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-oxidising 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 ( Nitrobacter) completes the oxidation process by converting the nitrites to nitrates, which constitute the completely oxidised state of N₂.
• The bacteria which initiate the 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 of over 5⁰c. Unless this requirements are satisfied, the disintegration of organic matter cannot proceed to completion and the end products represented by the humic acids now formed cause “souring” of the soil.
• Water draining from a soil in which these condition pertain will be acid in character, as for example, the acid waters draining from water-logged peat accumulations, in which the incomplete breakdown of the excessive amounts of vegetable organic matter is due to the absence of oxygen and of suitable bases.
• The plant remains are finally converted to true peat by anaerobic bacteria. Sandy soils, which are always characteristically low in mineral matter, may also under certain conditions accumulate organic matter, because in the absence of soil bases (lime) the decomposition of plant debris cannot proceed, and layers of peat may be formed.
• Similarly, even on heavy land plant debris may tend to accumulate, e.g., many old grass-lands have matted turves many inches thick which show little or no signs of decomposition if ploughed in unless lime is used to correct the soil acidity .
• Another example of failure of this biological scavenging process sometimes 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 acids 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 to 2 tons of lime per acre, combined with a period of rest from further sewage application.