14  The  Production  of  Plant  Food  in  the  Soil. 
they  effect.  Experiments  carried  out  by  the  writer  showed 
that  fertile  soils  absorbed  oxygen  more  rapidly  than  less 
fertile  soils,  indicating  a greater  amount  of  bacterial  activity. 
Increased  warmth,  food,  and  water  supply  up  to  a certain  point 
all  caused  increased  oxygen  absorption,  i.e.,  bacterial  activity  ; 
addition  of  lime  was  also  effective.  All  of  these  factors  cause 
increases  in  productiveness.  And  so  we  are  brought  back  to 
our  fundamental  principle  that  the  more  work  the  micro- 
organisms do  the  greater  is  the  productive  power  of  the  soil. 
What  would  happen  if  all  the  micro-organisms  were  killed  we 
do  not  know,  because  no  one  has  ever  succeeded  in  killing 
both  the  active  and  the  spore  forms  without  adopting  such 
drastic  measures  that  the  soil  itself  is  profoundly  altered.  But 
a vei-y  remarkable  result  follows  such  mild  methods  of  treat- 
ment as  kill  the  active  forms,  but  not  the  spores,  and  at  the 
same  time  do  not  much  alter  the  soil.  When,  for  example,  the 
early  soil  bacteriologists  heated  their  soils  to  212°F.  to  effect 
sterilisation,  they  observed  that  the  productiveness  was  increased. 
This  might  have  been  explained  as  the  result  of  a chemical 
decomposition,  but  no  such  simple  explanation  could  be  given 
for  another  fact  that  had  been  recorded.  For  fifty  years  or  more 
the  French  vine-growers,  and  following  them  the  Italian  vine- 
growers,  have  been  in  the  habit  of  injecting  carbon  disulphide 
into  the  soil  to  kill  phylloxera,  and  they  observed  that  the 
fertility  of  the  soil  was  thereby  increased.  Now,  carbon 
disulphide  not  only  supplies  no  plant  food  but  it  actually 
kills  the  micro-organisms,  and  the  problem  arose — If  micro- 
organisms play  so  important  a part  in  soil  fertility,  why 
should  it  be  a good  thing  to  kill  some  of  them  ? The  difficulty 
was  put  a stage  further  back  when  Hiltner  and  Stormer  made 
the  remarkable  discovery  that  the  numbers  of  bacteria  first 
fall,  but  then  increase  enormously  in  soils  that  have  been 
treated  with  carbon  disulphide.  Approaching  the  subject  from 
quite  a different  standpoint,  Dr.  Darbishire  and  the  writer 
found  that  treatment  of  the  soil  with  toluol,  chloroform,  carbon 
disulphide,  or  other  volatile  antiseptics,1  or  heating  to 
212°  F.,  led  to  an  increased  oxygen  absorption,  and  therefore 
bacterial  activity.  All  of  these  methods  of  treatment  increased 
also  the  productiveness  of  the  soil.  Thus  in  one  experiment 
the  heated  soil  produced  53  per  cent,  more  crop,  while  soils 
treated  with  antiseptics  gave  from  It!  to  38  per  cent,  more  crop 
than  untreated  soils.  Crop  increases  of  the  same  order  have 
been  obtained  in  numerous  other  experiments. 
1 The  treatment  consisted  in  adding  2 to  4 parts  of  antiseptic  to  every 
1,000  parts  of  soil,  leaving  it  to  act  for  a day  or  so,  and  then  spreading  out  the 
soil  till  the  antiseptic  had  all  evaporated.  Toluol  is  an  oil  obtained  from  coal- 
tar  and  is  very  much  like  benzine. 
