & aggregates together
with bacterial slime, fungal hyphae,
& root hairs (bigger aggregates are
harder to move)
 “aggregate stability”
• Converts fertilizers to stable forms, so they are
plants but
available to p
won’t wash away
• Binds heavy metals
in soil, so they don’t
wash into streams
• Promotes rapid plant growth
& deep root development
How can we enhance & restore soil
biodiversity, to improve plant growth, water
quality, and reduce runoff?
How does soil life provide stormwater
detention / infiltration?
• Builds soil structure,
moisture-holding capacity
• Increases surface
porosity
• Prevent /reduce compaction (keep heavy machinery off)
• Reduce intensive use of
pesticides & soluble fertilizers
UW trials, turf on glacial till soil
Compostamended
till soil –
up to 50%
reduction in
storm water
runoff
organic matter + soil organisms + time
creates 
soil structure, biofiltration, fertility, & stormwater
detention
- Most composts/plant materials are
40-70% organic content by this method.
?
Recommended soil amendment rates
(for low-organic
low organic soils or soil/compost topsoil mixes):
• 3% Soil Organic Matter (by lab test) for Tree soils
= 10-15% compost amendment into soil by volume
• 5-8% Soil Organic Matter (by lab test) for Turf/Landscape
=15-25% compost amendment by volume
• 10% Soil Organic Matter (by lab test) for Stormwater
bioretention soil mixes
47
= 30-40% compost amendment by volume
Air and water movement in soil
Structure
Clumps / clods peds
Density
weight / volume
pore space
Texture
sand / silt
clay
Soil Biology
gy
Organic matter
Carbon
Nutrients
N P K +
pH
Acidity
Air and water movvement / soil profile
Clearing up the confusion about “% organic”
“% Soil Organic Matter Content” in lab soil tests
is by loss-on-ignition method
• Incorporate compost into soil,
and mulch regularly,
to feed soil life
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Micro-pores
Macro-pores
Saturation Point
Field Capacity
Wilt Point
Plants and decaying organic matter in soil must respire
(bring in oxygen expire carbon)
Organic
matter
CO2 O2
Root
O2
CO2
Aerobic Soil
Good respiration
Organic
matter
Organic
matter
CO2
O2
O2
Root
CO2
Saturated Soil
Slow respiration
Root
Anaerobic Soil
No respiration
Plants will die more quickly with
too much water than too little !!!!!
Fine grained soil
I t f
Interface
Coarse grained
soil
Soil interface slowing the flow of water
Upper layer must become saturated
before water moves into lower layer
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9/6/2012
Analyzing Existing Site Soil
Compaction
Structure
Limited
d air and water movem
ment
Grading
Clumps / clods peds
Soil mixing
Density
weight / volume
pore space
Texture
sand / silt
clay
Reduced
Soil Biology
Organic matter
Carbon
Nutrients
N P K +
Salts
pH
Acidity
Fertilizer
Lower organic
matter
Water / Drainage
Higher pH
Too much / too little
Limitations of traditional soil analysis methods
for disturbed urban soils
Soil Survey
1.
2.
3.
• Soils vary across site: fill?,
native? Subsoil?
• Mixed or missing horizons –
topsoil layer often removed
• Sharp interface problems
(between native and fill soils)
• Compaction
C
ti
• Low ph, anaerobic?
• Low organic matter
• Debris, toxins?
The existing survey description
Changes since development
Consistency with existing conditions
Graded and compacted conditions
Imported soils
Non traditional ways to survey urban soil
Conclusions
Project soil borings
Old maps and photos
Read existing
topography
Regional geologic maps
Interviews
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9/6/2012
Compare urban tree leaf
growth with those from
nearby native soils.
Plants as indicators
of soil differences
and problems
Stressed tree
Poor soil
Healthy tree
Good soil
Chan
nges in soil type
Early Fall Color
indicating soil stress
Creating a soil profile with a Dutch auger
Adjacent constructions can reveal soil types and issues
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9/6/2012
Truck test
Clayey or silty soils?
Shoe test
Feel test
Find a reference soil in
or near the site that might
be undisturbed.
Under old trees, at
property lines, cemeteries,
parks etc.
The harder to clean your shoes,
tools, trucks; the greater the % clay
Use this to find remnant
soils.
Interface
Constantly smell
the soil! Sour
odor indicates
poor drainage
Make a soil survey map
Grey color, poorly
draining soil
Record all the information on a
drawing to show the different soil
types and soil issues
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