First two decades of 20th Century Scientist Contribution

Schreiner, shorey & coworkers, 1908 – 1930

40 different chemically individual compounds of organic nature – sterols, fats, org N – P cpds – HS not specific substances

Shmuk, 1924

Specific reserve of organic substances in the soil overlooked – soils studied have low humus content – not typical of majority soils

Oden, 1912, 14 , 19

Identified Humus Coal, FA, HA & Hymatomelanic acid

First two decades of 20th Century Scientist Contribution

Humus 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 colour

Fulvic 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

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Humic acid in the water extracts with Calcium Carbonate in the Lime Stone

Scientist Contribution

Humic Acid Dark Brown to Black colour, insoluble in alcohol soluble in alkali, eq. wt 340 58% C

Hymato- melanic Acid

Light coloured than HA, Similar to Ulmic acid, Chocolate Brown, soluble in alcohol, eq.wt.250 and 62% C

Mis-conceptions of Oden

FA not analogous to CA & ACAUA not identical to Hymatomelanic acidHS classification based on solubility provisional

First two decades of 20th Century Contd.

Scientist Contribution

Doyarenko, 1901Suzuki, 1906 – 08 Jodidi, 1910 – 13 Kelly, 1914

Protein nature of N fraction of HA was established

Sestini, 1902 HA condensation products of N cpds of polypeptide type – Aromatic nature established

First two decades of 20th Century Contd.

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First two decades of 20th Century ContdScientist Contribution

Maillard, 1912 – 17

Amino acids and carbodydrates reacted – Dark coloured HS of melanin type obtained – Condensation reaction – Microbes help in hydrolysis of Proteins & Polypeptides

Beijerinck, 1900 Perrier, 1913 Bertrand, 1898

Enzymatic activity of microbes in biochemical transformation of HS

First two decades of 20th Century Contd

Scientist Contribution

Trusov, 1914 – 15

Classified Plant Materials in to 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 synthesis

2. Cpds not readily utilized by microbes like lignins, tannins and amino acids of aromatic nature are direct sources of HS

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First two decades of 20th Century ContdTrusov identified three steps in HS

formation:

1. Hydrolytic decomposition with formation of simpler substances of aromatic nature

2. Oxidation of the latter with the formation of quinones which he called as hydroxy quinones

3. Further condensation of quinones into dark coloured complex products

Aromatic compounds by oxidation and condensation into HS have been confirmed

First two decades of 20th Century ContdTrusov

– Origin & Biochemistry of humus formation

Shmuk

– Chemical Nature, Structure and Physico – chemical properties of HS studied

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Humic Acid is a High Polymer Heterogenous Aromatic

Compound

Scientist Contribution

Shmuk

Humic acid most characteristic constituent of humus with specific nature – not chemically individual cpds – group of cpds with general structural features

Oden’s classification of HS based on solubility only provisional – isolated two fractions – one soluble and another insoluble from a chernozem soil

First two decades of 20th century Contd

Shmuk

HS 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) groups

N, constituent and not contaminant of HA – secondary origin – source being microbial plasma

First two decades of 20th Century Contd

The Steelink model of the Humic Acid Monomer.

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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 origin

2. Benzene Ring

Two 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

First two decades of 20th Century Contd

Defects of Shmuk’s works

1. Lignin was the main source of aromatic nature – But Tannins, Polyphenols also contain aromatic ring

2. Views on humin (alkali insoluble) and crenic & apocrenic acids as unstable mixture of plant residues in varying stages of decomposition differed from contemporary views

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Williams (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 HS

Humus synthesis not a chemical process – Any organic substance yields black liquids on acid addition – straw, saw dust etc

Vegetative cover, natural conditions important for humus formation

First two decades of 20th Century Contd

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 fungi

2. Meadow Grass Vegetation and bacteria, mainly anaerobic – Ulmic acid formed – anaerobic decomposition of grasses – stable crumb structure

3. Steppe Vegetation and aerobic bacteria – humic acids formed – deterioration of soil structure

First two decades of 20th century Contd

General Findings of 20th century

1. Existence of HS as natural products was convincingly demonstrated

2. Various plant materials during complex biochemical transformations served as source for HS

3. Kostychev’s ideas that products of bacterial synthesis were involved in the formation of HS was confirmed

First two decades of 20th century Contd

General Findings of 20th century

4. 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 HS

5. HA – condensation products of aromatic nature with N containing organic cpds

6. Enzymatic activity of microbes was involved not only during decomposition of plant residues but also during synthesis of HS

Further Investigations during 20th century

Chemistry of humus from coal, brown coal and peat studied by German Scientists – Fischer & Schrader, 1921-21, Fuchs, 1931-36 – Not strictly applicable to soil HS

Marcusson, 1922 – 27: Cellulose converted to oxycelluloses and uronic complexes, acquire aromatic nature and get converted into HS

Fischer and Schrader & Fuchs, 1920 – 30: Lignin undergoes number of complex transformations, mainly physico chemical – oxidation, condensation etc. gets converted into HS

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Further Investigations during 20th century

Arguments 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) group

2. 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

Further Investigations during 20th century

Weak 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 plants

2. Fischer and Schrader (1922) obtained HS by reacting lignin with alkali in autoclave – Unconvincing since experimental conditions are quite different from natural conditions

3. Nature of cpds participating in the synthesis of HS and exact mechanism of humus formation remain obscur

Further Investigations during 20th century

Lignin Theory Modified: Hobson & Page, 1932; Waksman & Coworkers, 1927 – 33)

1. Cellulose and Simple CHOs are rapidly decomposed

2. Hemicellulose and Proteins decomposed and incorporated into microbial plasma

3. Lignin resistant to decomposition

Two cpds important in Humus Formation

4. Lignin of Plant Tissues5. Protein resynthesized into microbial plasma

The interaction between the two result in formation of Ligno Protein Complexes