2 14 SOIL CONDITIONS AND PLANT GROWTH 



1. If the material is rich in energy supply but not in 

 nitrogen {e.g. carbohydrates) the organisms may assimilate 

 nitrates or ammonia already existing in the soil and thus re- 

 duce the amounts of these substances present (p. 266). These 

 circumstances are favourable to the nitrogen-fixing bacteria, 

 and if the temperature is sufficiently high there may be an 

 increase in the amount of nitrogen fixed (p. 203). The as- 

 similation effects are temporarily harmful but the fixation is 

 ultimately beneficial to the growing plant. 



2. If the material is rich in nitrogen {e.g. protein), the 

 organisms will produce considerable quantities of ammonia in 

 obtaining their energy supplies. This effect is wholly benefi- 

 cial to the plant (p. 185). 



3. If the air supply is insufficient — either generally or 

 locally — but other conditions are favourable, the organisms 

 will obtain some of their oxygen from the nitrates present in 

 the soil, and gaseous nitrogen will be evolved. This effect is 

 wholly harmful to the plant (p. 208). 



4. If, however, conditions are unfavourable {e.g. if there 

 is acidity or too low a temperature), the organisms will not 

 act ; the material will only partly decompose and it will ac- 

 cumulate as peat (p. 133). 



Summary of the Changes Taking Place and the Agencies 



Involved. 



It is unfortunate that no synthesis of a soil has yet been 

 effected, and consequently the preceding analysis of the changes 

 taking place cannot be tested by reconstructing the whole pro- 

 cess out of its constituent parts. On the whole the evidence 

 is satisfactory as to the general course of the changes, but in- 

 sufficient for sorting them out quantitatively and precisely. 

 The following scheme summarises them as completely as is 

 possible at present : — ^ 



^ It will be noticed that these processes show certain resemblances to those 

 of sewage purification beds, as worked out by Adeney (2) and Fowler {Bacterio- 

 logical and Enzyme Chemistry, London, 1911). The decomposition in the septic 

 tank, however, and especially in the percolating filter and the contact bed, ap- 



