Micro-organisms in soil and their ecological importance

Microorganisms in the Soil and their ecological importance

Generally, we consider the microorganisms that are present in the soil are known as the soil microbes. They are higher in the number and are more intimate to the soil particles and the cells of the root plants or we can say that they are in the soil either in independent form or the parasitic and the saprophytic form because the soil and the cells of the plant provide the food for their survival and the growth. Such microbes are mainly bacteria, actinomycetes, fungi, algae, and the soil protozoan which majorly play role in the decomposition of the soil.

a. Bacteria

Bacteria are the unicellular micro-organisms and are classified according to the nutritional pattern and their oxygen needs. Bacteria such as the autotrophic bacteria can oxidize ammonia, nitrates, sulphates, Sulphur, ferrous ion, Manganese ion, hydrogen gas and the carbon monoxides. Not only this, the autotrophic bacteria can oxidize the ammonium to the nitrates and then to the nitrates. For example, Blue-green bacteria or the Cyanobacteria fixes atmospheric nitrogen and release oxygen in the flooded rice field. Other examples are purple sulphur bacteria, green sulphur bacteria, halophiles etc.

The another form of the bacteria i. e Heterotrophic bacteria are those that depend on the organic matter for the nutrients. They are dominant in the soil and accounts for the decomposition of the organic matter. They may be present in an animal as well. They are in the parasitic or in the saprophytic form.

b. Actinomycetes

Actinomycetes are the gram-positive aerobic and the filamentous bacteria having the separate hyphae having elongated cells containing the nucleoid body. These are fungi-like bacteria as they occupy the intermediate position between the bacteria and the fungi. As they have profusely branching body, aerial mycelium and clumpy growth in the liquid culture their branching filamentous networks are very sensitive to the acidic soils, the actinomycetes have the ability to attack the complex and the resistant compounds like the cellulose and the chitin. They require air for the metabolism so the compost material should also be well aerated for the decomposition.

c. Fungi

Simply fungi are considered as those organisms that live on the dead or the living plant or the animal tissue as they have the ability to use the sun for their energy. They are the lignin decomposers. According to their nature fungi are divided into three groups as the ascomycetes, zygomycetes, and the basidiomycetes. The yeast falls under the ascomycetes group which do not play the major role in the formation of the soil but is used in the formation of the ethylene. The zygomycetes are very important in decomposing the residues, molds and are multicellular. The basidiomycetes include fungi like the mushroom that helps in decomposing the cellulose, lignin, and the protein. The fungi can oxidize the ammonium compounds and fix the nitrogen into the combined form.

d. Algae

Algae present in the soil is the chlorophyll-bearing organism. Green algae, blue-green algae, and diatoms are the chlorophyll-bearing algae and develop best in the moist soil. They are the producers of the new photosynthetic group and produce about 70 % of the oxygen we breathe.

e. Soil Protozoans

Protozoans are the unicellular organism without the true cell wall. The protozoa inject the other bacteria, fungi and the microbes and its digestion of the bacteria and the fungi influence the microbial population. The protozoans in the soil help to control the other minerals and also cause various diseases. E. Coli causes diarrhoea in human.

Factors affecting decomposition of the Organic matter

Various factors affecting the decomposition of the organic matter are:

a. Temperature

As the growth of the plant is restarted by the cold temperature, the growth of the microbes is adversely affected by the cold temperature. The bacteria requires an optimum temperature of 25 - 37 ⁰C. So they cannot tolerate the cold climate. The warm temperature provides the high growth and the activities of the plant as well as the microbes. Hence, increases the rate of the organic decomposition of the soil.

b. Soil moisture

For the microbes to proceed slight, wet environment is required. In the dry situation and the water saturated region, the growth of the plant, as well as the microbial activities, gets stopped.

c. Nutrients

From the human to the plants and each organism or the bacteria, nutrients is required. The lack of the nutrients reduces the growth and also slows down the decomposition process as the micro-organisms use the nutrients in the dead organic material before the plant roots can absorb it.

d. Soil pH

Micro-organisms cannot tolerate the most acidic as well as the more alkaline pH. Most common microbes survive in the pH between 6 to 8 . The severely inhibit in more acidic 8.5 pH soils but the plants can tolerate the extreme pH.

e. Soil texture

The fine-grained soil absorbs the nutrients easily. So many of the decomposing enzymes may be denatured. The organic substances like the protein, cellulose are also absorbed to the mineral surfaces by many kinds of the bonds like =O, -OH, -Fe, -Al. The organic molecules of the absorbed clays are particularly decomposed by the microorganism.Therefore, the process of the decomposition is reduced in the fine-grained soil than that of the coarse-grained soil. Besides the toxicity of the element, presence of the excessive soluble salts, organic phototoxic as the inhibitors of the decomposition.

Condition affecting the growth of the soil microbes

1. Growth of the soil microbes

The microbes are in the constant competition for the organically combining carbon and the other kinds of the nutrient in which the ability to get such nutrients depend on the moisture, temperature, soil acidity, soil nutrient level, suitable energy source and the competition by the other microbes.

a. Soil moisture

Dryness kills the microbes . Microbes cannot tolerate extreme dryness or the wetness. Many microorganisms enter into the dormant stage and they are mostly activated in the moist content near the field capacity value.

b. Soil acidity

The microbes do not survive in the pH greater than 8.5 and less than 4.5 . They survive in the optimum pH between 6 to 8. The bacteria and the actinomycetes are less tolerant of the acidic soil than the fungi.

2. Temperature

The microbes cannot tolerate the extreme warm as well as the extremely cold temperature. The majority of the microbes have the optimum activity temperature but some bacteria can survive in the extremely cold temperature like psychrophiles and can exist in the relatively high temperature like thermophiles.

a. Psychrophiles

They can survive at the temperature below 5 ⁰C but have the optimum temperature near that of the mesophiles.

b. Mesophiles

The optimum temperature for them is usually between 25-37⁰C which shows little growth above 40⁰C and may die at the higher temperature.

c. Thermophiles

They can tolerate 40- 75 ⁰C in which the optimum temperature is between 55 to 65⁰C . Example: Composting.

3. Nutrient level and the microbial composition

The ability of the nutrient is the essential factor for the growth of the soil microbes. The protozoa consume the other microbes as well. Some microbes also produce the antibodies that kill other microbes for gaining more nutrient. The optimum growth conditions for the microbes imply not only adequate temperature, moisture pH, and nutrients but also an absence or low level of the microbial enemies.

References:

Santra, S. (2004). Environmental Science . India: New Central Book Agency (p) Ltd.

T., R. (2008). Towards a Sustainable Future . India: PHI (p)Limited.

Miller, Jr. G. T. (2003). Living in the Environment. Wadsworth Publication.