The  Production  of  Plant  Food  in  the  Soil.  13 
so  that  they  need  neither  food,  air,  water,  nor  warmth,  but  can 
survive  treatment  that  would  kill  any  active  form.  The 
change  from  one  to  the  other  is  fairly  rapid,  and  can  take 
place  as  often  as  necessary  ; directly  food  or  water  conditions 
become  unfavourable  to  the  active  forms  they  change  into 
spores,  but  when  the  conditions  again  become  favourable  the 
spores  can  blossom  out  once  more  as  active  forms.  It  is  by 
no  means  a simple  matter  to  decide  whether  a particular 
organism  known  to  be  present  in  the  soil  is  there  in  an  active 
or  a spore  form.  But  in  spite  of  this  difficulty  something  has 
been  learned  about  the  life  in  the  soil,  and  a very  wonderful 
story  it  promises  to  be. 
Among  the  inhabitants  of  the  top  six  inches  of  soil  are 
bacteria  varying  in  length  from  sxsinjf  to  inch,  fungi 
and  moulds  which  are  larger  than  this,  algae,  and,  as  we  shall  see 
shortly,  protozoa  ; the  first  four  are  all  members  of  the  vegetable, 
and  the  last  of  the  animal,  kingdom.  There  are  also  larger  and 
more  organised  animals  like  eel-worms,  &c.  The  numbers  of 
the  micro-organisms  are  enormous  ; of  bacteria  alone  in  active 
and  spore  forms  there  are  from  four  to  ten  millions  per  gram 
of  soil — a gram  is  about  of  an  ounce,  and  is  roughly  a salt- 
spoonful — while  no  method  of  counting  the  other  organisms  has 
yet  been  devised.  The  active  forms  are  all  competing  for  food, 
and  we  can  imagine  that  the  struggle  for  existence  is  very  severe, 
but  the  power  of  transformation  into  spores  is  a great  advantage, 
saving  many  kinds  from  destruction.  In  the  struggle  no  one 
species  exterminates  the  rest ; just  as  elsewhere  under  natural 
conditions  a great  number  of  sorts  have  a chance  of  survival, 
so  in  the  soil  we  find  a highly  mixed  flora  and  fauna.  It 
would  be  a mistake,  however,  to  suppose  that  they  are  all 
there  expressly  for  the  purpose  of  making  plant  food  for  our 
benefit.  On  the  contrary  they  are  living  their  own  lives  just 
as  are  the  animals  in  an  uninhabited  country.  Some  of  them, 
it  is  true,  are  useful  to  us,  others  are  not.  Some  actually  use 
up  or  destroy  the  plant  food  that  has  already  been  made,  while 
others,  as  we  shall  see  later  on,  are  injurious  to  the  useful 
food-making  organisms.  Indeed  the  top  six  inches  of  soil 
might  almost  be  called  a separate  world,  so  vast  is  the  number 
and  variety  of  its  inhabitants,  and  if  a man  could  project  him- 
self into  it  and  see  what  was  going  on  he  would  have  such 
a tale  to  tell  as  would  put  to  shame  our  most  imaginative 
writers  of  romance. 
More  prosaic  methods,  however,  have  to  be  adopted  in  the 
laboratory.  One  that  has  proved  useful  is  the  measurement 
of  the  rate  at  which  oxygen  is  taken  up  from  the  air  by  the 
micro-organisms  of  the  soil.  They  require  oxygen  not  only 
for  their  breathing,  but  also  for  some  of  the  decompositions 
