🐜 Soil Biology

• Soil biology is the study of microbial and faunal activity and ecology in soil.
• These organisms include earthworms, nematodes, protozoa, fungi and bacteria.
• Soil biology plays a vital role in determining many soil characteristics yet, being a relatively new science, much remains unknown about soil biology and about how the nature of soil is affected.

Soil organisms

• Soil organisms are involved in nearly every aspect of soil quality
• Role of soil organisms are:
  • Soil N utilization by plants
  • Dead organic matters decomposed
  • Nutrient transformation
  • Physical (structure/aggregation) and chemical properties of soil.
  • The CO₂ content
  • Soil formation

    

Microorganisms

• Microorganisms are also classified based on their ability to grow in the presence or absence of molecular oxygen as aerobes and anaerobes.
• Based on temperature
  • Psychrophiles
  • Mesophiles
  • Thermophiles
• Microorganisms are also classified based on morphology, shape, size, biochemical transformations they carryout.
  • Soil organism are classified broadly soil flora and soil fauna.
  • These again may be subdivided into micro and macro.
  • Micro flora again is classified into Bacteria, Actinomycetes, Fungi and Algae.

Bacteria

• Single celled.
• The cells may be rod shaped or spherical.
• The rods may be about 1 µm wide and up to 3 µm long and about 2 µm in diameter.
• Bacteria are the most abundant group of microorganism in the soil.
• Their population in the soil is not uniform.

Classification of bacteria

• Based on O2 requirement: Aerobic and anaerobic
• Based on temperature
  • Facultative
  • Psychrophiles
  • Mesophiles
  • Theromophile
• Based on their food preparation:
  • Autotroph
  • Heterotroph
  • Chemoautotroph
  • Obligate chemoautotrophs: Prefer specific substrates
  • Nitrobacter – Nitrite as substrate
  • Nitrosomonas – Ammonia as substrate
  • Thiobacillus - Converts sulphur compounds to SO4
  • Ferrobacillus – Converts ferrous to ferric
• Based on symbiotic relationship:
  • Symbiotic N fixers:
    • Associated with a host plant.
    • Both the host and the bacteria get the benefit.
    • Fix atmospheric N.
  • Non symbiotic N fixers:
    • Bacteria present without the association of a plant.
    • But fix atmospheric N.
• Symbiotic, non-symbiotic and cellulose decomposers come under Heterotrophs
• Nitrifiers, denitrifiers Nitrate formers and sulphur oxidizers are autotrophs.

Role of Bacteria

• Bacteria carryout the decomposition of organic matter and synthesis of humus
• Enzymatic transformations are carried out by bacteria
• Bacteria oxidize or reduce many chemical reactions such as N fixation, sulphur oxidation nitrification etc.

👉🏻 Conditions affecting the growth of bacteria:

• Oxygen
• Moisture
• Temperature
• Organic matter
• Exch. Ca and pH
• High Ca concentration and pH 6.0-8.0 optimum.
• Some bacteria function at pH < 3
• Exchangeable Ca is more important than pH. The bacterial population may be 10^-8 -10^-9/gram.
• The biomass may vary from 450-4500 kg/ha.

Actinomycetes

• Unicellular like bacteria. Have same size as bacteria.
• Filamentous and profusely branched.
• Mycelial threads are smaller than those of fungi.
• No nuclear membrane as in bacteria.
• Also called as filamentous.
• Sensitive to acid soils.
• Potato scab a disease due to actinomycetes can be controlled by lowering the soil pH by applying Sulphur.
• Heterotrophic optimum temperature 25-30°C, pH 6.5 – 8.0.
• Actionomycetes are important for organic matter decomposition.
• Chitin and phospholipids are reduced to simple compounds.
• The aroma of freshly ploughed land at certain times of the year is probably due to actinomycetes as well as certain molds.
• Actinomycete population in soil exceed all other organisms except bacteria.
• Their proportion increases with soil depth. Their population and biomass are almost equal to that of bacteria.

Fungi

• Soil fungi may be parasitic or saprophytic.
• They possess filamentous mycelium composed of individual hyphae which are 5-20 µm in diameter and several centimeters in length.
• Most fungi are heterotrophic and hence they depend on the organic matter content of the soil.
• They are dominant in acid soils some can tolerate a pH upto 9.0.
• Fungi are strictly aerobic.
• Fungi are classified into Phycomycetes, Ascomycets, Basidiomycetes and fungi imperfecti.
• Fungi may also may be classificied as: Molds, Yeast, Mushrooms

Molds

• Molds are filamentous microscopic molds develop vigorously in all types of soils
• In acid forest soils - decomposing organic matter.
• The common genera - mucor, Fusarium and Aspergillus.
• Their average population - 10-200 billion / m².
• In humus formation and aggregate stabilization molds are more important than bacteria.
• They continue to decompose complex organic substances after bacteria and actinomycetes have stopped function.

Yeast

• Yeast is a group of fungi which exist as an unicellular organism.
• Reproduce by fission or budding.
• Used as food supplement and also for the production of alcoholic beverages.
• Yeast is not common in soils.
• They produce several plant diseases.

Mushroom

• Mushrooms are present in forests and grasslands where there are ample moisture and organic residues.
• Some mushrooms are edible.
• Mushrooms are also not common in cultivated soils.
• Their fruiting body is above the ground.

Algae

• Algae are filamentous u-10 µm in diameter. Population in soil around 1-10 billion / m2.
• Their mass in soil may be 50-600 kg/ha of furrow slice.
• Algae are photo autotrophs
• They are divided into 4 general groups. Blue green, Green, yellow green and diatoms
• Blue green algae are numerous in rice soils
• Blue green algae growing within the leaves of aquatic Fern. Azolla can also fix atmospheric N.

Soil Fauna

Macro Fauna- Earthworm

• About 1800 species of earthworms are known.
• There are 293 species in the genus Pheretima. The common earthworms found in India are Pheretima posthuma, P. elongater, Lampita mautritii etc.
• Their population may vary from 1,25,000 to 1000000/ha.
• They are active in monsoon season. The worms prefer organic matter as their food.
• They also consume larvae of small animals and bacteria mixed with earth.
• They ingest soil in large quantities which pass through the alimentary canal which has many grinder gizzard
• The ingested soil and organic matter are ejected in the form of castings.
• They make crores of burrows which make the soil porous.
• Earth worms prefer warm (21°C) well aerated soils. The casts have low C:N ratio.
• The burrows left in the soil increase aeration and drainage. They increase the size and stability of soil aggregates.
• The factors influencing the earth worm population and activity in the soil are availability of organic matter, soil pH, temperature and soil moisture.
• The biomass or live weight of earth worm may range from 110-1100 kg/ha.

Ants

• They have local effects. Some have the ability to break down woody materials.
• Some ants produce mounds and some have underground nests.
• There is considerable turnover of the soil due to these.
• The ants and termites can modify soil structure and till the soil.

Nematodes

• Commonly called as thread worms or eelworms. Present in almost all soils.
• They are microscopic most of the nematodes are Saprophytes
• They may feed on other nematodes, bacteria, algae and protozoa.
• The genus Heterodera can infest the roots of all plants.
• Heavy infestation may cause severe damage in vegetable crops.

Micro Fauna - Protozoa

• Single celled, larger than bacteria and are more complex.
• Soil protozoa may be amoeba, celiates and flagellates.
• Soils have more than 250 species. Live weight in soil ranges from 15-175 kg/ha.
• They cause a number of serious diseases in animals and plants.
• They thrive well in moist and well drained soils and on the surface.
• They are not abundant in soils to play a major role in the organic matter decomposition and nutrient release.

Rotifers

• About 100 species have been studied. They thrive under moist and swampy lands.
• Activities are confined to peat bogs and wet areas of mineral soils. Roots of higher plants
• Since the roots grow and die in the soil they supply food and energy for the soil microflora and fauna.
• They physically modify the soil as they push through cracks and make new openings.
• By removing moisture from the soil the plant roots create physical stress that stimulates soil aggregation.
• Plant roots exudates several chemicals which stabilize the soil structure.
• They decay and supply the material for the synthesis of humus.
• Roots constitute about 15-40% that of above ground crop.

Mycorrhizae

🚩 This will be covered in mannures lesson.