Understanding soil organic matter

Biologically

active

SOM

SOM is a complex mixture

Living organisms

Recent residues

Stabilized

SOM

Adapted from Magdoff and Weil (2003)

HUMUS??

http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html

This pie chart represents

organic matter in soil before

agriculture. After land has

been farmed for several

decades, much of the active

fraction is lost and stabilized

organic matter makes up more

than half of the soil organic

matter.

From the U of MN bulletin on SOM

http://www.extension.umn.edu/distribution/cropsystems/components/7402_02.html

From the U of MN bulletin on SOM

After

long-term

agriculture

Change in both the size of the

pie and the slices of the pie

The current OM level in a soil is a

result of the long-term balance

between organic inputs and outputs

So… shouldn’t yield enhancing

practices build SOM?

General rule of thumb for corn

grain, stover and roots each comprise

~1/3rd of the OM produced by corn

How much OM is returned to the soil

by a 200 bushel corn crop?

200 bushels*56 lbs/bu * 2 = 22,400 lbs/a/yr!

2/3rds of corn OM

Very little of the OM

supplied by corn residues

lasts more than a few years

on most farms

“The microherd”

Phil Brookes

Yield enhancing practices 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

accumulating.”

William Albrecht – 1938 Yearbook of Agriculture

Tillage + Lime + Drainage + N fertilizer =>

higher yields & higher decomposition rates

< 1 year

decades

centuries

What is

the

average

age of

SOM ??

Janzen (2006)

Ancient SOM

is most

abundant!

depleted in

most ag

soils

http://www.grida.no/climate/vital/graphics/large/12.jpg

Global

C cycle All# = GT

Gt = 109 t = 1 billion metric tons

Soil > Atmosphere + Vegetation

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

losses in SOM

2400

Why is SOM

important ??

What Does 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:

Most (but not all)

soil organisms

eat SOM

Some bacteria are CHEMOAUTOTROPHS

Chemoautotrophic bacteria obtain energy

through the oxidation of electron donors

other than C.

For example, the bacteria that oxidize ammonia

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).

SOM is the fuel

that energizes

most biological

processes in soil

SOM reduces bulk density

Magdoff and Weil (2004)

(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!

SOM increases plant available H20

Adapted from Brady and Weil (2002)

Do you remember this photo??

Better

sponge

SOM is a very important adsorbent in soil

Adapted from Brady and Weil (2002)

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:

Extreme chemical complexity

Resistance to further decomposition

High specific surface and negative charge

Dark color

Humus gives soil a darker color

Is this beneficial?

Have you

ever heard of

any humate

products?

Hydra-hume

There is growing evidence that mined humate products can

promote plant growth **BUT** are

not the same as SOM

Recent research has demonstrated that molecular

structure alone does not control SOM stability: in

fact, environmental and biological controls

predominate…

Nature, October 2011

The traditional concept of

giant stable humus

molecules in soil has been

rejected by most scientists

accumulate in

soil?

why does

mat org anic ter

So…

Understanding biochemical recalcitrance

(Giller, 2000)

aka digestibility

Understanding mineral protection

Magdoff and Weil (2004)

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 carbon

content

Understanding physical protection

Adapted from Carter (2002)

Mineral associated OM

Intra-

aggregate

POM

Free POM

Sen

sit

ivit

y t

o

man

ag

em

en

t

↑

↑

Soil microaggregates

Soil macroaggregate

OM OM

Is there organic matter inside macroaggregates?

Soil macro-

aggregates

form around

fresh organic

residues

Tillage

disrupts

aggregates

and

accelerates

decomposition

Tillage OM inputs

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

Geographic distribution of SOM

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. Five 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. A sandy loam rarely holds 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 lands that were 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

Temperature affects SOM production and destruction

Brady and Weil (2002)

70 F

Organic matter destruction

by aerobic organisms

Organic matter

synthesis by plants

Org

an

ic m

att

er

pro

du

cti

on

O

rga

nic

ma

tte

r c

on

su

mp

tio

n

OM

pro

du

cti

on

an

d c

on

su

mp

tio

n

Illinois

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?

Blackland soils of North Carolina

Lily (1981) > 1 million acres of Histosols

How

much

is

enough

??

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

C ~ 0.6*OM

% OM

Many biological products claim to contain

humic and fulvic acids

Fulvic Humic

Fulvic acid = soluble in strong base and still soluble when pH => 7

Humic acid = soluble in strong base but precipitates when pH => 7

TIDIC acid production system ☺

Humic and fulvic acids are

solubility fractions rather

than specific compounds

with specific biological

effects

Permanganate oxidizable C

a routine test for “active” soil C ??

Our analysis demonstrates the

usefulness of POXC in quickly and

inexpensively assessing

changes in the labile soil C pool.

Soil from a

long term

experiment in

Beltsville, MD

After

adding

water

1.4 % C 1.0% C

Relatively small differences in SOC

48 bu/a 140 bu/a

Large differences in soil function

16 % clay 39 % 49%

More OM is needed to stabilize fine textured soils

Adapted from Russell (1973)

16 % clay 39 % 49%

Aggregation changes more rapidly than total C

Jastrow (1996)

Years since prairie restoration

Janzen (2006)

Hydroelectric dam metaphor

Sufficient OM

optimizes the

beneficial effects of

OM

OM forms and

dynamics are more

important than the

total quantity

Managing SOM

well mixed vs.

stratified

Conventional tillage Conservation tillage

Adapted from House and Parmelee (1985)

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 ?

It is widely believed that tillage was 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…

Very few tillage studies have been sampled deeper than 1’

Many studies were only sampled 6” deep!

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

field operations

but has also

contributed

to some

environmental

problems

Pollution of

water resources Loss of SOM

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

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 carbon content and pH.

Hill top and backslope landscape positions

have been significantly degraded

but

catchment areas have deeper topsoil w/ more C.

Adapted from Bailey and Lazarovits (2003)

A systems approach

to SOM management

Well adapted crop

Nutrient

Management

Water

Management

SOM

Crop

residue

management

Erosion Control Practices

On-farm recycling of OM

Off-farm sources of OM

Crop Rotation

High residue crops

Cover crops

Forages

Innovative cover cropping

Actual C

Practically

attainable C

Potential C

(Dick and Gregorich, 2004)

Input factors

Many

factors

control soil

C content

Residue yield

Saturation deficit

Saturation of capacity

Actual C

Practically

attainable C

Potential C

(Dick and Gregorich, 2004)

Disturbance factors

Input factors

capacity factors

man

ag

em

en

t

= opportunity

Residue yield

Comparison of soil from fields and hedgerows with the

same soil type can help identify sites with the most

potential for building SOM