136 SOIL CONDITIONS AND PLANT GROWTH 



240^). It was traced to two causes : an accumulation of various pests, 

 and an abnormal development, especially in cucumber houses, of the 

 factor detrimental to bacteria. The properties of this factor show 

 that it is identical in character with that present in normal soil, and 

 strongly indicate its biological nature. No evidence of a soluble 

 toxin could be obtained. On the other hand some remarkably 

 interesting protozoa and allied organisms have been picked out from 

 these sick soils and described by Martin and Lewin (193) and T. 

 Goodey. Finally it has been shown that the whole trouble can be 

 cured by partial sterilisation and methods suitable for large scale 

 work have been investigated and are now in use in practice. Steam 

 heat at present proves most convenient, but the suitability and detailed 

 effects of lime have been studied by Hutchinson and McLennan (141) 

 and of various antiseptics by Buddin (640). 



The Action of the Plant on the Micro-organic Population of 



the Soil. 



The whole existence of the soil population is intimately bound up 

 with the growing plant : from this source it obtains its supplies of 

 energy and of those nitrogen compounds the changes in which form 

 the chief theme of soil bacteriology. These effects are mainly 

 exercised by the residues of dead vegetation ; there are, however, others 

 hardly less potent produced by the living plant. 



A growing plant removes a considerable amount of soluble material 

 from the soil, and thus modifies the composition of the soil solution 

 which serves as part of the medium in which the organisms live. 

 Further, the plant roots are continually giving off carbonic acid. It 

 might be supposed that this would cause the surrounding medium to 

 become strongly acid, but as a matter of fact the contrary happens 

 and the medium becomes alkaline. This has been known for some 

 time in the case of water cultures ; the explanation offered is that the 

 plant takes up the acid radicle of the sodium nitrate and leaves behind 

 the base, which immediately appears as the carbonate. Hall and 

 Miller (i 1 8) have obtained evidence of a similar action in the soil, the 

 calcium nitrate formed during nitrification being converted into 

 calcium carbonate while the nitrate radicle is taken by the plant. 

 These effects are favourable to micro-organisms ; others are unfavour- 

 able, such as the removal of moisture by the plant and the evolution 

 of carbon dioxide from the roots. 



We have now to ascertain how these various effects react on the 

 soil micro-organisms. Several attempts have been made to correlate 



