82 OUTLINES OF BACTERIOLOGY 



§3. BACTERIA IN THE SOIL. 



The number of bacteria in the soil is usually enormous, because not 

 only is organic material normally present, but also most of the soil- 

 bacteria belong to the spore-forming kinds, so that when food-stuff is 

 not available, or sufficient moisture be not present, they are able to 

 assume a resting state in the spore-condition until such time as more 

 favourable conditions hold. The bacterial contents of an ordinary soil 

 may range from 10,000 to 5,000,000 per gram, whilst polluted soil is, 

 like milk and sewage-water, an ideal nutrient medium, and conse- 

 quently can support an enormous number of bacteria. In such soils as 

 many as 100,000,000 and more per gram may be found. It must be 

 borne in mind that the soil is the depository of the remains of animal 

 and vegetable organisms, and further that these are constantly under- 

 going changes at the hands of a beneficent nature, to the end that they 

 may once more become useful to future generations of plants and 

 animals. Nature's agents in this are almost all of a bacterial kind. 

 Virgin soils have fewer bacteria than those that are cultivated, whilst 

 the latter have fewer than prepared soils. This must inevitably 

 result, because in prepared soils the fertilisers that are used serve 

 as excellent food-material to different kinds of bacteria, and, in fact, 

 this is the object of the cultivator, for the higher plants cannot utilise 

 the fertiliser until the composition of the latter has been changed 

 by the activity of bacteria. The number of bacteria in the soil of 

 densely populated places — whether populated by man or the lower 

 animals — is much greater than in that of sparsely populated localities, 

 owing to the larger amount of organic material resulting from the 

 greater number of deaths, the greater amount of waste-food, excretory 

 matters, etc. The number of bacteria rapidly diminishes as we 

 descend from the surface. Below 5 or 6 feet only a small number 

 of anaerobic bacteria are generally found, and 10 feet down the soil 

 is practically sterile, for at this depth there is no organic material 

 to support bacterial life. 



With regard to the diff'erent kinds of bacteria that are found in the 

 soil, there are saprophytic bacteria, which live on dead organic matters, 

 nitrite- and nitrate-bacteria, which feed on ammonia-compounds and 

 nitrites respectively, also nitrogen-bacteria, which win back from the 

 atmosphere the nitrogen gas which, in the free condition, has been 

 given off during the decomposition of various organic matters. Again, 

 denitrification bacteria are present. These break down nitrates, and 



