PUTREFACTION AND OXIDATION 119 



anaerobes, and are therefore found at some little depth in 

 the soil. In order that the soil should be kept in proper 

 condition for producing crops, it is essential that not only 

 these bacteria shall be present (in the lower layers), but 

 that in the upper layers there shall exist oxidation bacteria 

 that shall change the carbonaceous matter into carbonic 

 acid and the ammonia into nitrates. Plants cannot absorb 

 ammonia direct ; their nitrogen must be in the form of 

 nitrates before it is capable of assimilation. Hence we see 

 the necessity of maintaining due porosity in the soil, and 

 of not unduly loading it with more organic matter than the 

 organisms can successfully deal with. The failure of this 

 process of oxidation is well seen in a sewage farm where 

 sewage matter, either liquid or in the form of sludge-cake, 

 has been applied too freely to the land. Instead of large 

 crops being obtained, the reverse is found to be the case, 

 and the land is said to be ' sewage- sick.' 



The oxidation bacteria also play a very important part in 

 the purification of water artificially. 



The filter-beds at the waterworks are constructed of a 

 layer of large stones with unjointed pipes placed at intervals 

 at the bottom ; smaller stones are placed above these, then 

 gravel and rough sand, and lastly fine sand. 



The action of the bed is twofold. First it acts as a 

 mechanical strainer ; and its efficiency in this respect is 

 increased by the formation of a slimy layer derived from 

 the water which has been filtered, and consisting principally 

 of organic detritus and living bacteria. This layer in a 

 great measure prevents organisms from being washed 

 through into the filtrate. The second function of this 

 layer is to serve as a culture ground for oxidation bacteria, 

 which to a large extent tend to prevent the multiplication of 

 the other bacteria, and consequently their growth through 

 the filter. It is a matter of common experience among 



