SOIL ORGANIC MATTER

1. SOIL ORGANIC MATTER Organic matter is an essential and characteristic constituent of soils. It is present in a variety of forms ranging from undecomposed plant and animal remains to amorphous dark coloured substances which are stable products of decomposition known as humus. The sources of organic matter subjected to decay in the soil include: Plant remains; Animal remains and excretory products and Microbial cells.

2. The 10 source is the plant tissue. • The relatively un-decomposed organic matter on soil surface is referred to as litter. • The quantity of organic matter in soil varies widely. • Forest soils usually have more organic matter than soils in savanna. • Usually clay soils have more organic matter than sandy soils and • Generally top soils have higher amounts of organic matter than other horizons in the soil profile.

3. Soil organic matter usually has the following chemical components: • Celluloses and Hemicelluloses • Lignin • Proteins • Ether and Alcohol soluble constituents like fats and oils, waxes, resins, tannins and a number of pigments. • Water soluble fractions made up of sugars, amino acids and aliphatic acids.

4. Decomposition of Organic Matter • Microorganisms play a very important role in • the decomposition of plants and animal tissues. • There is usually rapid decomposition of proteins, • celluloses, hemicelluloses and soluble fractions • while lignin is decomposed slowly. • Usually a corresponding increase of microbial • products is recorded with decomposition of organic • materials. • As a result of the decomposition of plant and animal • residues the stable fractions of organic matter • known as humus is produced.

5. Humus is usually made up of resistant decomposition & microbial products and other compounds formed as a result of interaction of organic matter with each other and mineral components of the soil. • Factors Affecting the Rate of Decomposition of organic Matter • Generally, factors that affect activities of soil organisms also affect the rate of decay of organic matter. • These factors could be broadly grouped into three:

6. 1.The nature of the organic material been decomposed – age of plant, the kind of plant and chemical composition of the tissues are very important. • Younger plants generally decompose more rapidly than older ones. This is because they have a high content of water-soluble constituent; a high nitrogen content; narrower C-N ratio and a smaller percentage of lignin (wide C-N ratio is associated with slow decay). Plants have about the same amount of carbon (40%) but they have varying nitrogen content. The C-N ratio is wider in older plants.

7. 2. Soil factors such as aeration, temperature, availability of water, acidity and fertility level. • Decomposition of organic matter is usually stimulated in well aerated soil. • The temperature of the environment will affect species composition and the entire microbial population. This is because different organisms have different optimum temperature for growth. Usually there is a decline in the rate of decomposition of organic matter above 400C and at low temperature.

8. When moisture is at sufficient level, decomposition is enhanced but then the moisture level is too high the rate of decomposition declines since oxygen content will be reduced. • pH affects the growth of individual organisms • and enzymatic activities. Decomposition is most • rapid in neutral to alkali soils. Treatment of acid • soils with lime enhances mineralization of carbon. • Fertilization level: Supplemental carbon speeds • up humus mineralization. This is presumably • because large microbial population builds up • in response to the added substrate.

9. Also, nitrogen is very important for microbial growth thus if nitrogen in the substrate is poor, decomposition is slow. Carbon mineralization will be stimulated by supplemental nitrogen. • The quantity and the type of clay in the soil affect carbon mineralization. Clay adsorbs many organic substrates including extracellular carbohydrate splitting enzymes produced by microbes. It even adsorbs bacterial cells. In some instances kaolinites does not affect decomposition rate while montmorilomites alters the rate of decomposition of some substances.

10. 3.Climatic factors: Rainfall will have effect on the moisture content of the soil, temperature etc. It has indirect effects on soil factor. Importance of Organic Matter 1. The amount of humus in the soil directly influences the soil moisture content. Humus contains complex organic matter which helps to stick soil particles together so improving the soil structure and texture. This on the other hand improves the porosity of the soil and the rate of water percolation. 2. Humus provide a stock of exchangeable and available cations e.g. K, Ca, Mg.

11. Generally it is a source of many plant nutrients. Fresh organic matter makes phosphorous more readily available in acidic soils • The colour of the soil influences heat absorption. It is known that soils that are high in organic matter content have dark colour and dark soils are known to absorb heat. • Organic matter minimizes erosion hazards. This is because soil particles covered with humus stick together more firmly. • The Organic acids that are released during organic matter decomposition aid during the process of rocks and mineral weathering.

12. Fresh organic matter supplies food for the soil dwelling fauna like earthworms, large nematodes etc. • Organic matter helps in soil to buffer the soil against rapid chemical changes which may occur with addition of fertilizers and lime. • Living Population of Soil • Organisms inhabiting the soil include micro-organisms and macro-organisms. • Their activities result in various changes that take place in soil. • These contribute to the physical, chemical and biological properties of the soil.

13. Soil organisms may be classified according to their body size as microflora, microfauna, mesofauna and the macrofauna. Plant like organisms that are less than 200m in size are classified as micro flora. • Examples are fungi, bacteria and actinomycetes. • Animal like organisms that are less than 200m in size are classified as microfauna. Examples are protozoans and some nematodes. • Mesofauna are animals that are 200m-1cm in size e.g. termites, molluscs and most nematodes. • Macrofauna are animals whose length are measured in cm i.e. >1cm e.g. earthworms and vertebrates.

14. Soil Macroorganisms- These are the soil dwelling organisms that can be seen with the naked eyes. They include rodents, snails, insects e.g termites and earthworms. • Soil Microorganisms • Soil contains a wide variety of microorganisms belonging to 6 major groups: • These are: Bacteria; Fungi; Actinomycetes; Algae; Nematodes and Protozoa Assignment: Identify the activities of soil macroorganisms that affect soil fertility and structure.

15. BACTERIA • This forms the most abundant group of soil microorganisms. They are unicellular, prokaryotic, microscopic organisms (they have relatively simple structures with no organelles). Typically bacteria exist in 3 shapes: spherical, cylindrical and helical. • Spherical forms: these are organism commonly known as the cocci. They may exist in the following forms: • 0 Micrococcus(Single celled) • 00 00 Diplococcus (double celled)

16. streptococcus (in chains) Irregular clusters (staphylococcus) Cuboidal (Sarcina) Cylindrical bacteria: These organisms are commonly known as bacilli or rods. They vary in size from one specie to another. Some are slender and long while others are short and thick,

17. some have endospores while others do . not have spores. Some are flagellated while others have no flagella or monoflagellate. • Helical forms: These are made up of spiral shaped bacteria called spirilla. They are not common in the soil. There are intermediate forms between the spirilla and the rods known as the Vibrio e.g. organisms causing cholera.

18. Occurrence of bacteria in Soils • Soil inhabiting bacteria can be broadly grouped as Autochtonous and Allochtonous. • The Autochtonous species are indigenous species, • The allochtonous species may enter the soil through diseased plants or animals or through animal manure. • Many of the major biochemical transformation in soil are carried out by bacteria. • The dominant bacteria genera in soil are Arthrobacters and Bacillus.

19. Plant and animal pathogenic bacteria are also found in soils. e.g. of plant pathogenic species are Erwinia, Agrobacterium, Xanthomonas and Pseudomonas species. • Human / livestock pathogenic bacteria commonly found in soil include: Clostridium tetanii and Bacillus anthracis. • These are spore-bearing rods, which cause tetanus and anthrax respectively.

20. FUNGI • This is a group of multicellular, nucleated organisms with filamentous branched structures. • The extensive network of the fungi mycelia makes them a significant part of the soil biomass. • The mycelia is composed of individual hyphae, which could be septate or non-septate (i.e. divided into cells by cross wall or septa). • Taxonomy of soil fungi • Soil fungi are classified as Oomycetes, zygomycetes, Ascomycetes, Basidiomycetes and Deuteromycetes.

21. Importance of Soil Fungi • 1. Oomycetes- Advanced members of the group are obligate parasites of plants i.e. they cannot exist outside plant tissues e.g. Pythium. • 2. Zygomycetes • this include wide spread genera of soil fungi that are responsible for decomposition of plant debris and animal dung. e.g. Mucor, ; • a number of genera that infect roots of higher plants forming mycorrhiza (symbiotic relationship). e.g. Endogone • and predacious fungi, which occur in soil e.g Arthrobotrys

22. 3. Ascomycetes • Many are saprophytic soil organisms while others are plant pathogens. • Yeasts are unicellular ascomycetes which are also common in many soils. They are of great economic importance especially in brewing and baking industries. • Members of Ascomycetes such as Aspergillus and Penicillium species are of great economic importance being sources of antibiotics and enzymes. They are also spoilage microorganisms. • Others like Chaetomium sp play important role in breakdown of cellulolytic plant remains.

23. 4. Basidiomycetes • Some members of this group are wood destroying fungi. e.g. Polyporussp. • mushroom (Agaricus sp ) are of economic importance - eaten by men.) • Some basidiomycetes are important plant pathogens e.g. the rust. • 5. Deuteromycetes (Fungi imperfecti) • The group contains many fungi of importance including most of the fungal pathogens of man, many serious plant pathogens, many industrially important fungi and many common soil saprophytes. Examples are Fusarium, and Cladosporium.

24. ACTINOMYCETES • This is a transitional group between simple bacteria and fungi. Taxonomically, they are classified as bacteria. Soil Actinomycetes include Streptomyces ,Norcardia, Micromonospora and Actinomyces • Many actinomycetes produce toxic metabolites in the soil. e.g. of such antibiotics are streptomycin, tetracyclines and chloramphenicol. • Actinomycetes are heterotrophs i.e. are capable of utilizing many carbon sources ranging from simple sugars to polysaccharides and aliphatic hydrocarbons.

25. ALGAE: • These are photosynthetic and predominantly aquatic organisms (they occur in lakes, rivers, swamps etc) • However, many species also occur in surface soils as well as in lower horizons. Algae may be unicellular or may occur in short filament. Soil species are usually smaller then aquatic forms. • The Taxonomy of soil Algae • Soil Algae can be grouped into four namely: • Chlorophyta (the green algae) • Cyanophyta (the blue green Algae) • Bacillariophyta (diatoms) • Xantophyta (yellow green Algae)

26. The green Algae predominate temperate region, followed by the diatoms and the blue green Algae, while the blue green Algae are dominant to chlorophyta in tropical soil. Importance of Algae in soils 1. Algae are usually pioneering invaders of habitats where life has been eliminated by natural or artificial occurrences e.g. volcanic eruptions, erosion & bush burning. 2. Some cyanophyta (blue green Algae) are capable of utilizing molecular nitrogen for growth. In the process they enrich the environment with the combined nitrogen.

27. 3.The photosynthetic Algae liberate molecular oxygen in paddy soils thus providing part of the oxygen required by submerged roots of rice. They also enrich such soils with nitrogen. • PROTOZOA • These are primitive unicellular animals which may be parasitic or free living. • The free living forms are abundant in soils. Soil protozoa are classified into three groups based on their need for locomotion. • Sarcodina-amoeboid forms • Ciliophora-ciliated • Mastigophora-flagellated

28. NEMATODES • These are bilaterally symmetrical non- segmented pseudocoelomates. • Most of about 10,000 known species of nematodes are either free living in the sea or parasitic. • About 1,000 species are soil inhabitants. • Most soil nematodes are microscopic, transparent thread-like animals. • Typically the elongated body tapers at both ends with blunt heads and pointed tails. • They are usually found in the upper 10cm of soil profile.

29. The number of nematodes are higher in the vicinity of plant roots then elsewhere in the soil. Some of them are plant parasites extracting plants sap by piercing roots. E.g. Melodoigyne sp. • Majority about 70% of soil nematodes feed on other microorganisms such as fungi, bacteria, protozoa and algae. • They help in the regulation of microbial population and they serve as food sources in the ecosystem. • VIRUSES • This is a group of sub-microscopic non-cellular infectious agents that inhabit the soil.

30. They are obligate parasite of cellular organisms and thus are of considerable economic/medical importance (They cause diseases of plants, animals and man). A virus particle known as a virion is a complex molecule of proteins and nucleic acid with no metabolism of its own i.e. reproduction and other activities are possible only in the presence of suitable host. Virions vary in shape and may be helical, polyhedral or combination of both.

31. MACROORGANISMS • Animals of the rat family are present in large numbers in undisturbed soils. • Their tunnel digging activities improves aeration and water movement through the soil. • They feed on plant materials and release their waste products into soil thereby improving soil fertility. • Earthworms: usually live in well aerated and well drained soils that contains a lot of vegetable matter. • They improve soil aeration by their burrowing activities and ability to mix soil with vegetable matter.

32. Soil nutrient content is improved as soil materials that pass through their guts are mixed with ingested leaves and made into casts. • Stable soil aggregates are produced through the breakdown of the casts. • Snails: Feed on dying vegetation and fallen leaves. Their excretion improves soil nutrient level. • Millipedes and Centipedes: Feed on decaying remains. Their excreta is easily converted to humus.

33. Termites: They ingest soil and mix it with ingested vegetable matter. • They also create channels by burrowing through the soil thus improving soil aeration. • Many species make fine textured mounds from subsoil or surface soil. • The mounds improve the texture of soil when they break down. • The agricultural importance of macro-fauna rests on the contributions to fertility, soil structure and plant diseases.

34. Ecological Interrelationships in soil • In the rhizosphere, there may be interactions between plants and microorganisms or between different microbial populations. Such interactions could be positive or beneficial or they could be negative or detrimental. • Beneficial Interactions.1.Symbiosis:- This is the association of two organisms in which the two symbionts rely upon one another both benefiting from the relationship e.g. the Rhizobium - legume symbiosis in which case the legumes gain nitrogen from the nitrogen metabolizing bacteria and organic carbon is transferred to the Rhizobium from the CO2 metabolizing host.

35. Commensalism:- This is an association of two organisms in which one organism benefits while the other is unaffected. • Detrimental associations • Competition:- This involves rivalry between two organisms for limiting nutrients or common needs such as carbon, inorganic nutrient or even oxygen in soil. • Amensalism:-This is another form of competition which involves antagonism of one organism towards another resulting in the inhibition of another. Toxic metabolites are released into the environment and this suppresses other organisms..

36. Parasitism and Predation:- This is the direct attack of one organism on another organism. • A parasite lives in or on host causing it harm as it uses it as a source of energy and materials for biosynthesis, while • A predator tends to be free-living but uses its prey as source of energy/nutrients but not as habitat. • THE END