Understanding Soil Organic Matter

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Understanding Soil Organic Matter
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Joel Gruver NCSU Soil Science http//www.soil.ncs
u.edu/lockers/Gruver_J/
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SOM
Mineral particles
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SOM is a complex mixture
Living organisms
Biologically active SOM
Recent residues
Humus
Stable SOM
Adapted from Magdoff and Weil (2003)
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HUMUS

• Extreme chemical complexity
• Resistance to further decomposition
• High specific surface and negative charge
• Dark color

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Humus adsorbs ions and molecules
Adapted from Brady and Weil (2002)
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Humus increases plant available H20
Adapted from Brady and Weil (2002)
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Humus gives soil a darker color
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Organic matter functions mainly as it is
decayed and destroyed. Its value lies in its
dynamic nature. W. Albrecht, 1938
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The Soil Stomach
Bacteria Fungi Algae Protozoa Nematodes Microarthr
opods Enchytraeids Earthworms Ants, termites,
spiders Mollusks Others rodents, snakes,
voles, amphibians, etc.
Body size increasing
Microflora
Microfauna
Mesofauna
Macrofauna
Megafauna
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BACTERIA
cocci
bacillus
filamentous
spirilla
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FUNGI
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but most of the nutritional resources in soil
require digestion by the soil stomach
Roots are often the dominant soil organism by
weight
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Active OM energizes biologically mediated
processes
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The metabolic potential of soil microbial
communities greatly exceeds organic inputs to
soils
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Why does organic matter accumulate in soils ?
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Blechh !!! Tastes Bad !!!!
Biochemical recalcitrance
How do you expect to live off this stuff ???
Mineral protection
I cant get if off, you try !!
Fe
Al
We already are !!!!!!
Yuck !!!!! Sure is gritty !
Physical protection
Theres gotta to be a way inside !!!
Hey !! Theres good stuff in there !!!!
Adapted from Jastrow and Miller (1997)
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Understanding organic resource quality
(Giller, 2000)
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Chemical recalcitrance
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Relationship between fine mineral fraction and SOM
Magdoff and Weil (2004)
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Macroaggregates often form around particulate
OM. Processes that impact aggregation impact SOM
dynamics
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Location within the soil matrix affects SOM
dynamics
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Free POM
Sensitivity to management
Intra-aggregate POM
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Mineral associated OM
Adapted from Carter (2002)
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Climate affects SOM dynamics
25o C
Brady and Weil (2002)
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Landscape position affects SOM dynamics
Poorly drained
Interstream divide
Somewhat poorly drained
Moderately well drained
LANDSCAPE POSITIONS
Well drained
Poorly drained
Interfluve
Shoulder
Valley floor
SOIL DRAINAGE CLASSES
Backslope
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Blackland soils of North Carolina
Lily (1981)
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Geographic distribution of SOM
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How much is enough ??
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There are many ways to measure SOM
Total organic matter mass loss by ignition
Total C by several methods
Humic matter alkali extraction
Adapted from Strek and Weber (1985)
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Soil size fractions

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24.9
12.0
5.6
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Permanganate oxidizable C a routine test for
active soil C ??
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Permanganate field test for active C
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Crop growth response to using a rye cover crop on
soils with varying levels of the permanganate
active C test.
Data suggest little benefit of further increases
in active C beyond about 700 mg/kg. Soils with
less than 400 mg/kg usually show large response
to increasing SOM.
2.5 g soil in 20 ml 0.02 M KMnO4
From Lucas and Weil
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Effect of previous 20 years of rotations on SOM
and corn growth on Beltsville silt loam in
Maryland
Continuous bluegrass sod
Continuous corn with tillage
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Soils sampled from Ed Stricklings rotation plots.
25 yrs of conventional corn
20 yrs of bluegrass, then 5 yrs conventional corn
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After adding water to soils from the rotation
plots.
25 yrs of conventional corn
20 yrs of bluegrass, then 5 yrs conventional corn
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Aggregate stability is a key component of good
tilth
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Good Tilth
Poor Tilth
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Plasticity vs. Friability
Soils with good tilth are friable under a wider
range of moisture contents.
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Do not till or traffic on wet soils !!!
Structural damage
Soils with high C are more resistant to
structural damage !
(Watts and Dexter, 1997)
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More OM is needed to stabilize fine textured soils
16 clay
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16 clay
Adapted from Russel
(1973)
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SOM reduces bulk density
Magdoff and Weil (2004)
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Young particulate organic matter (POM)
contributes to disease suppression
Disease Suppression
Damping off
Adapted from Stone et al. (2004)
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Effect of compost maturity on tomato seedlings
Chen et al. (2004)
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Seasonal changes in mineralizable N
Adapted from Magdoff and Weil (2004)
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Managing SOM
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Systems approach
Bailey and Lazarovits (2003)
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Tillage affects the location of OM
Adapted from House and Parmelee (1985)
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What does tillage do to soil structure ?
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Microbial activity in contrasting tillage systems
Havlin et al. (1999)
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Potential for poor synchrony
Available for loss
Adapted from Robertson et al. (1998)
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The fence post principle
Adapted from Schriefer (1984)
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Spading machine
http//www.tramsales.com/vp.html
Cool animation !!!
http//www.timmenterprises.com/machines/spader2.ht
m
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Impact of high CN ratio residues on min-N
http//res2.agr.ca/stjean/publication/bulletin/nit
rogen-azote_e.pdf
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On-farm sources of OM
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The many benefits of surface residues
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Off-farm sources of OM
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Vermicompost
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Plant growth promoting rhizobacteria (PGPR) feast
on root exudates
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N and P from long-term applications of compost
Compost nutrient content 1.7 N 1.2 P
Tons of compost to supply 150 lbs of N/acre
Cummulative P build up in lbs/acre
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3000
60
2000
P in excess of crop removal
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1000
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3 6 9 12
3 6 9 12
YEARS
YEARS
Adapted from Magdoff and Weil (2004)
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Erosion Control Practices
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Crop Rotation
High residue crops
Cover crops
Forages
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Multi-functional cover crops
Cover Crops
Adapted from Magdoff and Weil (2004)
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Broccoli
Lettuce
Adapted from Sarrantonio (1994)
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Tomatoes
Adapted from Sarrantonio (1994)
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