7 Major Conditions Affecting Micro-Organisms in Soil

This article throws light upon the seven major conditions affecting activities of micro-organisms present in soil. The conditions are: 1. Moisture 2. Soil Reaction 3. Temperature 4. Nutrients 5. Organic Matter 6. Microbial Products 7. Microbial Enemy.

Condition # 1. Moisture:

Dryness kills many microbes and many micro-organisms develop resistant strains or enter a dormant stage. Micro-organisms are less active at the wilting percentage for plants (less water for their mobility) or at the other extreme saturated soil (very poor aeration) conditions.

Anaerobes, comprising a very small portion of soil microbes, grow best at saturated or waterlogged soil conditions. However, majority of soil micro-organisms are most active at field capacity soil moisture level.

Condition # 2. Soil Reaction:

The majority of soil organisms especially micro-organisms grow best at neutral soil reaction (pH 7.0) which is the pH of microbial cytoplasm (the cell material). Bacteria and actinomycetes are generally less tolerant of acidic soil reactions as compared to that of fungi which can grow a wide range of soil reactions.

An exception is the sulphur oxidising genus Thiobacillus (produces H₂SO₄) that tolerates soil pH even as low as pH 0.6. Localized micro-environments near roots or decomposing residues can produce localized lower pH than that of the whole soil.

Condition # 3. Temperature:

The activity of most of soil organisms accelerates with the rise in soil temperature. There are exceptions that can exist fairly well at very cold temperatures (psychrophiles—cold lovers) and others exists at high temperatures (thermophiles—heat lovers), but the majority of soil bacteria and actinomycetes show optimum activity at temperatures similar to those of the mesophiles (middle group organisms).

There are generally three groups based on temperatures as follows:

a. Psychrophiles:

Grow at temperatures <5°C (< 41°F), but have optimum temperatures near that of mesophiles.

b. Mesophiles:

Grow slightly near 0°C (32°F). Optimum temperatures are usually between 25 °C and 37°C (77-90°F).

c. Thermophiles:

This group of organisms can tolerate 45°-75°C (113°-167°F) with optima between 55° and 65°C (131°-149°F).

Condition # 4. Nutrients:

Micro-organisms have greater demands for nutrients for their growth and development, especially for N, P, S, and Ca, so the optimum amount of those nutrients should be present in the soil. Organisms compete for nutrients with other species of organisms and even with each other.

Condition # 5. Organic Matter:

Soil having moderate to high organic matter is most favourable for the optimum growth and development of most of the soil micro-organisms especially for the bacteria that live on the soil organic matter and become active.

Condition # 6. Microbial Products:

Some soil antibodies substances that retard or kill other beneficial micro-organisms in the soil. Besides, the competition for nutrients among different microbes can also be eliminated and thereby a particular group of micro-organisms gains more nutrients and consequently increases its activity.

Condition # 7. Microbial Enemy:

Many organisms act as predators and parasites. Sometimes they kill many of beneficial soil micro-organisms in soil and indirectly inhibit many important transformations like mineral nutrients transformation, decomposition of organic matter, fixation of nitrogen (gain of N) in soil etc. which affect plant growth and development.

Bacteriophages are viruses that parasites bacteria and cause their (bacteria) death and partial breakdown.