Som lecture 3

Slide 1

Further Investigations during 20th century

C52H46O10(OCH3)COOH(OH)4CO + H2NRCOOH

C52H46O10(OCH3)COOH(OH)4 C=NRCOOH + H2OAccording to Waksman;HS not specific cpds but a mixture of cpds non specific nature like cellulose, hemicellulose, fats, waxes etc.Artificial cpds formed by the action of alkali solutions on the soil Waksman in his book Humus (1937, page 157) There is no doubt now that this complex originates mainly from the lignin constituents of plant residues, by various biological and chemical modifications; this fraction may, therefore, be included in the lignin group

Further Investigations during 20th century

Tyurim (1937) Book on The Organic Matter of Soils HS specific cpds methods adopted by Waksman to determine humus composition not correct and unsuitable

Springer (1934 35) also criticized Waksman and supported that HS exist as cpds of specific nature Lignoprotein cpds do not acquire new properties like increased exchange reactions

Later many scientist showed:Non lignin cpds can form HS in the soilMicrobial activity & metabolism play a major roleAromatic cpds formed from aliphatic cpds including carbohydrates

Present Stage in Soil Humus Study

HS Structure, Composition and Properties Studied: X ray analysis, Electron Microscopy, Infrared Spectroscopy, Chromatography in conjunction with chemical methods

HS formation complex transformation organic & biochemical Role microbes & animal kingdom

Role of SOM in weathering of rocks and minerals, Soil Forming Processes, etc being studied

Interaction between SOM and Minerals

Participation in Physiological and Biochemical Processes in the plant

Books for Reading

Soil Organic Matter, its Nature, its Role in Soil formation and in Soil fertility. By kononova, M. M. 1966 2. Soil Components Vol.I. Organic Components By John E. Gieseking, 1975.

3. Soil Organic Matter By Schnitzer and Khan, 1978.

4. Soil Organic Matter and its Role in Crop Production By Allison

5. Chemistry of the Soil By F. E. Bear

6. Humus By S. A. Waksman

Books for Reading

7. Introduction to Soil Microbiology By Martin Alexander

8. Soil Biochemistry By A. D. Mclaren and G. H. Peterson, 1967

9. Study work on Soil Organic Matter and Soil Fertility By G. H. Peterson

10. Agrosphere Nutrient Dynamics, Ecology and Productivity By K. R. Krishna, 2003.

11. Humus Chemistry: Genesis, Composition and Reactions. By Stevenson, F. J., 1982.

12. Cycles of Soil: C, N, P, S and Micronutrients By Stevenson, F. J., 1985.

Soil Organic Matter

General Types of Organic MaterialsSpecific Types of SOM Substances and PoolsSOM DecompositionSoils and Soil Forming ProcessesFactors and Processes that affect SOM Content

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Soil Components

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Organic Matter (OM) versus Soil Organic Matter (SOM)

Organic Matter (OM) the material of which organisms are composed. The material may be alive or dead, and may be found in the atmosphere, in organisms, or in the water and soil.

OM may be from plant or animal tissues that contain C. There are many types of organic matter, and the C content varies widely.

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SOIL ORGANIC MATTER

Fraction of the soil composed of anything that once lived

The term "Soil organic matter" (SOM) has been used in different ways to describe the organic constituents of soil.

SOM as defined by Baldock and Skjemstad (1999) is "all organic materials found in soils irrespective of origin or state of decomposition".

Organic matter is the vast array of carbon compounds in soil.

Originally created by plants, microbes, and other organisms, these compounds play a variety of roles in nutrient, water, and biological cycles

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Soil Organic Matter - (SOM) includes all OM fixed in place on the soil, or already incorporated into the soil.

SOM inputs to the soil are from decomposing plants, vertebrates, invertebrates and microbes. Leachates from leaves (through rainfall) and exudates from plants and animals also added to SOM.

SOM exists in some stage of decomposition. Decomposition begins as soon as a plant part dies or a leachate or exudate is released into the soil. In fact, some leaching of substances from plant parts begin before plant death, and some roots decompose while the plant is still alive (grasses).

SOIL ORGANIC MATTER

Organic matter is the vast array of carbon compounds in soil. Originally created by plants, microbes, and other organisms, these compounds play a variety of roles in nutrient, water, and biological cycles.

Consists of:plant and animal remains in variable stages of decomposition

Metabolic Products of microbes utilizing organic residues as a source of energy root and microbial exudates

Products of secondary synthesis in the form of bacterial plasma

Humus HS consisting of HA, FA, Humin, Hymatomelanic Acid

SOIL ORGANIC MATTER

The first three categories form a group Consist of N and non N cpds CHOs, Proteins, Fats, Waxes, Organic Acids, Resins etc Constitute 10 15% of SOM

Cpds that did not come under any existing organic molecules Nature, Origin and Properties not fully known Humic Substances Constitute 85 90% of SOM

Thus SOM has two types of cpds:

Organic Substances of non specific nature

Natural Humic Substances of specific nature

SOIL ORGANIC MATTER

Humuswell-decomposed organic material

dark brown, amorphous

stable

poorly characterized chemically

~ 1-5 % of mineral soils

Distribution of humus forms in the soils of four great soil groups

colloids of polymeric nature structure: aromatic ring of di- or tri- hydroxy phenol type bridges: -O, -CH2-, -NH-, -N=, -S-

Humus - Structure

Proximate Constituents of SOM:

Carbohydrates Mono and Di Sachharides Cellulose, Hemi cellulose, Pectins, Pentosans, Mannans, PolyuronidesFats and Related cpdsProteins and their derivatives albumins, amino acids, amides, Purine, Pyridine and Pyrimidine cpds, Org cpds with NLignin and their derivatives precursors of lignin Decomposition productsTannins in simple and condensed formResins and terpenesOrganic acids, aromatic cpds, hydrocarbons, alcohol and related cpds

SOIL ORGANIC MATTER

Proximate Constituents of SOM: Shmuk, 1930

Carbohydrates Pentoses, Pentosans, Hexoses, cellulose & early decomposition productsHydrocarbons - ParaffinOrganic acids Oxalic acid, Succinic acid etcAlcohols MannitolEsters Glycerides of Caproic and Oleic acidAldehydes Vannins, Salicylaldehyde Resins N containing cpds Argenine, Choline, Creatinine, Histidine etc

SOIL ORGANIC MATTER

Other Proximate Constituents identified:

Allantonins Shorey, 1938Polyuronides Shorey & Martin, 1932; Waksman & Reuszer, 1932; Norman & Bartholomew, 1943Uronic acids Rudakov and Birkal, 1949Methylglyoxal (CH3COCOH) Enders, Hibbert, 1942 Primary Structural Element of ProtoligninsPolysaccharides of Bacterial Origin Fuller, 1947; Forsyth, 1950Amino sugars Glucosamine, Galactosamine Bremner and coworkers, 1954, 58Carbohydrates and Organic acids of low molecular wejght Nagar, 1953 and Martin, 1954

SOIL ORGANIC MATTER

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Methylglyoxal

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What constitutes Soil Organic Matter?

Litter: Macro organic matter (e.g. crop residues) that lies on the soil surface

Light fraction: Plant residues and their partial decomposition products that. reside within the soil proper

Microbial biomass: Cells of living microorganisms, notably bacteria, actinomycetes, and fungi

Faunal biomass: Tissues of animals (primarily invertebrates)

What constitutes Soil Organic Matter?

Below ground plant constituents: Primarily roots with lesser amounts of dead roots and exudates

Water-soluble organics: Organic substances dissolved in the soil solution

Stable humus: Humified remains of plant and animal tissues that have become stabilized by microbial and chemical transformations and/or by association with inorganic soil components

Organic Constituents of Soil

Non humic substances

Lipids (1-6%) Soluble in moderately hydrophobic solvents

Carbohydrates (5-25%)

Low concentrations of free sugars in the soil solution

Complex carbohydrates that can be extracted and separated from other organic constituents

Polymers of various sizes and shapes that are so strongly attached to clay and/or humic colloids that they cannot be easily isolated, purified, or identified

Organic Constituents of SoilProteins/peptides/amino acids (9-16%)

Low concentrations of free amino acids in the soil solution

Amino acids, peptides, and proteins bound to clay minerals and humic colloids

Mucopeptides and teichoic acids originating from bacterial cell walls

Other (trace)

Humic substances (up to 80%)

Most active fraction of humus Series of highly acidic, yellow-to black-colored, polyeloctrolytes call humic and fulvic acids

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Components of Soil Organic Matter

The Soil Biology Primer (Chapter 1): By Elaine R. Ingham and Small Organisms in the Soil Affect the Environment in Big Ways: www.iaswcd.org

Soils with high organic matter content are the most productive, store more water and contribute to a better environment.

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Composition of Soil Organic Matter

Hyphae Associated with Organic matter

Soil Type

Range

Representative Values

Black Soil

0.7 4.31

1.39

Red Soil

1.3 4.12

1.14

LateriteSoil

0.23 6.29

1.88

Alluvial Soil

0.28 3.18

0.95

Soil Organic Matter Content of Indian Soils

Alfisol

0.8 6.5

3.0

Aridisol

0.2 1.7

1.0

Histosol

20 98

80.0

Mollisol

1.5 6.5

4.0

Oxisol

1.5 5.0

3.0

Spodosol

1.5 5.0

3.5

Vertisol

1.5 3.0

2.0

Soil Organic Matter Content of Mineral Soils

SOM versus Soil Organic Carbon (SOC)

OM in not made up entirely of OC, there are other substances that make up about 40% of the weight.

% SOC x 1.724 = % SOM

% SOM x 0.58 = % SOC

The Van Bemmelen conversion factor is an average: The actual range in nature is between 0.2 and 3.0.

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Total Soil C versus SOC

Total Soil C = Soil Inorganic C + Soil Organic Carbon

Soil inorganic carbon is most common in semiarid and arid regions, but is also found in the eastern USA.

The main sources of inorganic C is from CaCO3 (a.k.a. calcite or lime) as calcareous dust fall, from weathering of limestone and concrete, agricultural application, construction in cities, traffic on gravel roads, exposure of unweathered calcareous rock in mine spoil and pits, and from marl formation in wetland soils by periphyton algae in/under calcium-rich waters.

We directly measure total C by combustion at 550C, then subtract the content of inorganic C measured by CO2 evolution to get the organic carbon portion.

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