T he  Production  of  Plant  Pood  in  the  Soil.  1 9 
Now  a certain  class  of  organisms  known  as  protozoa  possess 
all  these  properties.  Protozoa  are  the  simplest  members  of  the 
animal  kingdom,  and  are  so  small  that  they  can  only  be  seen 
under  the  microscope.  Like  bacteria  they  can  exist  in  active 
and  in  resting  forms,  the  latter,  known  as  cysts,  are,  however, 
much  more  easily  killed  than  spores.  They  are  widely  dis- 
tributed in  ditches,  ponds,  and  on  vegetable  matter,  but  their 
presence  in  soils  had  not  previously  been  suspected  and  no 
one  seems  to  have  looked  for  them.  An  examination  of  the 
untreated  soil  showed  that  they  were  invariably  present  in 
considerable  variety  ; Dr.  Hutchinson  found  a good  number 
straight  off,  and  over  two  dozen  sorts  have  so  far  been  found 
and  are  under  investigation  by  Mr.  Goodey.  In  the  partially 
sterilised  soils,  however,  one  small  kind  only  could  be  found, 
all  the  rest,  including  the  cysts,  having  been  killed  by  the 
treatment.  So  far  it  has  proved  difficult  to  determine  which 
of  the  protozoa  in  the  untreated  soil  occur  as  cysts  and  which 
as  active  forms,  but  experiments  in  culture  solutions  have 
shown  that  some  of  the  active  forms  are  very  detrimental  to 
bacteria  ; some  actually  feed  on  bacteria,  some  can  surround 
the  particles  of  organic  matter  and  thus  keep  off  bacteria,  and, 
as  they  are  about  a thousand  times  the  size  of  bacteria,  they 
must  all  be  severe  competitors  in  the  struggle  for  existence. 
Further,  they  possess  all  the  properties  of  the  injurious  factor 
as  given  above. 
There  may  also  be  other  large  organisms  in  the  untreated 
soil  harmful  to  bacteria. 
We  have  then  these  two  sets  of  facts  definitely  established 
by  experiment.  Soil  contains  something  detrimental  to 
bacteria,  capable  of  growth  and  therefore  presumably  living, 
and  possessing  other  definite  properties.  It  also  contains 
protozoa,  and  possibly  other  large  organisms,  possessing  all 
these  properties  and  detrimental  to  bacteria.  It  is  reasonable 
to  connect  these  two  sets  of  facts  and  conclude  that  the 
protozoa  are  the  injurious  agents  in  the  soil.  This  conclusion, 
of  course,  is  not  a definitely  established  fact  like  the  others ; 
it  is,  indeed,  simply  an  hypothesis,  but  we  can  accept  it  as  true 
until  it  is  disproved  because  the  evidence  in  its  favour  is 
strong  and  becomes  stronger  as  the  work  progresses. 
Thus  a little  more  light  has  been  thrown  on  the  life  in 
the  soil,  and  we  can  penetrate  a little  deeper  into  the  mysteries 
of  the  world  beneath  our  feet.  Besides  the  useful  bacteria 
that  break  down  the  organic  matter  and  make  plant  food, 
we  now  have  evidence  of  another  class  of  organisms  many 
times  larger  than  bacteria,  and  detrimental  to  them  in  several 
ways,  either  devouring  them,  keeping  them  from  their 
food,  or  competing  generally  in  the  struggle  for  existence. 
