The document discusses soil organic matter (SOM) dynamics in agricultural systems. It provides examples showing that only a small portion (around 10-17%) of crop residues and roots are retained as SOM in the long term. Factors like soil texture, historical vegetation, climate, landscape position, and management practices influence SOM levels by affecting the balance between organic matter inputs and losses through decomposition. Fine-textured soils in low-lying areas generally have higher SOM compared to coarse-textured or well-drained soils. Protecting organic materials from decomposition by physical protection within soil aggregates or association with mineral particles increases long-term retention as SOM.

1. Understanding soil organic matter
So when does
crop residue
become SOM?

2. Grain yields
have increased
dramatically
since WWII
Crop residue
production has
also increased
but not as
dramatically as
grain yields

3. *
200 bu/a
in 2014
IL corn yields have doubled
during my lifetime!

4. How much residue does a 200 bushel corn
crop return to the soil?

5. General rule of
thumb for corn
grain, stover
and roots each
comprise
~1/3rd of the
total biomass
33%

6. 200 bushels*~50 lbs/bu * 2 = 20,000 lbs residue/a/yr!
So how much of the ~10 tons/acre of dry residue
left by a 200 bu/a corn crop turns into
soil organic matter?
Why not 56? Why multiply by 2?

7. Estimating C inputs retained as soil organic matter from corn
Plant and Soil Volume 215, Issue 1, pp 85-91
M.A. Bolinder, D.A. Angers, M. Giroux, M.R. Laverdière
Abstract
In agroecosystems, the annual C inputs to soil are a major factor controlling
soil organic matter (SOM) dynamics. However, the ability to predict soil C
balance for agroecosystems is limited because of difficulties in estimating C
inputs and in particular from the below-ground part. The objective of this
paper was to estimate the proportion of corn residue retained as SOM. For
that purpose, the results of a 13C long-term (15 yr) field study conducted on
continuous silage corn and two silage corn rotations along with data from the
existing literature were analyzed. The total amount of corn-derived C (0–30
cm) was about 2.5 to 3.0 times higher for the continuous corn treatment (445
g m-2), compared to the two rotational treatments (175 and 133 g m-2 for the
corn-barley-barley-wheat and corn-underseeded barley hay-hay rotations,
respectively). Assuming that the C inputs to the soil from silage-corn was
mainly roots and would have been similar across treatments on an annual
basis, the total amount of corn-derived C for the two rotational treatments was
The results from this study indicate that ~ 17%
of corn root-derived C was retained long-term as SOM.
This is almost twice the ~ 10 % retention reported in the
literature for shoot-derived C and is in agreement with
many studies showing that
more root C is retained than shoot C.

8. The standard conversion from
SOM to C is %SOM / 1.72 = %C.
For soils or soil horizons containing large
amounts of relatively undecomposed plant
materials, a factor of 2 may be more
accurate (i.e., %SOM / 2 =%C).

9. crop
residue
carbon
CO2
Living organisms Microbial
compounds
often > 75%
After
1
year
Most of the C in
crop residues
quickly returns
to the
atmosphere
Plant compounds
SOM

10. The current OM level in a soil is the
result of the long-term balance
between organic inputs and outputs
So… shouldn’t yield enhancing
practices build SOM?

11. “The microherd”
Phil Brookes
Practices that enhance crop yield
also impact the soil stomach!!
When there is more
grass, I eat more!!

12. ”But with the removal of water through furrows, ditches,
and tiles, and the aeration of the soil by cultivation, what
the pioneers did in effect was to fan the former simmering
fires… into a blaze of bacterial oxidation and more
complete combustion. The combustion of the accumulated
organic matter began to take place at a rate far greater
than its annual accumulation. Along with the increased rate
of destruction of the supply accumulated from the past, the
removal of crops lessened the chance for annual additions.
The age-old process was reversed and the supply of
organic matter in the soil began to decrease instead of
accumulating.”
William Albrecht – 1938 Yearbook of Agriculture
Tillage + Lime + Drainage + N fertilizer =>
higher crop yields & higher decomposition rates

13. Mollisols (prairie soils) under long-term
agricultural management have ~ 50% less
organic C in their top 8” than native prairie soils.
Most of the loss of organic C is likely to have
occurred within 25 years of the original plowing
(as opposed to the last 50-100 years).
Organic C levels in IL soils have been relatively
stable over the past 50 years.

14. Soil Changes After Sixty Years of Land Use in Iowa
Jessica Veenstra, Iowa State University, 1126 Agronomy Hall, Iowa State
University, Ames, IA 50010
Soils form slowly, thus on human time scales, soil is essentially a non-renewable
resource. Therefore in order to maintain and manage our limited soil resources
sustainably, we must try to document, monitor and understand human induced
changes in soil properties. By comparing current soil properties to an archived
database of soil properties, this study assesses some of the changes that have
occurred over the last 60 years, and attempts to link those changes to natural and
human induced processes. This study was conducted across Iowa where the
primary land use has been row crop agriculture and pasture. We looked at
changes in A horizon depth, color, texture, structure, organic C content and pH.
Hill top and backslope landscape positions
have been significantly degraded w/ less organic C
but
catchment areas have deeper topsoil w/ more organic C.

15. Are crop
residues
different than
the residues
from native
vegetation ?
perennial roots > annual roots!

16. http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html
This pie chart represents
organic matter in soil with native
vegetation. Notice the large
active fraction.
When native vegetation is
converted to agriculture, the
active OM fraction normally
quickly declines. The stable
OM fraction changes much less.
From the U of MN bulletin on SOM

17. http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html
From the U of MN bulletin on SOM
After
long-term
agriculture
Why does old OM become dominant?
Smaller pie and different slices

18. < 1 year
decades
centuries
What is
the
average
age of
organic C
in soil ??
Janzen (2006)
Old C is most
abundant!
depleted in
most ag
soils
C dynamics

19. There is very clear evidence that atmospheric levels of CO2
are increasing and that the majority of the CO2 added to the
atmosphere in the last 3 decades has come from fossil fuels
Why do CO2
levels go up
and down
annually?
Prior to ~1980,
majority of CO2↑
came from
loss of SOM

20. Agricultural Production Affects Annual CO2 Cycle
Each year in the Northern Hemisphere, levels of atmospheric carbon
dioxide drop in the summer as plants grow, and then climb again as they
decompose. Over the past five decades, the size of this seasonal swing
has increased by ~50%, for reasons that aren’t fully understood.
Scientists recently evaluated global production statistics for 4 leading
crops – corn, wheat, rice and soybeans and found that production of
these crops in the Northern Hemisphere has more than doubled since
1960. This translates to ~ 1 billion metric tons of C captured and
released each year!
According to Dr. Josh Gray of Boston U:
“Croplands are ecosystems on steroids. They occupy about 6
percent of the vegetated land area in the Northern Hemisphere but
are responsible for up to a quarter of the total increase in seasonal
exchange of atmospheric carbon dioxide, and possibly more…that’s
a very large, significant contribution, and 2/3 of that contribution is
attributed to corn.“

21. http://www.grida.no/climate/vital/graphics/large/12.jpg
Global
C cycle
All# = GT
Gt = 109 t = 1 billion metric tons
Soil C > Atmosphere C + Vegetation C

22. 2400

23. Why is SOM
important ??

24. What Does Soil Organic Matter Do (for you)?
Nutrient cycling
Increases the nutrient holding capacity of soil (CEC).
Serves as a slow release form of nutrients for plants.
Chelates nutrients increasing their availability to plants.
Feeds soil organisms from bacteria to worms that excrete available nutrients
Water dynamics
Improves water infiltration.
Decreases evaporation.
Increases water holding capacity, especially in sandy soils.
Structure
Reduces crusting, especially in fine-textured soils.
Encourages root development.
Improves aggregation, preventing erosion and reducing compaction.
From the U of MN bulletin:
Fertilizer is not
a substitute
for SOM

25. SOM

26. Most
(but not all)
soil organisms
eat SOM

27. Some bacteria are CHEMOAUTOTROPHS
Chemoautotrophic bacteria obtain energy
through the oxidation of electron donors
other than C.
For example, the bacteria that oxidize ammonium
into nitrate, a important process called nitrification,
do NOT eat SOM
Many bacteria and all fungi
(as well as all other soil organisms)
are HETEROTROPHS
(which means that they eat organic matter).

28. SOM is the fuel
that energizes
most biological
processes in soil

29. (Watts and Dexter, 1997)
Structural
damage Soils with high OM
are more resistant to
structural damage !
Soils with more OM have less strength when dry
and more strength when moist!

30. SOM increases plant available H20
Adapted from Brady and Weil (2002)

31. SOM is a very important adsorbent in soil
Adapted from Brady and Weil (2002)

32. Humus gives soil a darker color
Is this beneficial?

33. Biologically
active
SOM
SOM is a complex mixture of
living, dead and very dead OM
Living organisms
Recent residues
Stabilized
SOM
Adapted from Magdoff and Weil (2003)
Historically often
called HUMUS

34. What is humus ???
Humus is organic matter that has been
transformed such that its original source
is no longer apparent… The diverse
products of “humification” have many
common characteristics:
 Resistance to further decomposition
 Complexation with fine mineral materials
 High specific surface and negative charge
 Dark color

35. HUMMUS
HUMUS
is NOT

36. The traditional concept of
highly complex humus
macro-molecules distinctly
different from bio-molecules has
been rejected by most scientists

37. Recent research has demonstrated that chemical
structure alone does not control SOM stability:
in fact, environmental and biological controls are
more important…
Nature, October 2011

38. Have you
ever heard of
any humate
products?
Hydra-hume

39. Descriptions of humate products often
refer to humic and fulvic acid content
Fulvic
acid Humic
acid
Fulvic acid = soluble in strong base and still soluble when pH => 7
Humic acid = soluble in strong base but precipitates when pH => 7
HA & FA are
solubility
fractions
NOT
specific
compounds

40. TIDIC acid production system ☺

41. Leonardite
source material for
commercial
humate products
Products differ
with respect to
the amount
and type of
processing

42. There is growing evidence that humate products can enhance
crop growth **BUT** are not the same as natural SOM

43. There are lots of humate products on the market.
Ask for research results and clear explanations.

44. Reputable companies should be able
to provide analytical results obtained
using this new standardized method

45. Humate products are not a substitute for good soil organic matter management

46. accumulate in
soil?
why does
matorganic ter
So…

48. Understanding biochemical recalcitrance
(Giller, 2000)
aka digestibility

49. C:N ~ 25
Faster
Slower
Decomposition

50. LIGNIN is the main molecule that makes
plants stiff – it decomposes much more
slowly than cellulose but research shows
that it eventually decomposes and does
not accumulate significantly in soil

51. Field-Grown Bt and non-Bt Corn:
Yield, Chemical Composition, and Decomposability
Sandra F. Yanni, Joann K. Whalen and Bao-Luo Ma
Abstract
Bt (Bacillus thuringiensis) corn (Zea mays L.) accounted for 74.5% of the
corn acreage in eastern Canada in 2009. Reports that Bt corn has
greater yield and lignin concentrations than unmodified corn have raised
questions about its effect on the soil ecosystem. Our objectives were to
evaluate the biomass of field-grown Bt and non-Bt corn, the chemical
composition of different corn components that remain as residues in the
field after harvest, and the effect of the Bt modification on residue
decomposition. Nine Bt corn hybrids and their near isolines were field-
grown in 2008 and 2009. Grain and stover yields were measured and
leaves, stems, and roots were collected and analyzed for lignin, C, and N
concentrations. Stem sections from a Bt/non-Bt corn pair were buried in
the field and sampled periodically during 1 yr. No difference in yield or
lignin concentrations due to the Bt gene was noted; however, N
concentration in Bt stems was significantly greater than in non-Bt stems
in 1 yr of the 2-yr study. Leaves had less lignin and a lower C/N ratio
than stems and roots in both years. In buried field litterbags, the decline
We conclude that the Bt gene does NOT affect the
chemical composition of corn residues in fields
without herbivory, and that Bt corn residue may
actually be MORE susceptible to decomposition than
non-Bt corn residue.
What about with herbivory???
SOME SURPRISING RESULTS

52. Charcoal is highly
resistant to
decomposition and
full of porosity

53. Terra Preta
soils contain
lots of ancient
charcoal
Typical
acid infertile
rain forest
soil

54. Can we make
new Terra Preta
soils by
amending
infertile soils
with BIOCHAR?
Scientists are trying to figure out…

55. Understanding Mineral Protection
Magdoff and Weil (2004)
Soil C content
is often
closely related
to fine mineral
content

56. Weak relationship between clay content and SOC
for 1261 agricultural soils in England and Wales
Webb et al.(2003)
Clay is clearly
not the only
factor controlling
C content in
these soils

57. Understanding physical protection
Adapted from Carter (2002)
Mineral protected OM
Intra-
aggregate
OM
Free OM
Over time SOM
becomes more and
more intimately
connected
to soil
mineral
particles

58. Soil microaggregates
Soil macroaggregate
OM
OM
Is there organic matter inside aggregates?

59. Soil macro-
aggregates
form around
fresh organic
residues
Tillage
disrupts
aggregates
and
accelerates
decomposition
TillageOM inputs

60. What is POM??
Mineral protected
Sand sized Silt and clay sized
http://www.grdc.com.au/growers/res_summ/pdfs/cso00029.pdf
Particulate OM = POM

61. Geographic distribution of SOM

62. What Determines Soil Organic Matter Levels?
The amount of organic matter in soil is the result of two processes: the addition of
organic matter (roots, surface residue, manure, etc.), and the loss of organic
matter through decomposition. 5 main factors affect both additions and losses.
Soil texture - Fine-textured soils can hold much more organic matter than sandy
soils for two reasons. First, clay particles form electrochemical bonds that hold
organic compounds. Second, decomposition occurs faster in well-aerated sandy
soils. Sandy loams rarely have more than 2% organic matter.
Historical vegetation - In prairies, much of the organic matter that dies and is
added to the soil each year comes from grass roots that extend deep into the soil.
In forests, the organic matter comes from leaves that are dropped on the surface
of the soil. Thus, farmland that was once prairie will have higher amounts of
organic matter deep in the soil than land that was previously forest.
Climate - High temperatures speed up the degradation of organic matter. In
areas of high precipitation (or irrigation) there is more plant growth and therefore
more roots and residues entering the soil.
Landscape position - Low, poorly-drained areas have higher organic matter
levels, because less oxygen is available in the soil for decomposition. Low spots
also accumulate organic matter that erodes off hill tops and steep slopes.
So what is the 5th factor?
MANAGEMENT

63. Interstream
divide
SOIL
DRAINAGE
CLASSES
Poorly
drained
Somewhat
poorly
drained
Moderately
well drained
Poorly
drained
Well
drained
Interfluve
Valley floor
Backslope
Shoulder
LANDSCAPE
POSITIONS
Landscape position affects SOM dynamics
Where does the most OM accumulate?
“flat black”
soils

64. Temperature affects OM production and decomposition
Brady and Weil (2002)
70 F
Organic matter
synthesis by plants
Illinois in 50 yrs?

65. How
much
is
enough
??

66. Have we learned anything in the last 77 years ?

67. Janzen (2006)
Hydroelectric dam metaphor
OM forms and
dynamics are more
important than total
quantity
but many soils
currently have OM
levels that limit
soil function
SOM is a very important source of nutrients in modern production systems but
we are less dependent on SOM to supply nutrients than we were in 1938.

68. There are many ways to “measure” SOM
Adapted from Strek and Weber (1985)
Total organic matter
by “loss on ignition”
Total C
by several wet and
dry oxidation
methods
Humic matter
by alkali extraction
OM/1.72 = C
% OM

69. Permanganate oxidizable C
a routine test for “active” soil C ??

71. “Our analysis demonstrates the
usefulness of POXC in quickly and
inexpensively assessing
changes in the labile soil C pool.”

72. Soil from a
long term
experiment in
Beltsville, MD

73. After
adding
water

74. 1.4 % C1.0% C
Relatively small differences in C

75. 48 bu/a 140 bu/a
Large differences in soil function

76. Aggregation changes much more rapidly than total C
Jastrow (1996)
Years since PRAIRIE RESTORATION
Aggregation
Total C

77. 16 % clay 39 % 49%
More OM is needed to stabilize fine textured soils
Adapted from Russell (1973)
16 % clay
39 % 49%

78. Comparison of soil from fields with the same soil type but
different OM levels can help identify sites with the most
potential for building SOM and improving soil function

79. Managing SOM

80. well mixed vs.
stratified
Conventional tillage Conservation tillage
Adapted from House and Parmelee (1985)

81. It is widely believed that tillage was the main cause of soil C loss when
natural ecosystems were converted to agriculture, and that substantial C
sequestration can be accomplished by changing from conventional tillage to
no-till. This is based on lots of experiments (and on-farm observations) where
soil C increased under no-till. However, sampling methods may have biased
the results. In essentially all cases where no-till was found to sequester C, soils
were only sampled to a depth of 1 foot or less…
What is meant by
the term
CARBON
SEQUESTRATION?
CARBON
SEQUESTRATION
in soil
CO2 -> SOM

82. Very few tillage studies have been sampled deeper than 1’
Many studies were only sampled 6” deep!
In the Upper Midwest, tillage reduces OM levels
near the soil surface but has much less impact
on SOM in the whole profile.
In warmer climates, tillage more negatively
effects OM levels in the whole profile.

83. Effect of tillage on microbial activity
Havlin et al. (1999)
Tillage
Which tillage system has
more total microbial
activity ?
Conventional tillage
Which system releases
more CO2 when crops need
CO2 ?

84. Elevated OM levels at the soil surface are beneficial
even if no greater OM accumulates at depth

85. Artificial drainage has greatly increased the number of
days when soils in the Upper Midwest are suitable for
field operations
but has also
contributed
to some
environmental
problems
Pollution of
water resources?
Loss of SOM??

86. Original soil surface of a Histosol (muck soil) in FL

87. Adapted from Bailey and Lazarovits (2003)
A systems approach
to SOM management
Well adapted crop
Nutrient
Management
Water
Management
SOM

88. Crop
residue
management
Increase residue production
(especially roots) and minimize
soil disturbance

89. Crop Rotation
High residue crops
Cover crops
Forages

90. Erosion Control Practices

91. Erosion is a major cause of reduced SOM levels at the soil surface
http://tucson.ars.ag.gov/isco/isco10/SustainingTheGlobalFarm/P241-Kimble.pdf
Erosion status

92. On-farm recycling of OM
Most IL
fields never
receive
manure

93. Off-farm sources of OM

94. ~ 60% of the
biosolids generated
in the US are land
applied. Farmers get
nutrients and OM
for free!

95. Innovative cover cropping
Its possible to ~double the months
of active plant growth in IL
4 8

96. A good way
to grow
more roots!

97. Actual C
Practically
attainable C
Potential C
(Dick and Gregorich, 2004)
Input factors
Many
factors
control
SOM
content
Residue yield

98. Saturation deficit
Saturation of capacity
Actual C
Practically
attainable C
Potential C
(Dick and Gregorich, 2004)
Disturbance factors
Input factors
capacity factors
management
= opportunity
Residue yield

99. Fencerows are often a good place to check
a soil’s capacity for C accumulation

100. Some
effects of
higher OM
quantity
and/or
quality
occur
relatively
quickly
Other
effects
take
longer

101. Fields or parts of fields with the lowest OM content
(relative to their potential) will benefit the most
from practices that build SOM.