296 SOIL CONDITIONS AND PLANT GRO WTH 



soils, but it is highly probable that much of the apparent 

 discrepancy will disappear with further investigation. 



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

 tion 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 pro- 

 duced by the living plant. 



A growing plant removes a considerable amount of soluble 

 material from the soil, 1 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 acid, but 

 as a matter of fact the contrary happens and the medium be- 

 comes 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 (i2O#), and subsequently Maschhaupt (192) 

 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, involving as they do an accumulation 

 of calcium compounds in the surface soil, are favourable to 

 micro-organisms ; others are unfavourable, such as the removal 

 of moisture by the plant and the evolution of carbon dioxide 

 from the roots. Some investigators have supposed that the 

 plant secretes part of the mineral matter which it has taken up 

 but no longer needs, and if so this would affect the micro- 

 organisms. 



Burd, in California (62^), recognises three periods in the 



For amounts see Appendix II., p. 357. 



