Understanding soil organic matter
So when does
but not as
IL corn yields have doubled
during my lifetime!
How much residue does a 200 bushel corn
crop return to the soil?
General rule of
thumb for corn
and roots each
~1/3rd of the
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?
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
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.
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).
Living organisms Microbial
often > 75%
Most of the C in
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?
Practices that enhance crop yield
also impact the soil stomach!!
When there is more
grass, I eat more!!
”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
William Albrecht – 1938 Yearbook of Agriculture
Tillage + Lime + Drainage + N fertilizer =>
higher crop yields & higher decomposition rates
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.
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
catchment areas have deeper topsoil w/ more organic C.
perennial roots > annual roots!
This pie chart represents
organic matter in soil with native
vegetation. Notice the large
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
From the U of MN bulletin on SOM
Why does old OM become dominant?
Smaller pie and different slices
< 1 year
in soil ??
Old C is most
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
Prior to ~1980,
majority of CO2↑
loss of SOM
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.“
All# = GT
Gt = 109 t = 1 billion metric tons
Soil C > Atmosphere C + Vegetation C
What Does Soil Organic Matter Do (for you)?
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
Improves water infiltration.
Increases water holding capacity, especially in sandy soils.
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
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)
(which means that they eat organic matter).
SOM is the fuel
processes in soil
(Watts and Dexter, 1997)
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!
SOM increases plant available H20
Adapted from Brady and Weil (2002)
SOM is a very important adsorbent in soil
Adapted from Brady and Weil (2002)
Humus gives soil a darker color
Is this beneficial?
SOM is a complex mixture of
living, dead and very dead OM
Adapted from Magdoff and Weil (2003)
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
Resistance to further decomposition
Complexation with fine mineral materials
High specific surface and negative charge
The traditional concept of
highly complex humus
different from bio-molecules has
been rejected by most scientists
Recent research has demonstrated that chemical
structure alone does not control SOM stability:
in fact, environmental and biological controls are
Nature, October 2011
ever heard of
Descriptions of humate products often
refer to humic and fulvic acid content
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
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
Field-Grown Bt and non-Bt Corn:
Yield, Chemical Composition, and Decomposability
Sandra F. Yanni, Joann K. Whalen and Bao-Luo Ma
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
Charcoal is highly
full of porosity
lots of ancient
Can we make
new Terra Preta
Scientists are trying to figure out…
Understanding Mineral Protection
Magdoff and Weil (2004)
Soil C content
to fine mineral
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
C content in
Understanding physical protection
Adapted from Carter (2002)
Mineral protected OM
Over time SOM
becomes more and
Is there organic matter inside aggregates?
What is POM??
Sand sized Silt and clay sized
Particulate OM = POM
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?
Landscape position affects SOM dynamics
Where does the most OM accumulate?
Temperature affects OM production and decomposition
Brady and Weil (2002)
synthesis by plants
Illinois in 50 yrs?
Have we learned anything in the last 77 years ?
Hydroelectric dam metaphor
OM forms and
dynamics are more
important than total
but many soils
currently have OM
levels that limit
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.
There are many ways to “measure” SOM
Adapted from Strek and Weber (1985)
Total organic matter
by “loss on ignition”
by several wet and
by alkali extraction
OM/1.72 = C
Permanganate oxidizable C
a routine test for “active” soil C ??
“Our analysis demonstrates the
usefulness of POXC in quickly and
changes in the labile soil C pool.”
Soil from a
well mixed vs.
Conventional tillage Conservation tillage
Adapted from House and Parmelee (1985)
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
CO2 -> SOM
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.
Effect of tillage on microbial activity
Havlin et al. (1999)
Which tillage system has
more total microbial
Which system releases
more CO2 when crops need
Elevated OM levels at the soil surface are beneficial
even if no greater OM accumulates at depth
Artificial drainage has greatly increased the number of
days when soils in the Upper Midwest are suitable for
but has also
Loss of SOM??
Original soil surface of a Histosol (muck soil) in FL
Adapted from Bailey and Lazarovits (2003)
A systems approach
to SOM management
Well adapted crop
Increase residue production
(especially roots) and minimize
High residue crops
(Dick and Gregorich, 2004)
Saturation of capacity
(Dick and Gregorich, 2004)
Fencerows are often a good place to check
a soil’s capacity for C accumulation
Fields or parts of fields with the lowest OM content
(relative to their potential) will benefit the most
from practices that build SOM.