Som lecture 2


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History of Soil Organic Matter Studies Continued, Developments during 20th Century, Contributions by Shmuk, Trusov, Berzelius and others on Soil Organic Matter

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Som lecture 2

  1. 1. First two decades of 20th Century Scientist ContributionSchreiner, 40 different chemicallyshorey & individual compounds of organiccoworkers, nature – sterols, fats, org N – P1908 – 1930 cpds – HS not specific substancesShmuk, 1924 Specific reserve of organic substances in the soil overlooked – soils studied have low humus content – not typical of majority soilsOden, 1912, Identified Humus Coal, FA, HA &14 , 19 Hymatomelanic acid
  2. 2. First two decades of 20th Century Scientist ContributionHumus Coal Similar to sprengel’s humus coal, Berzelius and Mulder’s humin & Ulmin, Insoluble in water, acid alkali, partially soluble during fusion with alkali and black colourFulvic acid Similar to Berzelius Crenic and Apocrenic acids – yellow to yellow cinnamonic in colour – soluble in alkali alcohol – sensitive & resistant to oxidation (CA & ACA), 55% C content
  3. 3. Humic acidin thewaterextractswithCalciumCarbonatein theLime Stone *Image via Bing
  4. 4. First two decades of 20th Century Contd. Scientist ContributionHumic Acid Dark Brown to Black colour, insoluble in alcohol soluble in alkali, eq. wt 340 58% CHymato- Light coloured than HA, Similarmelanic Acid to Ulmic acid, Chocolate Brown, soluble in alcohol, eq.wt.250 and 62% CMis- FA not analogous to CA & ACAconceptions UA not identical toof Oden Hymatomelanic acid HS classification based on solubility provisional
  5. 5. First two decades of 20th Century Contd. Scientist ContributionDoyarenko, Protein nature of N fraction of1901 HA was establishedSuzuki, 1906– 08 Jodidi,1910 – 13Kelly, 1914Sestini, 1902 HA condensation products of N cpds of polypeptide type – Aromatic nature established
  6. 6. *Image via Bing
  7. 7. First two decades of 20th Century Contd Scientist ContributionMaillard, 1912 Amino acids and carbodydrates– 17 reacted – Dark coloured HS of melanin type obtained – Condensation reaction – Microbes help in hydrolysis of Proteins & PolypeptidesBeijerinck, Enzymatic activity of microbes1900 Perrier, in biochemical transformation of1913 HSBertrand,1898
  8. 8. First two decades of 20th Century Contd Scientist ContributionTrusov, 1914 Classified Plant Materials in to– 15 two types:1. Cpds readily utilized by microbes such as cellulose, hemicelluloses, simple sugars and organic acids are indirect sources of HS – first incorporated into microbial plasma and then utilized in HS synthesis1. Cpds not readily utilized by microbes like lignins, tannins and amino acids of aromatic nature are direct sources of HS
  9. 9. *Image via Bing
  10. 10. First two decades of 20th Century ContdTrusov identified three steps in HS formation:1. Hydrolytic decomposition with formation of simpler substances of aromatic nature2. Oxidation of the latter with the formation of quinones which he called as hydroxy quinones3. Further condensation of quinones into dark coloured complex products Aromatic compounds by oxidation and condensation into HS have been confirmed
  11. 11. First two decades of 20th CenturyTrusov Contd– Origin & Biochemistry of humus formationShmuk– Chemical Nature, Structure and Physico – chemical properties of HS studied
  12. 12. Humic Acid is a High PolymerHeterogenous Aromatic Compound *Image via Bing
  13. 13. First two decades of 20th century Contd Scientist ContributionShmukHumic acid most characteristic constituent of humus with specific nature – not chemically individual cpds – group of cpds with general structural featuresOden’s classification of HS based on solubility only provisional – isolated two fractions – one soluble and another insoluble from a chernozem soil
  14. 14. First two decades of 20th Century ContdShmukHS are intermediate between crystalloids and colloids – but possess properties of colloids like adsorption, swelling etc.HA formed esters with alcohol and Benzyl chloride – Presence of Carboxyl and Hydroxyl (Phenolic) groupsN, constituent and not contaminant of HA – secondary origin – source being microbial plasma
  15. 15. The Steelink model of the Humic Acid Monomer.*Image via Bing
  16. 16. First two decades of 20th Century ContdShmuk - Established aromatic nature of humic acid – Found two components in HA (Hoppe Seyler(1889) in peats)1. An aromatic N containing compound of microbial origin2. Benzene RingTwo components in chemical linkage and not a mixture – Shmuk’s ideas together with that of Trusov (Oxdn of aromatic cpds into quinones and subsequent condensation) led to present day theory of humus – condensation of an aromatic cpd with N cpd of protein origin
  17. 17. First two decades of 20th Century ContdDefects of Shmuk’s works1. Lignin was the main source of aromatic nature – But Tannins, Polyphenols also contain aromatic ring2. Views on humin (alkali insoluble) and crenic & apocrenic acids as unstable mixture of plant residues in varying stages of decomposition differed from contemporary views
  18. 18. *Video by Signzit via YouTube
  19. 19. First two decades of 20th Century ContdWilliams (1897, 1914)Humus formation – biologically interrelated phenomena – equilibrium between reciprocal processes – life & death; symbiosis & antibiosis; synthesis of organic cpds in plants, their decomposition by microbes & resynthesis into HSHumus synthesis not a chemical process – Any organic substance yields black liquids on acid addition – straw, saw dust etcVegetative cover, natural conditions important for humus formation
  20. 20. First two decades of 20th century ContdWilliams (1897, 1914) - Three types of biological associations;1. Woody Vegetation and Microbes – Fungi, Actinomycete and anaerobic bacteria – podzol formation by crenic acids – woody vegetation decomposed by anaerobic fungi2. Meadow Grass Vegetation and bacteria, mainly anaerobic – Ulmic acid formed – anaerobic decomposition of grasses – stable crumb structure3. Steppe Vegetation and aerobic bacteria – humic acids formed – deterioration of soil structure
  21. 21. First two decades of 20th century ContdGeneral Findings of 20th century1. Existence of HS as natural products was convincingly demonstrated2. Various plant materials during complex biochemical transformations served as source for HS3. Kostychev’s ideas that products of bacterial synthesis were involved in the formation of HS was confirmed
  22. 22. First two decades of 20th century ContdGeneral Findings of 20th century4. HS were considered to be a product of a two stage process – Decomposition of original plant residues into simpler compounds and subsequent synthesis of complex HS5. HA – condensation products of aromatic nature with N containing organic cpds6. Enzymatic activity of microbes was involved not only during decomposition of plant residues but also during synthesis of HS
  23. 23. Further Investigations during 20th centuryChemistry of humus from coal, brown coal and peat studied by German Scientists – Fischer & Schrader, 1921-21, Fuchs, 1931-36 – Not strictly applicable to soil HSMarcusson, 1922 – 27: Cellulose converted to oxycelluloses and uronic complexes, acquire aromatic nature and get converted into HSFischer and Schrader & Fuchs, 1920 – 30: Lignin undergoes number of complex transformations, mainly physico chemical – oxidation, condensation etc. gets converted into HS
  24. 24. *Image via Bing
  25. 25. Further Investigations during 20th centuryArguments in favour of Lignin Theory:1. Similarity in the structure of lignin and HA – Presence of aromatic ring and functional groups such as Methoxyl (OCH3) and Phenolic Hydroxyl (OH) group2. Fischer and Schrader (1922) obtained HS by reacting lignin with alkali in autoclave. Under similar conditions Cellulose yielded only weak coloured low molecular weight compounds (Willstatter & Zeichmeister, 1913)3. Lignin was relatively resistant to microbial action while cellulose was rapidly decomposed to low molecular weight cpds
  26. 26. Further Investigations during 20th centuryWeak points of Lignin Theory:1. Similarity in the structure of lignin and HA – Presence of aromatic ring – Also present in substances of non lignin origin such as tannins & Polyphenols in plants2. Fischer and Schrader (1922) obtained HS by reacting lignin with alkali in autoclave – Unconvincing since experimental conditions are quite different from natural conditions3. Nature of cpds participating in the synthesis of HS and exact mechanism of humus formation remain obscur
  27. 27. Further Investigations during 20th centuryLignin Theory Modified: Hobson & Page, 1932; Waksman & Coworkers, 1927 – 33)1. Cellulose and Simple CHOs are rapidly decomposed2. Hemicellulose and Proteins decomposed and incorporated into microbial plasma3. Lignin resistant to decompositionTwo cpds important in Humus Formation1. Lignin of Plant Tissues2. Protein resynthesized into microbial plasma The interaction between the two result in formation of Ligno Protein Complexes