18 
The  Production  of  Plant  Food  in  the  Soil. 
Millions  of  bacteria  in 
one  gram  of  soil 
Ammonia  and  nitrate 
present  in  one  million 
parts  of  soil 
At 
After 
After 
At 
After 
After 
begin- 
39 
101 
begin- 
39 
101 
ning 
days 
day  6 
ning 
days 
days 
Partially  sterilised  soil  . 
2 
43 
41 
10-7 
25-7 
34-8 
„ „ + 0-5  per 
cent,  of  original  soil  . 
2 
596 
70-6 
10-7 
26-7 
42-8 
At 
After 
After 
At 
After 
begin- 
20 
61 
begin- 
61 
Gain 
ning. 
days 
days 
ning. 
days 
Partially  sterilised  soil  . 
2 5 
28 
60 
24-8 
49  1 
24-3 
,,  „ + water  ex- 
tract  containing  bacteria  from 
original  soil  .... 
25 
613 
166-6 
280 
71-7 
437 
A clue  is  furnished  by  the  experiment  just  quoted,  which 
shows  that  the  organisms  of  the  untreated  soil  can  multiply 
much  more  rapidly,  and  effect  more  decomposition,  in  the 
partially  sterilised  soil  than  in  their  own  soil.  This  proves 
that  the  partially  sterilised  soil  is  a better  dwelling  place  for 
bacteria  than  the  original  soil  was  ; it  shows — and  this  was 
supported  by  other  experiments  which  I need  not  set  out — 
that  the  original  soil  contains  something  detrimental  to 
bacteria  which  is  put  out  of  action  by  heat  or  by  antiseptics. 
This  “ something  ” was  not  a chemical,  for  it  was  found  to  be 
capable  of  growth  ; thus  when  5 per  cent,  of  the  original  soil 
was  added  to  partially  sterilised  soil  there  was  at  first  a rise 
in  bacterial  numbers,  due  to  the  invasion  of  the  un weakened 
stocks,  but  then  a fall,  showing  that  the  injurious  factor  had 
begun  to  manifest  itself.  Thus  it  grows,  but  not  so  quickly 
as  bacteria.  It  cannot  pass  through  a filter  and  does  not  appear 
in  the  water  extract  of  the  untreated  soil,  although  bacteria 
do,  as  a previous  experiment  has  shown.  It  is  therefore 
larger  than  bacteria.  It  is  killed  by  drying  the  soil  for  a long 
time  but  not  for  a few  hours  ; it  is  killed  by  long  exposure  of 
the  soil  to  hot  sunshine,  or  by  heating  for  an  hour  at  122°  F., 
but  not  at  110°  F.,  and  it  never  revives,  but  only  reappears 
when  some  of  the  untreated  soil  is  added.  A number  of 
poisons  also  kill  it.  When  soil  is  treated  by  any  of  these 
methods  there  is  a gain  in  bacterial  numbers,  in  the  amount 
of  plant  food  made,  and  in  the  crop-producing  power.  On 
the  other  hand,  the  injurious  factor  is  favoured  by  dampness 
and  organic  matter,  conditions  which  obtain  on  sewage  farms 
or  in  certain  greenhouses. 
