VPE 511: Environment and Environmental Hygiene (2+1)

SOIL BACTERIA

• Ground water lying near the surface will usually contain free living bacteria derived from the upper few inches of the soil complex.
• These bacteria include aerobic forms which break down organic material into its simple component elements of carbon, nitrogen and hydrogen. This resolution process is further carried on by two groups of nitrogen oxidizing organisms which convert ammoniacal nitrogen to acid radicles that in combination with existing soil bases form nitrites and nitrates.
• The first of these bacterial groups (Nitrosomonas) converts ammonia to nitrites whilst the second (Nitrobactor) completes the oxidation process by converting the nitrites to nitrates, which constitute the completely oxidized state of nitrogen.
• The bacteria which initiate breakdown of organic material as well as those involved in the oxidation of nitrogen require for their efficient functioning the provision of adequate moisture, oxygen , suitable bases and an environmental temperature over 5⁰c.
• This process of converting the soil organic matter to nitrites and nitrates is called biological scavenging.
• If the process is interrupted it leads to accumulation of acid radicals which cause souring of the soil. Water draining from such soil in which these condition pertain will be acid in character.
• Another example of failure of this biological scavenging process sometime occurs in connection with land treatment of sewage tank liquor, where as a result of excessive application of the sewage liquor the alkaline bases in the soil become exhausted and consequently the conversion of the acid formed by the nitrifying bacteria to nitrates and nitrites does not occur and they accumulate in the soil rendering it “sewage sick”. Such soil may have its purifying properties restored by the addition of 1-2 tons of lime per acre, combined with a period of rest from further sewage application.