The  Production  of  Plant  Food  in  the  Soil. 
17 
increased  production  of  ammonia.  Further,  the  evidence  all 
goes  to  show  that  this  increased  production  is  the  result  of  the 
increased  numbers  of  bacteria.  Thus  the  curves  in  Fig.  2 are 
of  the  type  associated  with  bacterial  rather  than  chemical 
change,  and  in  any  case  all  the  antiseptic  has  gone  before  the 
experiment  began.  Soil  which  has  been  completely  sterilised 
by  heating  to  260°  F. — at  which  temperature  spores  as  well 
S0'1 
lVvea^° 
•#0?- 
Soil  treated  with  Toluol 
Untreated  Soil. 
Fig.  2- 
*u  days. 
-Curve  showing  the  rate  at  which  ammonia  is  formed  in  partially 
sterilised  soils. 
as  active  forms  are  killed — shows  no  increase  in  ammonia 
content  when  it  is  kept,  nor  is  there  any  such  increase  if  a 
little  toluol-  is  left  in  the  soil,  or  if  the  water  supply  is 
insufficient  for  bacterial  needs.  And  so  the  problem  reduces 
itself  to  finding  why  the  new  bacteria  can  bring  about  so 
much  more  decomposition  than  the  old. 
At  first  it  was  thought  that  partial  sterilisation  killed  all 
the  weaker  races,  leaving  only  the  stronger,  which  multiplied 
vigorously  when  they  had  the  field  to  themselves.  But  this 
explanation  broke  down  when  it  was  found  that  the  new  races 
were  individually  less  potent  than  the  old.  Nor  were  the  new 
races  more  efficient  in  making  plant  food  by  reason  of  the 
grouping  of  the  species  or  the  type  of  the  flora.  They  were, 
indeed,  less  efficient,  and  when  some  of  the  old  bacteria 
were  added  to  the  partially  sterilised  soil — as  was  done  by 
inoculating  with  some  of  the  original  soil — there  was  a marked 
increase  not  only  in  numbers  of  bacteria  but  also  in  the 
amount  of  decomposition  effected. 
Therefore  we  are  driven  to  the  conclusion  that  the  extra 
plant  food  produced  by  the  new  races  is  due  simply  to  their 
increased  numbers,  and  we  have  now  to  find  why  they  can 
multiply  so  rapidly  in  the  partially  sterilised  soil  even  though 
they  have  been  rather  weakened  by  the  sterilisation  treatment. 
VOL.  71.  C 
