Soils, soil
organisms, soil
amendments, and
their relation to
plant health
Thanks to Craig Cogger
Extension Soil Scientist WSU-Puyallup
http://www.puyallup.wsu.edu/soilmgmt
Outline
• Introduction to soils
• Local soil types
• Soil organisms
• Choosing organic amendments
• Compost quality
• How much organic amendment
to use
Soil Components
Mineral Matter
Pore Space
Organic
Matter
The soil
ecosystem
Residue decomposition
Nutrient cycling
Aggregation and porosity
Enhance plant growth
Break down contaminants
•Water Movement
How quickly water moves
through soil
Water Holding Capacity
How much water a soil can hold
available for plant growth
Soil pores and
water movement
•Macropores:
Infiltration and
drainage
•Capillary pores: Available
water
•Micropores: Unavailable
water
Soil properties that
affect porosity
•Soil texture
•Soil structure
•Compaction and
•Organic matter
disturbance
Soil Particle Sizes
Sand
Silt
Clay
.05-2 mm
.002-.05 mm
<.002 mm
Coarse Fragments
>2 mm
Hand texture technique
Soil Structure
Aggregation of sand, silt, and clay particles
Structure affects:
Macroporosity
Infiltration
Aeration
• Growth
Formation of soil
structure
of roots and movement of
organisms create pores and aggregates
• Soil organisms break down organic
residues, producing glues that
stabilize aggregates
• Fungi provide structural support to
aggregates
• Physical, chemical processes also
involved
Urban and
suburban soils
•Compaction:
Loss of
structure and macropores
•Cuts: Loss of topsoil, less
structure, shallow depth
•Fills: Unstructured “dirt”
Disturbed soil:
Cut and
compacted.
Effects of
development on soils
• Increased bulk density
• Resistance to root penetration
• Loss of structure
• Reduced porosity
• Reduced infiltration
• Reduced rooting depth
• Reduced nutrient and water
availability
Consequences
•Increased
plants
•Increased
stress on
risk of runoff
and erosion
Prescription
•Incorporate
matter
organic
Expected benefits of
organic matter
• Physical: Improved bulk density, structure,
porosity, permeability,
• Biological: More activity
• Available water: Increase depends on soil
and irrigation regime
• Runoff: Better structure and porosity
reduces runoff and erosion
• Nutrients: Significant for some materials
Soil Organisms
Bacteria, fungi, actinomycetes, protozoa,
nematodes, arthropods, earthworms
Pictures courtesy M. Fauci and D. Bezdicek
Roles of soil organisms
Mary Fauci
Residue decomposition
Nutrient cycling
Aggregation and
porosity
Contaminant breakdown
Nitrogen fixation
Enhance root function
Pathogens
Predators
Soil food web and nutrient cycling
Phytophagous
nematodes
Roots
Predacious
mites
Collembolans
Mycorrhizae
Nematode
Feeding mites
Cryptostigmatid
mites
NoncryptoStigmatid mites
Fungi
Detritus
Fungivorous
nematodes
Predacious
nematodes
Omnivorous
nematodes
Flagellates
Amoebae
Bacteria
Bacterivorous
nematodes
Organic matter
stimulates soil
organisms
•Formation of soil structure
•Nutrient cycling
•Plant disease
suppression/stimulation
Choosing
organic
amendments
Organic materials:
• Fertilizer
Fertilizers vs. Soil amendments
vs. mulches
1. High nutrient content and availability.
2. Main benefit is nutrients.
3. Relatively small amounts applied.
• Soil amendment
1. Low nutrient content and availability.
2. Main benefit is organic matter.
3. Large amounts applied.
• Mulch
1. Negative available nutrients
2. Applied to surface to control weeds and conserve
moisture
Carbon:Nitrogen ratio
•Ranges from <5:1 to >500:1 in
organic materials
•Low C:N supplies N to plants
•High C:N ties up N by
biological immobilization
Types of organic
amendments
Hot stuff – C:N <10:1
Cool stuff – C:N 15:1 to 25:1
Woody stuff – C:N > 30:1
Hot stuff C:N < 10:1
•Rapid N availability
•Use as a fertilizer
•Over application leads
to excess
nutrient levels in soil -potentially harming crop and water
quality.
Examples:
•Poultry manure
•Packaged organic fertilizers
•Fresh grass clippings
•Fresh, undiluted rabbit manure
•Heat-dried biosolids
Cool stuff, C:N 15:1 to 25:1
• Slow N availability
• Can add large amounts without risk of
over-fertilization
• Use as a soil amendment
• Expect some N immobilization (tie-up)
shortly after application.
Examples:
•Compost
(yard debris, most
manures, biosolids)
•Mixed fresh yard debris
•Cover crop residues
•Dairy manure solids
Woody stuff, C:N > 30:1
•N immobilization
•Need to add N along
with organic
amendment
•Use as mulch or bulking agent for
compost
Examples:
•Straw
•Sawdust
•Paper waste
•Horse manure
rich in bedding
Compost
Quality
Why use compost
as a source of
organic matter?
• Locally produced, recycled material
• Home, farm, or commercial
• Can usually be applied at high rates to
increase organic matter benefits
• Hot composting kills pathogens
•Biological
What is
composting?
transformation of raw
organic materials into biologically
stable, humus-rich substances
suitable for growing plants
What can compost
be made of?
•Feedstocks
include
 yard debris
 wood waste
 biosolids
 dairy solids
 feedlot manure
 poultry manure
 fair waste
 and more
Compost Quality
•Quality
depends on specific use
(landscape incorporation vs. mulch
vs. potting mix component)
Compost Quality:
Important things to
know
• Moisture
• Particle size
• Organic matter
• Nutrient availability
• Salts
• Biological stability
• Contaminants
Compost moisture
affects handling
•Dry
compost (< 35% moisture) is
dusty
•Wet compost (> 60% moisture) is
clumpy
Compost particle size
• Particle size < 1 inch is good for
incorporation in landscape beds
Compost organic matter
•Typically 40 to 60%
•If a compost contains
large
amounts of soil, the organic
matter content will be lower (this
may be true of backyard and
feedlot composts)
Keys to compost
nutrient availability
•Carbon to Nitrogen
•Biological stability
(C:N) ratio
Soluble salts
•Less
of a problem in humid
climates than in arid climates
•General recommendation is
soil:compost blend < 2.5 to 6
mmho/cm, depending on sensitivity
of plants
Biological Stability
•Unstable
compost can harm plants
(phytotoxic compounds include
organic acids and high levels of
ammonia).
•Indicators include color, odor,
very low or high C:N, stability
test kits.
• Inerts
Compost
contaminants
(plastic etc.) affect aesthetic
appeal.
• Metals (lead, cadmium etc.) tend to
be low in Northwest composts.
• Pesticides: Clopyralid was a concern in
some composts, but no problems have
been reported since 2001.
Other organic
amendments
• Cover crops
• Yard debris (leaves, grass clippings)
• Uncomposted manures (horse, dairy
solids, rabbit, goat, etc.)
• Class A biosolids (such as Tagro)
• Food waste (coffee grounds, vegetable
trimmings)
Amending
soil with
organic
materials
How much to add?
• Physical benefits are most apparent
with high rates of amendments.
• Materials must have low nutrient
availability to avoid potential N
leaching when high rates are used.
• Most research has been done on
agricultural soils.
• Maximum rates studied are about
1/3 by volume.
Landscape
plantings
• Most research has focused on
amended planting holes.
• Little or no benefit of amending
holes.
• Not much data available for
planting beds.
• Recommend 1/3 by volume based
on results from ag research and
field experience.
Annual Beds
•Establishing
raised beds. You can
use up to 30 to 50% by volume of
suitable material. Expect
settling.
•Annual amendments. One half to
one inch per year to maintain OM.
Organic Mulches
•Cover ground, reduce erosion
•Reduce growth of weeds
•Reduce evaporation
•Buffer surface soil temperature
•Decompose to become part of soil
organic matter
Organic Mulches
for Landscapes
• Coarse, woody material (bark, wood chips)
are good for weed control in landscapes
• Apply 3” deep, keep away from trunks
• Compost mulches may not control weeds
well after the first few months
• Woody mulches may slightly reduce N
availability to plants in first year after
application
Which mulch
where?
•Landscapes: Woody mulches
•Annual gardens: Mulch in winter
with compost or straw, or grow
cover crops (living mulch)
•Turf: Do not mulch