THE FERTILITY OF THE SOIL II 



the control of the farmer ; the more thorough the cultiva- 

 tion, the better the drainage and aeration, and the higher 

 the temperature of the soil, the more rapidly will the 

 nitrates be produced,/ As it was then considered that 

 the plant could only assimilate nitrogen in the form 

 of nitrates, and as nitrogen is the prime element neces- 

 sary to nutrition, it was then easy to regard the fertility 

 of the soil as determined by the rate at which it would 

 give rise to nitrates. Thus the bacteria of nitrification 

 became regarded as a factor, and a very large factor, in 

 fertility. This new view of the importance of the living 

 organisms contained in the soil further explained the 

 value of the surface soil. I remember how, when I was 

 an undergraduate, the Master of Balliol told me that he 

 had just been to DR. GILBERT'S first Sibthorpian lecture, 

 and had been chiefly struck by the demolition of the 

 fallacy which leads people instinctively to regard the 

 good soil as lying deep and requiring to be brought to 

 the surface by the labour of the cultivator. This con- 

 fusion between mining and agriculture probably origi- 

 nated in the quasi-normal idea that the more work you 

 do the better the result will be ; but its application 

 to practice with the aid of a steam plough, in the days 

 before bacteria were thought of, ruined many of the 

 clay soils of the Midlands for the next half century. 

 Not only is the subsoil deficient in humus, which is the 

 accumulated debris of previous applications of manure 

 and vegetation, but the humus is the home of the 

 bacteria which have so much to do with fertility. The 

 discovery of nitrification was only the first step in the 

 elucidation of many reactions in the soil depending upon 

 bacteria, for example, the fixation of nitrogen itself. 

 A supply of combined nitrogen in some form or other 



