Download What is Soil Organic Matter? - AgEBB

THE VALUE OF SOIL ORGANIC MATTER
A.
B.
Which soil would you
pick to grow plants in
your garden?
What is Soil Organic Matter?
• Definition:
“The organic fraction of soil including plant,
animal, and microbial residues, fresh and at
all stages of decomposition… and highly
carbonized compounds such as charcoal,
graphite and coal”
Soil organic matter = (2 x C) + O, H, N, P, S,
etc.
FUNCTIONS OF SOIL ORGANIC MATTER
SOIL ORGANIC MATTER AND SUSTAINABILITY
• Provides soil nutrients and enhances soil
nutrient availability over several growing
seasons.
• Improves soil physical, chemical and
biological properties for plant growth.
• Buffers changes in soil properties due to
disturbance or variation in other
environmental factors (e.g. climate).
• Reduces environmental contamination and
soil loss.
• Possibly reduces need for external inputs and
enhances internal recycling when crop
residues and/or organic soil amendments are
applied.
RESERVOIR OF PLANT NUTRIENTS
• 95% or more of total N in soil is
organic N
• If soil contains 0.2 % total N then:
0.2 kg N/100 kg soil * 95% * 2,000,000
kg soil/hectare plow layer
= 3800 kg organic N/hectare plow
layer or 3393 lb organic N/acre
plow layer
CARBON IN SOIL
• What are the major forms of C in soil?
Organic residues
CO2 (60-80%)
Soil
Soil organisms
(3-8%)
Nonhumic
substances (3-8%)
Soil humus (15-35%)
Organic residues in soil
Complex humic
substances (10-30%)
FORMS OF SOIL ORGANIC MATTER
Particulate organic matter and sand particles under magnification
AGE OF SOIL ORGANIC CARBON
Sample Description
Grassland soil (North Dakota)
Virgin soil
Manured orchard
Bridgeport loam, Wyoming
Surface sod layer
Clay fraction from virgin soil
Continuous wheat plot
Mollisol, Saskatchewa
Surface soil
B horizons
Buried horizons
Mean Residence Time
--- years --1,175 ± 100
880 ± 74
3,280
6,690
1,815
545 to present
700 to 4,000
5,900 and 8,410
MANAGEMENT PRACTICES THAT
DECREASE SOIL ORGANIC CARBON
•
•
•
Frequent tillage
Burning for clearing
Removal of crop
residues
• Inadequate
fertilization
• Soil erosion
EFFECTS OF TILLAGE ON SOIL OM
LOSS OF SOIL ORGANIC C WITH TILLAGE
Sanborn Field
EFFECTS OF LAND-USE ON SOIL OM IN
THE TROPICS
Research conducted on the
Pacific island of Guam
LOSS OF SOIL ORGANIC MATTER
“Soil organic matter is one of our most important
national resources; its unwise exploitation has been
devastating; and it must be given its proper rank in
any conservation policy as one of the major factors
affecting the levels of crop production in the future”
William A. Albrecht
University of Missouri (1938)
LONG-TERM SUSTAINABLE MANAGEMENT AND
SOIL CARBON
Lowland Taro System in Palau
PRACTICES THAT INCREASE SOIL
ORGANIC MATTER
• Adopt conservation tillage, residue
management and mulch farming
• Apply organic wastes to soil
• Use N and other nutrient sources more
efficiently
• Convert marginal and degraded lands to
restorative land uses
• Adopt soil conservation practices
• Eliminate summer (bare) fallow and
incorporate legumes and other appropriate
cover crops in rotations
Kimble et al., 2002
CONSERVATION TILLAGE
Tillage
No-Till
EFFECT OF ROTATIONS ON SOIL OM
SOIL C DISTRIBUTION WITH DIFFERENT
LAND MANAGEMENT PRACTICES
Landscape
position
GR
TOC
AF
CR
GR
POM C
AF
CR
--------------------------- g kg-1 soil --------------------------
DOC
AF
GR
CR
-------- mg kg-1 soil ------
Upper
19.5
24.5
19.5
3.54
6.60
3.03
106.8
123.3
80.1
Middle
22.5
23.5
22.0
5.42
5.76
4.52
105.7
147.2
75.0
Lower
25.5
24.0
18.5
7.12
7.38
3.28
133.5
124.6
72.4
LSD(0.10)
------------- 2.4 ------------
----------- 2.68 ------------
------------ 22.2 ------------
Greenley Agroforestry Watershed Research
in Northeastern Missouri
RESIDUE QUALITY AND DECOMPOSITION
• Carbon to nitrogen of the
residue > 25 may result in
N deficiency in soil for
plant growth and slow down
the rate of decomposition.
• Plant residues containing
high proportions of lignin
and tannins decompose relatively slower than other
plant residues. Woody materials contain high
proportions of lignin. Oak leaves contain relatively high
tannin content.
• The smaller the pieces of residue, the faster it will
decompose.
CO2 efflux (µ mol m-2 sec-1)
COMPACTION AND DECOMPOSITION
60
Non--compacted
Non
45
Check
High litter
Non--compacted
Non
30
Compacted
Compacted
15
0
May 11
July 20
Sampling date
Compaction
also decreased
soil moisture
and increased
soil
temperature
EFFECTS OF GM CROPS
Transgenic Crop
Climate
Management
practices
Pesticides
Fertilization Tillage
(Animal waste)
Nutrient export
Crop residues
Root exudates
Soil properties:
Microbially-mediated
nutrient transformations:
Rhizosphere
• Mineralization/immobilization
• Oxidation/reduction
• Fixation/gaseous loss
• Solubilization
Gene transfer
• Clay type and content
• pH
• Soil structure
• Soil organic matter
DIFFICULTY IN CREATING A CARBON
BALANCE???
Biosphere 2, designed to be an artificial, selfcontained, self-supporting ecosystem. All water, air
and nutrients were recycled in the structure. Had
problems with maintaining atmosphere fit for life
partially due to role of soil organic matter.
http://www.bio2.edu
GLOBAL CARBON CYCLE AND C SEQUESTRATION
In Gt or Gt/year
1 Gt =1 Pg
= 1015g
Historic net loss of 66 – 90 Gt C from soil
REASONS
FOR CONSERVING
SOIL ORGANIC C
ANY
QUESTIONS?