238 THE LIVING ORGANISMS OF THE SOIL [chap. 



time to attain its full action within the soil. Both the 

 required antiseptic and the method of applying it are 

 now matters under systematic investigation. 



The existence of " sick " soils is not confined to glass 

 houses, and one such case, that of sewage sick soil, has 

 been investigated and shown to be due to the presence 

 of an excess of protozoa. In the purification of sewage 

 by its application to cultivated land on which crops are 

 grown, it is found that the productive power of the land, 

 which is at first enormously enhanced by the abundant 

 supply of water and soluble nitrogenous compounds in 

 the sewage, after a time begins to fall off. If the land 

 is anything heavier than almost pure sand, the physical 

 condition suffers greatly, the surface becomes covered 

 with a gelatinous skin of algae and other organisms, 

 percolation is impeded by this skin and by the defloccu- 

 lation of the clay by the alkaline sewage water, in 

 consequence the effluent is very imperfectly purified 

 by oxidation. To some extent the condition of the soil 

 may be ameliorated by resting the land, especially if 

 lime is applied to flocculate the clay, and if the skin is 

 destroyed by roughly ploughing and leaving the surface 

 to dry out; but still the soil remains comparatively 

 ineffective as a purifier of the sewage. Russell and 

 Golding have shown that this condition of sickness is 

 caused by the accumulation of active protozoa in the 

 sewage-laden soil, in which the conditions are naturally 

 favourable for their development. At first the soil 

 bacteria multiply 'rapidly and purify the sewage by 

 breaking down the nitrogenous materials ; the numbers 

 of the protozoa, however, grow up to the new bacterial 

 content and then limit them, so that this powerful 

 purifying taction with the production of plant food 

 becomes similarly limited. Treatment with toluene 



