BIOLOGICAL PROPERTIES

Soil flora and Soil fauna are terms used to describe the organisms that live within soils. The flora is composed primarily of microorganisms such as bacteria, actinomycetes, algae and fungi (though some fungi are visible to the unaided eye). The fauna is composed of micro fauna such as protozoans, meso fauna such as nematodes and arthropods such as mites, centipedes, springtails, and macro fauna such as insects and worms, mice, moles etc.

The soil flora and fauna play important roles in agriculture and horticulture, performing such roles as decomposing organic materials, moving nutrients through soil layers, fixing atmospheric nitrogen, and improving soil structure.

Some parts of the soil flora and fauna are plant pathogens, while others are antagonists. Without the activities of soil organisms, organic materials would accumulate and litter the soil surface, and there would be no food for florcitas. The soil biota includes

• Megafauna: size range 20 mm upwards, e.g. moles, rabbits, and rodents.

• Macrofauna: size range 2-20 mm, e.g. woodlice, arañas, earthworms, beetles, centipedes, slugs, snails, ants.

• Mesofauna: size range 100 micrometre-2 mm, e.g. mites and springtails

• Microfauna and Microflora: size range 1-100 micrometres, e.g. yeasts, bacteria, fungi, protozoa, roundworms, and rotifers.

Of these, bacteria and fungi play key roles in maintaining a healthy soil. They act as decomposers that break down organic materials to produce detritus and other breakdown products. Soil detritivores, like earthworms, ingest detritus and decompose it.

 Bacteria

• Bacteria are single-celled organisms, and are the most numerous members of the soil, with populations ranging from 100 million to 3 billion in a gram.

• They are capable of very rapid reproduction by binary fission (dividing into two) in favourable conditions.

• One bacterium is capable of producing 16 million more in just 24 hours. Most soil bacteria live in close contact with to plant roots and are often referred to as rhizobacteria.

• Bacteria live in soil water, including the film of moisture surrounding soil particles, and some are able to swim by means of flagella.

• The majority of the beneficial soil-dwelling bacteria need oxygen (and are thus termed aerobic bacteria) and those that do not require air are referred to as anaerobic, and tend to cause putrefaction of dead organic matter.

• Aerobic bacteria are most active in a soil that is moist and neutral soil pH, and where there is plenty of food (carbohydrates and micronutrients from organic matter) is available.

The important roles that bacteria play are:

Nitrification

•  Nitrification is important  part of the nitrogen cycle wherein certain bacteria decomposition of proteins are able to transform nitrogen in the form of ammonium and then into nitrates, which are available to growing plants

Nitrogen fixation

•  In another part of the cycle, the process of nitrogen fixation constantly puts additional nitrogen into biological circulation.

•  This is carried out by free-living nitrogen-fixing bacteria in the soil or water such as Azotobacter, or by those which live in close symbiosis with leguminous plants, such as rhizobia.

•  These bacteria form colonies in nodules on the roots of peas, beans, and related species.

•  These are able to convert nitrogen from the atmosphere into nitrogen-containing organic substances.

Denitrification

•  While nitrogen fixation converts nitrogen from the atmosphere into organic compounds, a series of processes called denitrification returns an approximately equal amount of nitrogen to the atmosphere.

•  Denitrifying bacteria tend to be anaerobes, or facultatively anaerobes including Achromobacter and Pseudomonas.

•  The putrefaction process caused by oxygen-free conditions converts nitrates and nitrites in soil into nitrogen gas or into gaseous compounds such as nitrous oxide or nitric oxide.

Actinobacteria

• • •  Actinobacteria are critical in the decomposition of organic matter and in humus formation, and their presence is responsible for the sweet "earthy" aroma which is associated with a good healthy soil.

    •  They require plenty of air and a pH between 6.0 and 7.5, but are more tolerant of dry conditions than most other bacteria and fungi.

Fungi

•  A gram of garden soil can contain around one million fungi, such as yeasts and moulds. Fungi have no chlorophyll, and are not able to photosynthesise; besides, they can't use atmospheric carbon dioxide as a source of carbon.

•  Therefore they are chemo-heterotrophic, meaning that, like animals, they require a chemical source of energy rather than being able to use light as an energy source, as well as organic substrates to get carbon for growth and development.

•  Many fungi are parasitic, often causing disease to their living host plant, although some have beneficial relationships with living plants.

•  In terms of soil and humus creation, the most important fungi tend to be saprotrophic, that is, they live on dead or decaying organic matter, thus breaking it down and converting it to forms which are available to the higher plants.

•  Some fungi are able to break down cellulose and lignins.

•  Fungi spread underground by sending long thin threads known as mycelium throughout the soil; these threads can be observed throughout many soils and compost heaps.

•  From the mycelia the fungi is able to throw up its fruiting bodies, the visible part above the soil (e.g., mushrooms, toadstools and puffballs) which may contain millions of spores.

•  When the fruiting body bursts, these spores are dispersed through the air to settle in fresh environments, and are able to lie dormant for up to years until the right conditions for their activation arise or the right food is made available.

Mycorrhizae

•  Those fungi that are able to live symbiotically with living plants, creating a relationship that is beneficial to both, are known as Mycorrhizae (from myco meaning fungal and rhiza meaning root).

•  The mycorrhiza obtains the carbohydrates that it requires from the root, in return providing the plant with nutrients including nitrogen and moisture. Later the plant roots will also absorb the mycelium into its own tissues.

•  Beneficial mycorrhizal associations are to be found in many of our edible and flowering crops.

•  These include at least 80% of the brassica and solanum families (including tomatoes and potatoes), as well as the majority of tree species, especially in forest and woodlands

Role of soil microorganisms in the decomposition of organic matter

•  Soil microorganism plays an important role in completing the carbon cycle. They convert organic matter into simple substances, liberate CO2 and water, increase the concentration of nitrogen content and bring down the ratio between C and N in soil.

•  This improves the soil fertility. The released CO2 goes back to complete the cycle and this ensures that there is no major lock-up in organic tissues.

• Mineralization into CO2, NH4, NO3, NO2

• Immobilization of C, N, P, S in microbial cells

• The C: N ratio is ~ 10:1. C: N ratio – is a tool to predict the rate of decomposition. If the C: N ratio is wider, decay will be slower.

 Ecological significance of soil microorganisms

• Soil microorganisms are very important as almost every chemical transformation taking place in soil involves active contributions from soil microorganisms.

• In particular, they play an active role in soil fertility as a result of their involvement in the cycle of nutrients like carbon and nitrogen, which are required for plant growth.

•  For example, soil microorganisms are responsible for the decomposition of the organic matter entering the soil (e.g. plant litter) and therefore in the recycling of nutrients in soil.

• Certain soil microorganisms such as mycorrhizal fungi can also increase the availability of mineral nutrients (e.g. phosphorus) to plants.

• Other soil microorganisms can increase the amount of nutrients present in the soil. For instance, nitrogen-fixing bacteria can transform nitrogen gas present in the soil atmosphere into soluble nitrogenous compounds that plant roots can utilise for growth.

• These microorganisms, which improve the fertility status of the soil and contribute to plant growth, have been termed 'bio-fertilizers' and are receiving increased attention for use as microbial inoculants in agriculture.

• Similarly, other soil microorganisms have been found to produce compounds (such as vitamins and plant hormones) that can improve plant health and contribute to higher crop yield.

• These microorganisms (phytostimulators) are currently studied for possible use as microbial inoculants to improve crop yield.

• In contrast to these beneficial soil microorganisms, other soil microorganisms are pathogenic to plants and may cause considerable damage to crops.

• Large numbers of pathogenic microorganisms are routinely found in the soil and many of them can infect the plant through the roots.

• However, certain native microorganisms present in the soil are antagonistic to these pathogens and can prevent the infection of crop plants.

• Antagonism against plant pathogens usually involves competition for nutrients and/or production of inhibitory compounds such as secondary metabolites (antimicrobial metabolites and antibiotics) and extra cellular enzymes.

• Other soil microorganisms produce compounds that stimulate the natural defence mechanisms of the plant and improve its  resistance to pathogens.

• Collectively, these soil microorganisms have been termed 'bio-pesticides' and represent an emerging and important alternative (i.e. biological control) to the use of chemical pesticides for the protection of crops against certain pathogens and pests.

• Azospirillum induces the proliferation of plant root hairs which can result in improved nutrient uptake

• Macro-organisms are only a small part of a soil’s make up, but they are important for healthy plants and nutrient-rich soil.

• In gardening, macro-organisms are the animals, mostly invertebrate, that live in the soil, and which are generally visible to the naked eye.

• Many benefit the soil by helping to break down minerals, soil particles and nutrients. They also add other elements to the soil: egg casings, digested soil, digested plant and animal (some prey on others) materials.

• Since their life cycles are short, when they die, their bodies also add nutrients to the soil. These additional elements break down quickly in the soil, adding to the overall richness of the soil.

• Beneficial macro-organisms include earthworms, many species of nematodes, and other beneficial insects.

• Destructive macro-organisms include larvae of some insect species.