Composts
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What is compost and why use it?
What organisms are involved in the composting
process?
What chemical changes occur during composting?
What are the optimal conditions for composting
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Does composting kill harmful pathogens (plant
and human), nematodes and weed seeds?
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Temperature
Moisture
Oxygen
Safety regulations
Compost production systems
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Small scale
Commercial
What is compost – why use it?
Composting is the
decomposition of plant
remains and other onceliving materials to make an
earthy, dark, crumbly
substance that is excellent
for adding to houseplants or
enriching garden soil.
•compost improves soil structure, texture, aeration - increases
the soil's water-holding capacity.
•Compost loosens clay soils and helps sandy soils retain water.
•improves soil fertility and stimulates healthy root development
•Organic matter provides food for microorganisms - nitrogen,
potassium, and phosphorus mineralized
The Science of Composting
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Composting is the natural process in which living
organisms decompose organic matter into
inorganic matter in the soil.
The organisms feed on the organic material and
through respiration generate the energy that they
use for movement, growth, reproduction or
stored energy.
The organism excrete inorganic material that
enriches the soil.
When the organisms die, their bodies add to the
organic matter in the compost pile.
Fresh Organic
Materials
+
Oxygen
Microbes,
Moisture,
and Time
Compost
+
Carbon
Dioxide
+
Energy
Slide credit: Tom Richard, Penn State University
Organisms use carbon as a source of
energy and nitrogen to grow and
reproduce.
•Too little N:
•there will be few
microorganisms,
and decomposition
will be slow.
•Too much N:
•some will turn to
ammonia that will
volatilize, creating
an odor.
Experimental test - effect of C:N ratio on nitrogen
retention in compost
Initial C:N ratio
Final
nitrogen
%
Nitrogen
conservation
%
1
20
1.44
61.2
2
20.5
1.04
51.9
3
22
1.63
85.2
4
30
1.21
99.5
5
35
1.32
99.9
Factors affecting the compost process
C:N
ratio
Aeration
Size and
texture
NB. Moisture level also critical
Moisture level is also critical
•Optimum moisture content
40-60%
•Feels moist to touch, but
when squeezed only
produces few drops
Ideal conditions for composting
Parameter
Moisture
C:N ratio
Oxygen
Temperature
Bulk density
pH
OK
40-65%
20-40:1
>5%
43-66 C
1000 lbs/yd
5.5-9.0
Ideal
45-60%
25-35:1
>10%
54-60 C
1000 lbs/yd
6.5-8.0
The Science of Composting
Temperature (C)
Three Phases of Thermophilic
Composting
60
50
40
30
20
10
0
0
10
20
30
40
Time (days)
Composting goes through three distinct phases that can
be characterized by temperatures.
•
•
•
Mesophilic Phase (moderate temperature)
Thermophilc Phase (high temerpature)
Mesophilic Phase (moderate temperature again)
The Science of Composting
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Mesophilic Phase 1 (10-40
•
•
•
•
•
•
•
•
•
•
Phase (>40 0 C)
Can last from several days to several months depending on size of
system
Mixed population of heat loving organisms
High heat helps breakdown of proteins, fats, “tough” plant material
like cellulose
High temperature (>55 0C) kill weeds and pathogen harmful to
humans
Higher temperature (>600C) kill organism needed for decomposition
Mesophilic
•
C)
Lasts only a few days
Explosive growth of bacteria and fungi
Rapid breakdown of soluble sugar and starches
Thermophilic
•
0
Phase 2 (10-40 0 C) “Curing Phase”
Can last several months
Bacteria, fungi, actinomycetes( mix between bacteria and fungus,
give “earthy” smell) predominate. Invertebrates active.
Supply of organic material has decreased. Remaining organic
material is slowly broken down.
Additional chemical reactions take place to make remaining organic
material more stable
The Science of Composting:
Chemistry
Important factors in compost chemistry
 Carbon-Nitrogen Mix (C/N Ratio)
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Carbon provides energy source and building material
for 50% of composting organisms’ cells
Nitrogen important in formation of proteins, nucleic
acids, amino acids, enzymes etc. for organisms
30:1 Carbon to Nitrogen optimum mix (decreases in
curing phase)
Brown and woody carbon
Green and moist nitrogen
The Science of Composting:
Chemistry
Important factors in compost chemistry
 Oxygen
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Needed to oxidize carbon for energy
Without oxygen will produce rotten egg smell
pH Level
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Acids form as organisms digest organic material and
lowers pH
Lower pH encourages fungi and the break down of
“tough” matter
If pH too low (<4.5) limits microorganisms’ activity
Changes in Ammonium-N
distillation method; fresh sample
-1
NH4-N (mg kg )
4000
FA
PA
3000
2000
1000
0
0 20 40 60 80 100 120 140
Composting time (days)
Changes in Carbon to Nitrogen Ratio
24
FA
PA
C: N
20
16
12
8
0 20 40 60 80 100 120 140
Composting time (days)
Dan Sullivan and Linda Brewer
Changes in cation exchange capacity
CEC
-1
(cmol kg compost-C)
(ash-free; pH 7)
500
400
300
FA
PA
200
100
0
0
20 40 60 80 100 120 140
Composting time (days)
Temperature
Fatty acids
NH3
emitted
pH
days
Compost chemistry - pH
The Science of Composting:
Physics
Important factors for compost physics:
 Temperature
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3 Phases
Want to maintain temperature between 55-600C
Temperature impacted
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Heat generated by organism
Heat lost to environment through conduction, convection
and radiation  shape and size of pile
Moisture content (specific heat and heat capacity of
water)
The Science of Composting:
Physics
Important factors for compost physics:
 Particle size
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Microorganism activity occurs on surface of
organic material
The more surface area for organisms to attack,
the quicker the decomposition  want smaller
particles
Flip-side: The smaller the particles, the more
dense and compact the material resulting in poor
oxygen circulation
The main players
1. Bacteria:
major decomposers, breakdown
simpler forms of organic material
2. Actinomycetes:
degrade complex organics such as
cellulose, lignin, chitin, and proteins –
earthy” smell, long “spider webs”
filaments
3. Fungi:
Break down tough debris, too dry, too
acidic or too low in nitrogen for
bacteria to eat
What do microbes in compost do?
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Consume organic matter to grow
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Mineralize nutrients
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Organic to inorganic forms (protein to NH4)
Transform nutrients
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Stabilize organic matter
Aerobic oxidation produces CO2
Anaerobic produces reduced compounds
organic acids, alcohols
Nitrification – pH and temperature sensitive
NOTE: invertebrates not important in high temperature
composting, only in cold
Compost Quality
Compost Maturity and Nitrogen Release
Characteristics in Central Coast Vegetable
Production
July 2002
CA Integrated Waste Management Board
Marc Buchanan, PhD
Compost Maturity
Defined as various levels of maturity based upon:
C:N ration of 25 or less, plus at least one from each of the following:
Group A (tests to determine compost stability)
 CO2 evolution or respiration
 Oxygen demand
 Dewar self-heating test
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Group B (tests to further determine maturity in reference to
potentially phytotoxic compounds)
 NH4-NO3 ratio
 NH3 concentration
 Plant test (seed germination and growth)
 Volatile organic acids concentration
Compost Maturity
Very Immature
C/N ratio greater than 25, and/or stability test is greater
than 12, and/or NH4 is greater than 500 and no nitrate
present.
Immature
Unstable compost
Odors likely
High toxicity potential
Immobilization (tie-up) of available nitrogen
Moderately Mature
Stability test greater than 6 and less than 8 and/or when
nitrate is detected and is greater than 25 ppm N.
Mature
Cured compost
Odor production not likely
Limited toxicity potential
Positive impact on available soil nitrogen
Very Mature
Well-cured compost
No continued decomposition
No odors
Compost Quality Measures
Test
Rating
VM
M
IM
CO2 Test Stability 1
(respiration rate)
C / unit organic matter/ day
<2
2–8
>8
BIO-C CO2 Stability 2
C / unit organic matter / day
<2
2–8
>8
NH4-/NO3-N Ratio
< 0.5
0.5 – 3
>3
Seed Germination
% of control
> 90
80 – 90
< 80
Plant Vigor Tests
% of control
> 90
80 – 90
< 80
C:N below 25
COMPOST QUALITY INDEX for Commercial Products
Based on 13 monthly samples for green waste, 11 for blend, and
1 poultry manure compost.
%
%
Ammonium Nitrate
C:N Stability1 Stability germination vigor
-N ppm
-N
2
ppm
ratio
Compost
Green
Waste
average
range
14
7-29
Blend
average
range
12.7
8-18
VUS - VS
VUS VS
7
S
S
VUS -VS
US - VS
92
85-100
90
75100
244
22-579
287
0-784
13.6
0.03 - 122
59
20-95
38
0-94
745
0-2477
155
0-485
14.9
0 -130
0
0
5578
0.0
nd
Poultry
Compost Type
Maturity
Crop
Yield
Sandy Soil Class (Coarse Sandy Loam and Fine Sandy Loam Textures)
Blend
Green Waste
VM
VM
VM
VM
M
IM
IM
IM
MM
Baby Lettuce
Baby Lettuce
Baby Spinach
Baby Spinach
Frisse
Baby Spinach
Baby Spinach
Baby Chard
Baby Mustard
o
+
o
?
o
+
o
+
MM
IM
IM
MM
IM
MM
IM
MM
M
M
VIM
Baby Lettuce
Baby Lettuce
Baby Lettuce
Baby Spinach
Baby Spinach
Baby Spinach
Baby Spinach
Baby Spinach
Baby Chard
Frisse
Baby Mustard
+
+
o
o
o
?
-
+ = Increased yield
O = No difference vs check
- = Decreased yield
? = no-compost treatment not included
Loamy Soil Class (Sandy Clay Loam Texture)
Blend
M
MM
MM
Lettuce
Lettuce
Celery
+
+
+
Green Waste
M
IM
M
Lettuce
Lettuce
Celery
o
o
+
Inorganic N Released (lb N/acre)
Inorganic N release – Spring and summer 2000
Poultry
400
Blend [M-18]
300
Green [M-1]
200
Check
100
0
0
50
100
150
Days Afte r Incorporation
200
250
60
Blend
% Nitrogen Released
50
Green
40
30
20
10
0
-10
-20
VIM MM MM
M
M
M
VM VIM VIM MM
Compost Maturity
M
M
VM
Conclusions
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The maturity index appears to be a useful tool to assess compost quality.
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Mature composts most consistently result in positive crop yield response
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Chicken manure and green waste blends provide more available N
than pure green-waste composts.
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Immature and very mature composts may reduce inorganic soil N
sufficient to have negative impact on production in conventionally farmed soils
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High NH4-N levels associated with immature manure-based composts can
reduce seed germination and crop productivity.
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Mature composts can provide significant N dependent on timing
of applications.
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Maturity status of compost is a significant predictor of compost quality
for vegetable production.
Growing Issue
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How effective is composting at killing
pathogens?
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Weeds, plant disease organisms
Human pathogens!!!!
U.S.A. Composting Regulations
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Biosolids, Class A compost (U.S. EPA 40 CFR Part 503)
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Time-temperature relationship (PFRP)
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Vector Attraction Reduction
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14 days, 40 C minimum, 45 C average temperature
Pathogen testing criteria
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Static aerated pile, 3 days > 55 C
Turned windrow, 15 days > 55 C, turned at least 5 times
Fecal Coliforms < 1000 MPN/g TS
or Salmonella < 3 MPN/4g TS
USDA National Organic Program §205.203 (c)
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Time –temperature 55 C – 70 C (CFR Part 503)
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Turned windrow 15 days with at least 5 turns
In-vessel or static aerated system 55 C – 70 C for 3 days
C:N ratio 25:1 – 40:1 (NRCS code 317 composting facility)
Why turn windrows at least 5 times in 15 days?
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Prevent regrowth of Salmonella
Non-uniform heating
Turn cooler material into insulated center
> 55 C
Done properly it works, but can
less intensive regimes work too?
NOSB Compost Task Force
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Composting regulations too prescriptive
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Manage compost to reach 55 C for 3 days
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Vermicompost
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Aerobicity maintained by adding thin layers every 1-3 days
70-90% moisture
12 months for outdoor windrows, 4 months for wedge systems
or indoor containers, 2 months for vertical flow reactors
Processed manure
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Heat to 65 C for 1 hour
Dry to < 12% moisture
Negative for Salmonella and fecal coliforms
Animal pathogen destruction
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Meet current time-temperature standards
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Will not eliminate all weed seeds or all plant pathogens
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Attain sanitation target for particular end-use,
quality assurance testing
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Use technology that is financially attainable
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Composting process
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Contain and treat leachate
Exclude vectors
Avoid pathogen regrowth conditions
Avoid recontamination of product
Compost production systems

Small scale
Compost piles
need to be at
least one cubic
to hold the
heat from
decomposition
Passive composting
Commercial composting
Large scale
 Passive aeration with turning or actively
aerated systems
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Feedstock conditioning - grinding
Moisture management - most important factor to
stabilize biological and chemical properties
Processing – turned windrows
Compost blankets to moderate moisture
www.vanierselcompost.com/. ../productie.htm
Composting - windrow
Turning helps
aeration and
to move
material from
edge into hot
center region
CO2
Hot
Cool
O2
Graphic credit: Tom Richard, Penn State University
Composting – Static forced air
CO2
Hot
O2
Cool
Graphic credit: Tom Richard, Penn State University
•Air forces heat
outwards
•Some systems
can switch
direction to
keep base core
at high enough
temperature
•Also helps
control odor
Processing - forced aeration
Hot ammonia kills!!!
Contain, treat leachate
Compost blankets – beware moving from fresh to curing
www.vanierselcompost.com/. ../productie.htm
Cure compost with 40-50% moisture
to promote competitive microorganisms and
avoid salmonella regrowth
CASFS Farm compost piles 2008
Compost Temperatures
60
50
Degrees C
40
#1
#2
#3
#4
30
20
10
0
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Date
(*C)
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