Som lecture 3


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Recent Studies on Soil Organic Matter, Nature, Composition, functions of Soil Organic Matter

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Som lecture 3

  1. 1. Further Investigations during 20th centuryC52H46O10(OCH3)COOH(OH)4CO + H2NRCOOHC52H46O10(OCH3)COOH(OH)4 C=NRCOOH + H2OAccording to Waksman;1. HS not specific cpds but a mixture of cpds non specific nature like cellulose, hemicellulose, fats, waxes etc.2. Artificial cpds formed by the action of alkali solutions on the soilWaksman 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
  2. 2. Further Investigations during 20th centuryTyurim (1937) – Book on The Organic Matter of Soils – HS specific cpds – methods adopted by Waksman to determine humus composition not correct and unsuitableSpringer (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 reactionsLater many scientist showed:1. Non lignin cpds can form HS in the soil2. Microbial activity & metabolism play a major role3. Aromatic cpds formed from aliphatic cpds including carbohydrates
  3. 3. Present Stage in Soil Humus Study HS Structure, Composition and Properties Studied: X ray analysis, Electron Microscopy, Infrared Spectroscopy, Chromatography in conjunction with chemical methodsHS formation complex transformation – organic & biochemical – Role microbes & animal kingdomRole of SOM in weathering of rocks and minerals, Soil Forming Processes, etc being studiedInteraction between SOM and MineralsParticipation in Physiological and Biochemical Processes in the plant
  4. 4. Books for Reading1. Soil Organic Matter, its Nature, its Role in Soil formation and in Soil fertility. By kononova, M. M. 19662. 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 Allison5. Chemistry of the Soil By F. E. Bear6. Humus By S. A. Waksman
  5. 5. Books for Reading7. Introduction to Soil Microbiology By Martin Alexander8. Soil Biochemistry By A. D. Mclaren and G. H. Peterson, 19679. Study work on Soil Organic Matter and Soil Fertility By G. H. Peterson10. 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.
  6. 6. Soil Organic Matter1. General Types of Organic Materials2. Specific Types of SOM Substances and Pools3. SOM Decomposition4. Soils and Soil Forming Processes5. Factors and Processes that affect SOM Content
  7. 7. Soil Components*Image via Bing
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  9. 9. Organic Matter (OM) versus Soil Organic Matter (SOM)Organic Matter – (OM) the material of whichorganisms are composed. The material maybe alive or dead, and may be found in theatmosphere, in organisms, or in the waterand soil.OM may be from plant or animal tissues thatcontain C. There are many types of organicmatter, and the C content varies widely.
  10. 10. *Image via Bing
  11. 11. *Image via Bing
  12. 12. SOIL ORGANIC MATTERFraction of the soil composed of anything that once livedThe term "Soil organic matter" (SOM) has been used in differentways to describe the organic constituents of soil.SOM as defined by Baldock and Skjemstad (1999) is "all organicmaterials found in soils irrespective of origin or state ofdecomposition".Organic matter .is the vast array of carbon compounds in soil.Originally created by plants, microbes, and other organisms, thesecompounds play a variety of roles in nutrient, water, and biologicalcycles
  13. 13. SOIL ORGANIC MATTERSoil Organic Matter - (SOM) includes all OM fixed inplace on the soil, or already incorporated into the soil.SOM inputs to the soil are from decomposing plants,vertebrates, invertebrates and microbes. Leachates fromleaves (through rainfall) and exudates from plants andanimals also added to SOM.SOM exists in some stage of decomposition.Decomposition begins as soon as a plant part dies or aleachate or exudate is released into the soil. In fact, someleaching of substances from plant parts begin before plantdeath, and some roots decompose while the plant is stillalive (grasses).
  14. 14. SOIL ORGANIC MATTEROrganic matter is the vast array of carbon compounds in soil.Originally created by plants, microbes, and other organisms, thesecompounds play a variety of roles in nutrient, water, and biologicalcycles.Consists of:plant and animal remains in variable stages of decompositionMetabolic Products of microbes utilizing organic residues as asource 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
  15. 15. SOIL ORGANIC MATTERThe first three categories form a group – Consist of N andnon N cpds – CHOs, Proteins, Fats, Waxes, Organic Acids,Resins etc – Constitute 10 – 15% of SOMCpds that did not come under any existing organicmolecules – Nature, Origin and Properties not fully known –Humic Substances – Constitute 85 – 90% of SOMThus SOM has two types of cpds:1. Organic Substances of non specific nature2. Natural Humic Substances of specific nature
  16. 16. Humus well-decomposed organic materialdark brown, amorphousstablepoorly characterized chemically~ 1-5 % of mineral soils
  17. 17. Distribution of humus forms in the soils of four great soil groups
  18. 18. Humus - Structure•colloids of polymeric nature•structure: aromatic ring of di- or tri- hydroxyphenol type•bridges: -O, -CH2-, -NH-, -N=, -S-
  19. 19. SOIL ORGANIC MATTERProximate Constituents of SOM:1. Carbohydrates – Mono and Di Sachharides – Cellulose, Hemi cellulose, Pectins, Pentosans, Mannans, Polyuronides2. Fats and Related cpds3. Proteins and their derivatives –albumins, amino acids, amides, Purine, Pyridine and Pyrimidine cpds, Org cpds with N4. Lignin and their derivatives – precursors of lignin – Decomposition products5. Tannins in simple and condensed form6. Resins and terpenes7. Organic acids, aromatic cpds, hydrocarbons, alcohol and related cpds
  20. 20. SOIL ORGANIC MATTERProximate Constituents of SOM: Shmuk, 19301. Carbohydrates – Pentoses, Pentosans, Hexoses, cellulose & early decomposition products2. Hydrocarbons - Paraffin3. Organic acids – Oxalic acid, Succinic acid etc4. Alcohols – Mannitol5. Esters – Glycerides of Caproic and Oleic acid6. Aldehydes – Vannins, Salicylaldehyde7. Resins8. N containing cpds – Argenine, Choline, Creatinine, Histidine etc
  21. 21. SOIL ORGANIC MATTEROther Proximate Constituents identified:1. Allantonins – Shorey, 19382. Polyuronides – Shorey & Martin, 1932; Waksman & Reuszer, 1932; Norman & Bartholomew, 19433. Uronic acids – Rudakov and Birkal, 19494. Methylglyoxal (CH3COCOH) – Enders, Hibbert, 1942 – Primary Structural Element of Protolignins5. Polysaccharides of Bacterial Origin – Fuller, 1947; Forsyth, 19506. Amino sugars Glucosamine, Galactosamine – Bremner and coworkers, 1954, 587. Carbohydrates and Organic acids of low molecular wejght – Nagar, 1953 and Martin, 1954
  22. 22. *Image via Bing *Image via Bing
  23. 23. Methylglyoxal*Image via Bing
  24. 24. What constitutes Soil Organic Matter?Litter: Macro organic matter (e.g. crop residues) that lieson the soil surfaceLight fraction: Plant residues and their partialdecomposition products that. reside within the soil properMicrobial biomass: Cells of living microorganisms, notablybacteria, actinomycetes, and fungiFaunal biomass: Tissues of animals (primarilyinvertebrates)
  25. 25. What constitutes Soil Organic Matter?Below ground plant constituents: Primarily roots withlesser amounts of dead roots and exudatesWater-soluble organics: Organic substances dissolved inthe soil solutionStable humus: Humified remains of plant and animal tissuesthat have become stabilized by microbial and chemicaltransformations and/or by association with inorganic soilcomponents
  26. 26. Organic Constituents of Soil•Non humic substances Lipids (1-6%) oSoluble in moderately hydrophobic solvents Carbohydrates (5-25%) oLow concentrations of free sugars in the soil solution oComplex carbohydrates that can be extracted and separated from other organic constituents oPolymers 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
  27. 27. Organic Constituents of Soil Proteins/peptides/amino acids (9-16%) Low concentrations of free amino acids in the soil solution oAmino acids, peptides, and proteins bound to clay minerals and humic colloids oMucopeptides 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
  28. 28. Components of Soil Organic MatterSoils with high organic matter content are the most productive, store more water and contribute to a better environment. The Soil Biology Primer (Chapter 1): By Elaine R. Ingham and Small Organisms in the Soil Affect the Environment in Big Ways: 29
  29. 29. *Image via Bing
  30. 30. Composition of Soil Organic Matter
  31. 31. Hyphae Associated with Organic matter
  32. 32. Soil Organic Matter Content of Indian SoilsSoil Type Range Representative ValuesBlack Soil 0.7 – 4.31 1.39Red Soil 1.3 – 4.12 1.14Laterite Soil 0.23 – 6.29 1.88Alluvial Soil 0.28 – 3.18 0.95
  33. 33. Soil Organic Matter Content of Mineral SoilsAlfisol 0.8 – 6.5 3.0Aridisol 0.2 – 1.7 1.0Histosol 20 – 98 80.0Mollisol 1.5 – 6.5 4.0Oxisol 1.5 – 5.0 3.0Spodosol 1.5 – 5.0 3.5Vertisol 1.5 – 3.0 2.0
  34. 34. SOM versus Soil Organic Carbon (SOC)1. OM in not made up entirely of OC, there are other substances that make up about 40% of the weight.2. % SOC x 1.724 = % SOM3. % SOM x 0.58 = % SOC4. The Van Bemmelen conversion factor is an average: The actual range in nature is between 0.2 and 3.0.
  35. 35. 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 550 C, then subtract the content of inorganic C measured by CO2 evolution to get the organic carbon portion. 37