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The population of microorganisms in the soil are affected or influenced by many factors including soil moisture, pH, temperature, aeration, and amounts of organic and inorganic nutrients.

  • pH: pH denotes the hydrogen ion concentration of any environment. While some microbes can thrive in environments with higher pH levels (alkaline), others manage to thrive in environments with lower pH (acidic). However, it is noteworthy that the optimal growth of most microbes may be inhibited at higher pH levels and lower pH levels. Thus, most microbes including those that live in the soil thrive at neutral pH a level (7.0) that is neither too acidic nor too alkaline in nature. 
  • Soil moisture: Moisture is a critical factor for microbial growth in any environment including the soil. It is required for the growth and multiplication of vegetative microbial cells. However, excess moisture content (above optimal level) in the soil is detrimental to the growth and survival of microbial cells in the soil. This is because, excess water supply in the soil limits the exchange of gases (e.g. oxygen) required for optimal microbial growth. Excess water supply in the soil (especially to levels when the soil becomes water-logged) prevents the supply of available oxygen in the soil; and this condition deprives aerobic microbes in the soil the amount of aeration they require for growth and multiplication. Porous (leaky) soils absorb more water than less-porous soil; and this makes soils that are porous in nature to easily become waterlogged than the later (i.e. less porous soils) due to the accumulation of water by the former. Porous soils can easily support the growth of anaerobic microbes than less-porous soils because of the water-logged nature of the former than the later.
  • Amount of organic and inorganic nutrients: Microorganisms require both inorganic and organic nutrient molecules for their optimal growth in any environment including the soil. These microbes can naturally acquire these nutrients from the organic plant and animal matter which they degrade or decompose in the soil. Organic and inorganic nutrients required for microbial growth can also be acquired or sourced from both organic (biofertilizers) and inorganic fertilizers that are use to improve the fertility of the soil. It is however noteworthy that the application of manure (especially inorganic fertilizers) alters the microbial community in the soil; and this adversely affects the microflora of the soil composition. Adequate organic and inorganic matter from the soil is critical for optimal microbial growth in the soil. Microbial population in the soil is usually large in the rhizosphere region of the soil – where nutrient materials such as amino acids and vitamins released from the plant root increases the microbial population and their metabolic activities in the soil. When these critical growth nutrients are released by plant tissues, the growth of the plants in the soil is positively influenced and improved upon due to microbial activities and/or metabolism – which release important microbial products that improve the general fertility of the soil. This phenomenon is generally known as the rhizosphere effect of the soil.             

Further reading

Jee C and Shagufta (2007). Environmental Biotechnology. APH Publishing Corporation, Darya Ganj, New Delhi, India.

Latha C.D.S and Rao D.B (2007). Microbial Biotechnology. First edition. Discovery Publishing House (DPH), Darya Ganj, New Delhi, India.

Maier R.M, Pepper I.L. and Gerba C.P (2000). Environmental Microbiology. Academic Press, San Diego.

Mishra B.B, Nanda D.R and Dave S.R (2009). Environmental Microbiology. First edition. APH Publishing Corporation, Ansari Road, Darya Ganj, New Delhi, India.

Paul E.A (2007). Soil Microbiology, ecology and biochemistry. 3rd edition. Oxford: Elsevier Publications, New York.

Pelczar M.J., Chan E.C.S. and Krieg N.R. (2003). Microbiology of Soil.  Microbiology, 5th Edition. Tata McGraw-Hill Publishing Company Limited, New Delhi, India.

Pepper I.L and Gerba C.P (2005). Environmental Microbiology: A Laboratory Manual. Second Edition. Elsevier Academic Press, New York, USA. 

Roberto P. Anitori (2012). Extremophiles: Microbiology and Biotechnology. First edition. Caister Academic Press, Norfolk, England.

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