46 An Account of the Soil [pi'. i 



dragging the materials into the soil and effecting a 

 proper admixture : moulds and bacteria are the import- 

 ant decomposing agents. Fortunately all these organ- 

 isms require substantially the same soil conditions as 

 plants: thus they need air, water, proper temperature, 

 and food, absence of injurious substances, presence of 

 chalk, etc. The soil population is, however, very com- 

 plex and the organisms are not all equally useful ; there 

 is indeed evidence that some are distinctly detrimental. 

 Hitherto no method of discrimination has been adopted 

 and all organisms good and bad have been allowed to 

 grow in the soil without any intentional interference: 

 methods are now being worked out in horticultural 

 practice for controlling the soil population so as to en- 

 courage the useful forms and repress the others. These 

 methods are based on the fact that the useful forms are 

 less easy to kill than the others, and therefore if the soil 

 is heated, treated with mild poisons, dried by the sun, 

 or frozen for long periods during the winter, the sur- 

 vivors are on the whole more useful to the cultivators 

 than the original lot. The process is known as Partial 

 Sterihsation and is under investigation at Rotliamsted^. 

 We may now summarise the processes described in 

 this chapter. 



1. Decomposition of plant and other proteins gives 

 rise to ammonia, which is then oxidised to nitrates. The 

 production of ammonia is called ammonification, and 

 the oxidation to nitrates nitrification. 



2. The nitrates are taken up by growing plants and 

 built up into protein. Certain soil organisms can effect 

 the same change in presence of carbohydrates. 



^ See Reports on Partial Sterilisation in the Journal of the Board of 

 Agriculture, 1912, 1913 and 1914. 



