This document discusses the importance of soil organic matter. It states that soil organic matter affects chemical and physical soil properties and overall health. It is made up of living and dead biomass and humus. Soil organic matter content typically ranges from 1-6% and provides benefits like improved structure, water retention, and nutrient availability. Maintaining or increasing soil organic matter through practices like reduced tillage, cover crops, and reducing erosion can improve soil quality and sustainability.

1. ROLE OF ORGANIC MATTER
IN MAINTENANCE OF SOIL
FERTILITY
P. Dhamodharan
2019502025
Department of agronomy

2. Soil organic matter
• Soil organic matter - the product of on-site biological decomposition -
affects the chemical and physical properties of the soil and its overall
health.
• Its composition and breakdown rate affect: the soil structure and
porosity; the water infiltration rate and moisture holding capacity of
soils; the diversity and biological activity of soil organisms; and plant
nutrient availability.
• It is related to moisture, temperature and aeration, physical and
chemical properties of the soils as well as bioturbation (mixing by soil
macrofauna), leaching by water and humus stabilization
(organomineral complexes and aggregates).

3. Components of SOM

6. Significance of SOM
• The mass of SOM in soils as a percent generally ranges from 1 to 6% of the
total topsoil mass for most upland soils.
• Soils whose upper horizons consist of less than 1% organic matter are mostly
limited to desert areas, while the SOM content of soils in low-lying, wet areas
can be as high as 90%.
• Soils containing12-18% SOC are generally classified as organic soils.
• A good soil should have an organic matter content of more than 3.5 per cent.

7. Organic matter “transformation”

8. Soil Composition
50% Pore Spaces 50% Solids
1-5%

9. Soil Organic Matter
living biomass
10-20%
Organic matter is 1-6%
of total soil mass active fraction
10-20%
humus
60-80%
“The living, the dead, and the very dead”
Vermont Agric Exp Sta Bullletin 135, 1908

10. Soil organic matter
encomposses all
organic components
of the soil:
Image: soils.usda.gov
• Living organisms
• Fresh residues
• Decomposing OM
• Stabile OM

11. Soil organic matter
• Living organisms (plant roots,
fungi, bacteria, worms)
• Fresh residues (crop residues,
sloughed roots, dead insects,
animal manures, microbial
secretions)
• Decomposing (decaying
leaves & stalks; partly
recognizable &
partially stabilized OM)
Brady and Weil, 1996
• Stabilized OM
(transformed, recalcitrant OM
or “humus”)
SOM dynamic = changing = transforming

12. What is Humus?
Humus is the end result of organic matter decomposition
(break-down) and transformation (build-up) into a complex
- dark-brown, amorphous-heterogenous (non- crystalline,
non-uniform) structure that no longer resembles the
decaying matter of origin
- is resistant to further microbial decay
- has chemical and physical properties of great importance
to soils and plants.

13. Humus properties:
1. Humus particles become bonded to clay-silicate
surfaces, leading to the formation of clay-humus
complexes.
2. Humus stores and releases soil N.
3. Humus possesses buffering capacity
4. Humus possesses cation exchange capacity
5. Humus possesses anion exchange capacity
6. Humus adsorbs pesticides and other agricultural
chemicals.
Soil Fertility Management for Sustainable Agriculture
James F. Power and Rajendra Prasad, CRC Press, 1997

14. % Organic Matter (dry weight) via Lab Test
Image: Rodale Institute
5%OM 1%OM

15. SOM Colorimetric Field Test Using Sodium Hydroxide/EDTA
K-State Soil Test Kit
www.ksre.ksu.edu

16. Soil quality is the capacity of a soil to function (in a farm or ecosystem)
and thereby sustain productivity, maintain environmental quality, and
promote plant and animal health
Physical Chemical
NPK S
Ca Mg
Fe Mn
BiologicalSoil Health
Aim is to manage
for “balance”
between all three
soil components

17. Function of Soil Organic Matter and the Effect on Soil Properties, CSIRO 2004

18. James B. Nardi, Life in the Soil, 2007

19. EPS – Extracellular
polysaccharides
Sand, silt, clay
Clay-humus
Bacteria & fungi
Soil Aggregate
The interaction of soil particles, biology & biochemistry
Soil Mineral, Organic Matter, Microorganism Interactions, P.M. Huang (2004)

22. Benefits of Organic Matter
Nutrient Supply:
Organic matter is a reservoir of nutrients that can be released to the soil.
Each percent of organic matter in the soil releases 20 to 30 pounds of
nitrogen, 4.5 to 6.6 pounds of P2O5, and 2 to 3 pounds of sulfur per year.
The nutrient release occurs predominantly in the spring and summer, so
summer crops benefit more from organic-matter mineralization than winter
crops.
Soil Structure Aggregation:
• Organic matter causes soil to clump and form soil aggregates, which improves
soil structure.
• With better soil structure, permeability (infiltration of water through the soil)
improves, in turn improving the soil's ability to take up and hold water.

23. Water-Holding Capacity:
• Organic matter behaves somewhat like a sponge, with the ability to absorb
and hold up to 90 percent of its weight in water.
• A great advantage of the water-holding capacity of organic matter is that
the matter will release most of the water that it absorbs to plants.
• In contrast, clay holds great quantities of water, but much of it is unavailable
to plants.
Erosion Prevention:
• Data used in the universal soil loss equation indicate that increasing soil
organic matter from 1 to 3 percent can reduce erosion 20 to 33 percent
because of increased water infiltration and stable soil aggregate formation
caused by organic matter.

24. Transistion period of development

25. Improve Soil Organic Matter Levels
• Reduce or Eliminate Tillage: Tillage improves the aeration of the soil and
causes a flush of microbial action that speeds up the decomposition of
organic matter. Tillage also often increases erosion. No-till practices can help
build organic matter.
• Reduce Erosion: Most soil organic matter is in the topsoil. When soil erodes,
organic matter goes with it. Saving soil and soil organic matter go hand in
hand.
• Soil-Test and Fertilize Properly: You may not have considered this one. Proper
fertilization encourages growth of plants, which increases root growth.
Increased root growth can help build or maintain soil organic matter, even if
you are removing much of the top growth.
• Cover Crops: Growing cover crops can help build or maintain soil organic
matter. However, best results are achieved if growing cover crops is combined
with tillage reduction and erosion control measures.

27. How do organic matter levels change?
• Most organic matter losses in soil
occurred in the first decade or two
after land was cultivated.
• Native levels of organic matter may
not be possible under agriculture, but
many farmers can increase the
amount of active organic matter by
reducing tillage and increasing
organic inputs.
An illustration of soil organic
matter losses and gains in
response to tillage.

29. Nitrogen Released from Organic Matter
In surface 7-inch depth of soil (2,000,000 lbs)
% OM Stable OM lbs/ac Total N lbs/ac Lbs N/ac Released
in Silt Loam Soil
1.0 20,000 1,000 15-30
1.5 30,000 1,500 22-45
2.0 40,000 2,000 30-60
2.5 50,000 2,500 37-75
3.0 60,000 3,000 45-90
3.5 70,000 3,500 52-100
4.0 80,000 4,000 60-120
4.5 90,000 4,500 67-135
5.0 100,000 5,000 75-150
Soil Fertility and Corn Production, Univ of Missouri Agr Exp Sta Bull 583 (1952)

30. Transformation of raw organic matter into stabilized compost occurs in
a succession of temperature & biological processes
Phases of Compost Heating, Cooling, and Maturation
FiBL / IFOAM Training Manual on Organic Agriculture in the Tropics

31. USDA – Natural Resources Conservation Service
“Goals for SOM Management”
1. Till the soil as little as possible
2. Grow as many different species of plants as possible
through rotations & diverse mixtures of cover crops
3. Keep living plants in the soil as long as possible with
crops & cover crops
4. Keep the soil surface covered with residue year
round

32. The soil organic matter content rises more quickly for the very depleted soil (starting at 0.5% organic
matter) than for the 1% organic matter content soil, because so much more organic matter can be stored in
organo-mineral complexes and inside very small and medium-size aggregates.

33. ORGANIC MATTER INCREASES THE AVAILABILITY OF NUTRIENTS
Directly
• As organic matter is decomposed, nutrients are converted into forms that plants can use directly.
• CEC is produced during the decomposition process, increasing the soil’s ability to retain calcium,
potassium, magnesium, and ammonium.
• Organic molecules are produced that hold and protect a number of micronutrients, such as zinc and
iron.
Indirectly
• Substances produced by microorganisms promote better root growth and healthier roots, and with a
larger and healthier root system plants are able to take in nutrients more easily.
• Organic matter contributes to greater amounts of water retention following rains because it improves
soil structure and thereby improves water-holding capacity.
• This results in better plant growth and health and allows more movement of mobile nutrients (such as
nitrates) to the root.

34. Importance of SOM
• Nutrients from decomposing organic matter.
• Addition of nitrogen.
• Storage of nutrients on soil organic matter.
• Protection of nutrients by chelation.
• Protection of the Soil against Rapid Changes in Acidity
• Stimulation of Root Development
• Darkening of the Soil
• Protection against Harmful Chemicals
Corn grown in nutrient solution with
(right) and without (left) humic acids.

35. Role of Soil Organic Matter in Maintenance of Soil Productivity:
• Cation Exchange Capacity
• Nutrient Retention and Release
• Soil Structure and Bulk Density
• Water Holding
• Biological Activity
• Effect of Farming Practices on Soil
• Tillage
• Crop Rotations
• Fertilization

36. Summary
Organic matter is an important soil property. It is associated with a variety of other
important soil physical, chemical and biological characteristics.
Cool climate favours the retention of organic matter to a greater extent than is the
case in warmer regions.
It is not easy to show a clear link between soil organic matter concentrations and
crop yield and quality. Many other factors affect yield and quality to a greater extent.
Nevertheless, there is sufficient evidence linking organic matter and yield to suggest
that farmers should strive to maintain or increase organic matter in their soils.
Maximising inputs of organic matter by incorporating crop residues and including
green manures and pastures in the rotation, where practical, should be a goal for all
farmers.

37. THANK YOU