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
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
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
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
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.
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
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.
SOIL ORGANIC MATTERFraction of the soil composed of anything that once livedThe 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
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).
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 decompositionMetabolic Products of microbes utilizing organic residues as asource of energy – root and microbial exudatesProducts of secondary synthesis in the form of bacterial plasmaHumus – HS consisting of HA, FA, Humin, Hymatomelanic Acid
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
Humus well-decomposed organic materialdark brown, amorphousstablepoorly characterized chemically~ 1-5 % of mineral soils
Distribution of humus forms in the soils of four great soil groups
Humus - Structure•colloids of polymeric nature•structure: aromatic ring of di- or tri- hydroxyphenol type•bridges: -O, -CH2-, -NH-, -N=, -S-
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
What constitutes Soil Organic Matter?Litter: Macro organic matter (e.g. crop residues) that lieson the soil surfaceLight fraction: Plant residues and their partialdecomposition products that. reside within the soil properMicrobial biomass: Cells of living microorganisms, notablybacteria, actinomycetes, and fungiFaunal biomass: Tissues of animals (primarilyinvertebrates)
What constitutes Soil Organic Matter?Below ground plant constituents: Primarily roots withlesser amounts of dead roots and exudatesWater-soluble organics: Organic substances dissolved inthe soil solutionStable humus: Humified remains of plant and animal tissuesthat have become stabilized by microbial and chemicaltransformations and/or by association with inorganic soilcomponents
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
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
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: www.iaswcd.org 29
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.
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