26 THE FERTILITY OF THE SOIL [ch. 



no useful purpose whatsoever. ^Vhether or not 

 further knowledge will show that it is really an 

 essential part of the scheme we cannot say ; our en- 

 deavour now is to reduce it as much as possible. 



More recent investigations made at Rothamsted 

 and elsewhere have brought out the striking fact that 

 conditions which are injurious to active life in the 

 soil often bring about increases in the numbers 

 of bacteria and in productiveness, while conditions 

 favourable to active life often lead to decreases in 

 bacterial numbers and in productiveness. 



This apparent paradox was solved by showing that 

 two groups of organisms occur in the soil : the useful 

 makers of plant food, and another set detrimental to 

 them but, fortunately, more easily killed and slower 

 in multiplying. When adverse conditions appear the 

 detrimental forms suffer more than the useful forms : 

 thus long severe frost, hot dry summer, heat, treat- 

 ment with mild poisons that can subsequently be 

 removed, all put them out of action temporarily, if 

 not permanently, and so lead to greater bacterial 

 activity and greater productiveness. The detrimental 

 forms are provisionally identified with the protozoa 

 in the soil, of which numbers have now been found. 



