Compost Benefits
David M. Crohn
Department of Environmental Sciences
University of California, Riverside
Marketing
Healthy Soils for
Healthy Plants
Soil Nutrients: macronutrients
Total N
–Organic N
–Available N (nitrate and
ammonia-N)
Total P
Total K
Soil Nutrients : micronutrients
Boron
Chlorine
Cobalt
Copper
Iron
Manganese
Magnesium
Molybdenum
Sulfur
Zinc
Soil properties
Organic matter
Water holding
capacity
Bulk density
Structure
Texture
ECe
pH
pH
Low pH =
acid soil
High pH =
alkaline soil
Neutral pH =
7
ECe = salinity
ECe stands for Electrical Conductivity
Salts conduct electricity and this is used to
measure them in soils
High ECe soils may have trouble
germinating seeds and supporting growth
ECe is a water management indicator, more
than a soil property
Texture
The sand,
silt, and claysized particles
determine
soil texture
Not affected
by structure
or OM
Bulk density
Soil mass .
Soil volume
Dense soils
–
–
–
–
Retard water movement
Hold less water
Impede air exchange
Stop root development
Fig. Engle et al. WSUE
Water holding
capacity
Available Water Capacity
by Soil Texture
Textural Class
Available
Water (in/ft
soil)
Coarse sand
0.25–0.75
Fine sand
0.75–1.00
Loamy sand
1.10–1.20
Sandy loam
1.25–1.40
Fine sandy
loam
1.50–2.00
Silt loam
2.00–2.50
Silty clay loam
1.80–2.00
Silty clay
1.50–1.70
Clay
1.20–1.50
How
Composts Benefit
Soils
Why compost?
To eliminate disease
organisms
Animal ● Plant ● Human
To produce a stable and
safe soil amendment
Nutrients ● Odors ● Phytotoxins
Organic matter: What you are selling
Hargrove and Luxmore
SOM: Matter of life and
death
Carbon
Source of energy for microbes
Not all forms are equally
available
– sugars (more available)
– fats, waxes, proteins
– cellulose
– lignin (less available)
C
Starches
Used by plants and animals for
intermediate energy storage.
A carbohydrate (CH2O)x
Readily metabolized by microbes
Decomposed during composting
Cellulose
Most abundant organic compound on Earth
Examples: paper, cotton, cellophane, rayon
Like starch, a carbohydrate composed of
chains of glucose molecules
(polysaccharides), but joined by -linkages
Cellulose is used by plants for construction
of cell walls
Tend to be structured in straight lines
Can be metabolized by bacteria, fungi
Ruminants, termites use bacteria to break
down cellulose
Starch
and
Cellulose
Lignin
Second most abundant organic
compound on Earth
Has a complex structure
Along with cellulose is used by plants
for construction of cell walls
Can be metabolized by fungi
Lignin
molecule
C
Soil Nutrients: macronutrients
Total N
–Organic N (slow release )
–Available N (variable)
Total P (often rich)
Total K (variable)
Bacteria
80 – 90% of the microbial community is bacteria
Small: 0.5 – 3 m diameter
High surface:volume ratio. Important
for moving nutrients in and waste products out
C:N ratios on the order of 4.3:1 to 6:1
Soil species differ from active
compost species
– 0 - 40C: mesophyllic
– 40-65 C: thermophilic
Fungi
Fungi include molds and yeasts
Larger than bacteria
Grow more slowly
Able to metabolize lignin
C10H17O6N
C:N ratios on the order of 9:1
Fungi on
lignin
N
Nitrogen
Vital nutrient for both microbes and plants
Microbes are better than plants at accumulating
N
C:N ratio
– Compost feedstock initially about 35:1
– About 10:1 to 20:1 in cured product
– About 8:1 in a soil
N
Soil Nutrients : micronutrients
Boron
Chlorine
Cobalt
Copper
Iron
Manganese
Magnesium
Molybdenum
Sulfur
Zinc
(Variable,
depending on
source)
pH
Low pH =
acid soil
High pH =
alkaline soil
Neutral pH =
7
(buffers pH)
ECe = salinity
ECe stands for Electrical Conductivity
Salts conduct electricity and this is used to
measure them in soils
High ECe soils may have trouble germinating
seeds and supporting growth
ECe is more of a water management indicator
(Contains salts, some of which are nutrients.
Improves tilth so salts can leach away)
Texture (no effect)
The sand,
silt, and claysized particles
determine
soil texture
Not affected
by structure
or OM
Bulk density (decreases density)
Soil mass .
Soil volume
Dense soils
–
–
–
–
Improves water movement
Holds more water
Increases air exchange
Promotes root development
Bacteria and fungi
Bacteria are much
smaller than fungi
Both decompose
composts in the
soil
Compost in soil
Encourages the formation of soil
aggregates
Aggregates are soil clusters held together
as a result of compost decomposition
– Fungal hyphae
bind particles
together
– Bacterial
polysaccharides
serve as glue
Fig. Engle et al. WSUE
Aggregated soils
Hold water while allowing air to
penetrate
Facilitate drainage and salt removal
Allow roots to penetrate
Are more stable, resisting erosion
– Sheet
– Rill
Water holding
capacity
Available Water Capacity
by Soil Texture
Textural Class
Available
Water (in/ft
soil)
Coarse sand
0.25–0.75
Fine sand
0.75–1.00
Loamy sand
1.10–1.20
Sandy loam
1.25–1.40
Fine sandy
loam
1.50–2.00
Silt loam
2.00–2.50
Silty clay loam
1.80–2.00
Silty clay
1.50–1.70
Clay
1.20–1.50
Overcome worry…
…with knowledge and information
Photo credit: aur2899
Compost Use Index
Derived from focus groups asking experts and
users what they care about.
Measureable using STA/TMECC values
Designed to indentify quality composts for
specific uses
Not all parameters matter for all uses
Currently implemented for
–
–
–
–
–
Avocado
Lettuce
Tomato
Grape
Strawberry
Use Index Parameters
C:N Ratio
Organic Matter
Salinity (EC)
Total N
Total P
Stability
Phytotoxicity
pH
Boron
Chloride
Sodium
Manmade Inert
Impurities
Particle Size
Distribution
Safety
– Metals
– Pathogens
Carbon:Nitrogen Ratio
Low C:N do not tie up N and are
biologically most active
High C:N Ratios (>25) imply lignin and
cellulose which resist decay.
– Good for mulches.
– May immobilize N in soils, stealing N from
plants
Use
Mulch
Soil
amendment
C:N ratio
 15 : 1
 20 : 1
Organic Matter
Carbon based
Derived from the tissue of living things
Substrate for soil microbes that improve soil
structure
Helps hold plant nurtients
> 65% O.M. suggests immaturity
Use
Mulch
Soil
amendment
OM
 25 %
 25 %
 65 %
Salinity (EC)
Most significant issue for arid and semi-arid areas
All charged ions, including nutrients, contribute to salinity
Compost and soil salinities are measured differently.
– Compost: EC5:1 (5:1 water to compost)
– Soil: ECe (saturate paste extract)
Plant toxicity levels are in terms of ECe
Formula needed to predict the ECe of a Soil/compost mix
–
–
–
–
Soil texture
Soil organic matter content
Compost organic matter content
Mixing depth
Salinity is a transitory property
Salts added with irrigation water
usually far exceed compost salts
Soil/compost
Crop
mix ECe, dS/m
Avocado
4
Lettuce
1.3
Tomato
2.5
Grape
1.5
Strawberry 1
Macronutrients
Total N
– Source of crop nutrients
– Too much ammonia can harm crops
Total P
– Not a crop health concern
– Eutrophication concern
Maturity
Stability
– Unstable composts are still decomposing
rapidly. This emits CO2. And odors.
Use
C:N ratio
Mulch
 8 mg CO2/g OM/day
Soil amendment  8 mg CO2/g OM/day
Phytotoxicity
– Germination test
Use
C:N ratio
Mulch
 80 % emergence
Soil amendment  80 % emergence
pH
Describes the acidity of the compost
Low pH (acid) composts may not be
mature.
Phytotoxic organic acids produced during
composting must be decomposed as they
are phytotoxic.
High pH (alkaline) composts may be
contaminated.
Use
C:N ratio
Mulch
6.0  pH  8.5
Soil amendment 6.0  pH  8.5
Boron toxicity thresholds (Hanson et al., 1993, Maas, 1986).
Boron
Toxic B Levels: Concentration
in saturated
Crop
extract, mg L-1
Avocado
0.5 – 0.75
Lettuce
0.5 – 0.75
Tomato
4.0 – 6.0
Grape
0.5 – 0.75
Strawberry
0.75 – 1.0
Naturally present in some
soils and irrigation waters. More available under
acid conditions. Controlled through leaching.
Once commonly added to plywood glue, therefore
found in MSW, demolition debris, coal or wood
ash feedstock
More available under lower soil pH conditions
Hard to say how a compost Use
C:N ratio
Mulch
 100 mg kg-1
will affect available soil
Soil
 100 mg kg-1
levels
amendment
Toxic C Levels
Chloride
Crop
Avocado
Lettuce
Tomato
Grapes
Strawberry
Concentration
in saturated
extract, meq L-1
5 – 7.5
n.a.
n.a.
30 – 40
5 – 7.5
Toxic to some crops,
but not to others.
Almost all composts can meet the 1% dry
wt. limit
Crop
Avocado
Lettuce
Tomato
Grape
Strawberry
Chloride content
 0.2 % dry weight
 1 % dry weight
 1 % dry weight
 1 % dry weight
 0.2 % dry weight
Sodium
Sodicity
Sodium disperses clay particles
collapsing soil structure
This is offset by compost organic matter,
calcium and magnesium
Phytotoxicity
Crop
Sodium content
Avocado
 0.2 % dry weight
Lettuce
 1 % dry weight
Mainly woody
Tomato
 1 % dry weight
plants
Grape
 1 % dry weight
Strawberry
 1 % dry weight
Manmade Inert Impurities
Plastic, in particular, can litter the soil.
Remember that runoff water is often reused
and plastic can clog water systems
Use
Inerts
Soil amendment  0.75 % dry weight
Mulch
 0.75 % dry weight
Particle size distribution
Mulches should have larger particles than
soil amendments.
Large particles in soils can tie up nutrients.
Too many small particles in mulches can
support weed development.
Use
Soil amendment
Mulch
Size Characteristics
 95% passing though a 5/8” screen
 70 % passing through a 3/8” screen
 99% passing though a 3” screen
 25 % passing through a 3/8” screen
Metals
Current numbers based on assumption of 100 years of
biosolids applications at 4.5 t/ac and a detailed risk
assessment.
Compost applied at higher rates than biosolids so those
number are not conservative.
Mulches are applied at higher rates still, though less often.
Constituent
Arsenic (As)
Cadmium (Cd)
Chromium (Cr)
Copper (Cu)
Lead (Pb)
Mercury (Hg)
Nickel (Ni)
Selenium (Se)
Zinc (Zn)
Amendments
(ppm)
41
39
1200
1500
300
17
420
36
2800
Mulches
(ppm)
3.0
2.9
221
111
22.1
1.3
31
7.4
206
Pathogens
Many diseases are spread through a fecal-oral transmission
route. Both humans and animals can deposit infectious
bacteria, viruses, and helminths.
Because there are so many infectious organism out there,
we use indicator organisms. Because E. coli O157:H7 is so
dangerous, suggest checking when applied to produce
eaten fresh.
Use
Avocado
Lettuce
Tomato
Grape
Strawberry
Size Characteristics
Fecal coliforms: < 1,000 MPN per gram of total solids
Salmonella sp. < 3 MPN per 4 g of total solids
Fecal coliforms: < 1,000 MPN per gram of total solids
Salmonella sp. < 3 MPN per 4 g of total solids
E. coli O157:H7: non detect
Fecal coliforms: < 1,000 MPN per gram of total solids
Salmonella sp. < 3 MPN per 4 g of total solids
Fecal coliforms: < 1,000 MPN per gram of total solids
Salmonella sp. < 3 MPN per 4 g of total solids
Fecal coliforms: < 1,000 MPN per gram of total solids
Salmonella sp. < 3 MPN per 4 g of total solids
E. coli O157:H7: non detect
My Advice to Growers on
Choosing a Compost
Trust your judgment!
Compost should have an earthy smell
An off smell suggests that the compost is
not ready yet. It may damage young plants
Composts have different amounts of organic
matter. 25 – 75%. More is usually better
Local stuff will be cheaper to deliver
Value the reputation of the composter
STA certification
Soil amendments:
modify soil properties
Improve tilth
Improve water holding capacity
Improve aeration
Improve infiltration capacity
Adjust pH
Promote microbial activity
Mulches:
cover the soil
Protect soils against erosion
Conserve water by slowing
evaporation
Control weeds
Control certain plant diseases
Decorate landscapes
Organic fertilizers:
slow release sources of
Nitrogen
Phosphorus
Potassium
Micronutrients
Summary on uses
Mulches, soil amendments and
organic fertilizers are uses, not
products.
Composts may serve any of these.
Different composts have different
properties, and will therefore be more
suitable for some uses than for others.
Think long-term.
Focus on the soil
and your plants
will thrive.