jAJ4.  I,  1897.] 
THE  TROPICAL  AGRICULTURIST 
475 
THE  GERM  LIFE  OF  THE  SOIL. 
Formerly  we  were  wont  to  reg.ircl  the  soil  as  com- 
posed of  dead,  inert  matter;  now,  however,  thanks 
to  recent  bacteriological  research,  we  know  better. 
Most  of  it,  it  is  true,  is  composed  of  dead  matter; 
but  this  is  so  inextricably  and  intimately  permeated 
by  microbio  life  that  it  can  hardly  be  regarded  as 
dead.  What  all  the  functions  of  these  microbes  are 
we  do  not  as  yet  fully  know,  but  many  effect  the 
decomposition  of  organic  matter ; which,  indeed,  they 
have  been  shown  to  be  among  the  eai'liest  agents 
in  giving  rise  to.  The  fertility  of  a soil  is  directly 
due  to  their  initiative,  for  they  elaborate  in  varioi.a 
ways  the  food  materials  of  tho  plant,  and  convert 
the  latter  into  forms  suitable  for  assimilation.  As 
was  pointed  out  in  a recent  article  on  this  subject, 
soils  may  be  said  to  be,  in  a very  direct  sense,  the 
product  of  their  work  through  tho  long  past  ages. 
Till  recently,  it  was  believed  that  these  organisms  re- 
quired for  the  development  organic  matter ; but  one 
of  the  most  important  discoveries  in  this  domain  which 
has  recently  been  made  goes  to  prove  that  some 
of  them,  at  least,  are  able  to  subsist  on  a purely 
mineral  food.  These  latter  microbes  are  of  very  wide 
occurrence,  and  are  found  even  on  bare  rock  surfaces. 
Although  we  know  as  yet  but  very  little  with  regard 
to  the  methods  .in  which  the  decomposition  of 
tho  material  of  the  soil  is  effected,  it  would  seem 
as  if  the  ultimate  resirlts  obtained  are  due  to  a 
highly  interesting  system  of  co-operation.  Thus  to 
one  class  is  due  the  initial  stage  in  decomposition  ; 
while  another  class  carry  out  the  work  started  by 
the  former  bacteria  to  a further  stage  of  development, 
and  so  on. 
Few  of  us  probably  grasp  the  importance,  from 
an  economic  pomtofview,  of  the  process  of  the  putre- 
faction of  organic  matter  constantly  going  on  in 
the  soil.  It  is  one  of  the  great  factors  in  maintain- 
ing the  circulation  of  matter,  a law  on  which  the 
perpetuity  of  all  life  depends.  There  is  in  this  uni- 
verse only  a certain  definite  amount  of  matter  suit- 
able for  the  formation  of  new  animal  and  vegetable 
life  ; hence,  to  permit  of  the  formation  of  new  forms 
of  such  life,  it  is  necessary  that  the  dead  matter  should 
become  broken  up,  and  rendered  available  for  this  pur- 
pose. Were  this  law  inoperative,  the  result  would 
of  course  be  that  the  earth’s  surface  would  be- 
come encumbered  with  the  accumulation  of  dead 
organic  matter  to  so  great  an  extent  that  animal  life 
would  be  impossible.  Hence  it  is  that,  in  this  respect, 
bacteria  perform  services  to  humanity,  as  well  as  to  all 
other  forms  of  life,  of  incalculable  value. 
With  regard  to  the  occurrecce  and  distribution  of 
these  organisms  in  the  soil,  investigations  have  shown 
that  it  is  almost  entirely  in  their  surface  portion  they 
are  to  be  found,  and  that  the  deeper  we  go  the  less 
numerous  do  they  become.  Among  the  factors 
determining  their  abundance,  the  season  of  the  year  is 
one  of  the  most  important.  Since  among  the  condi- 
tions ixnder  which  bacteria  live  one  of  the  most 
important  is  the  existence  of  a certain  temperature,  we 
find  that  they  are  most  abundant  during  the  summer 
time.  Indeed,  from  spring  to  autumn  there  is  a 
steady  increase  in  the  number.  They  may  be  divided 
into  different  classes,  according  to  the  nature  of  their 
products.  A large  class  oxidise  the  ingredients  of  the 
soil  by  assimilating  the  organic  matter,  converting 
it  into  carbonic  acid  and  water,  and  sometimes  actually 
giving  off  oxygen.  We  have  another  class  whose 
action  is  of  a completely  opposite  character.  These 
organisms  exercise  a reducing  influence  that  is 
not  helpful  to  the  processes  of  agriculture.  To 
this  class  belong  those  which  give  rise  to  a loss  of 
nitrogen  from  its  valuable  compounds,  and  which 
thus  impair  the  fertility  of  the  soil.  But,  happily, 
the  work  of  this  class  of  organism  is  largely  neutra- 
lised by  that  of  a recently  discovered  class  which 
enrich  the  soil  in  this  valuable  fertilising  ingredient 
by  fixing  the  free  nitogen  of  the  air,  and  thus 
bringing  it  within  the  scope  of  the  plant.  Of  these 
three  types  of  organisms  examples  may  be  cited ; 
and  in  doing  so  we  shall  select  such  as  have  to 
do  with  either  the  fixation  or  elaboration  of  that 
most  important  plant-food,  nitrogen. 
In  the  first  place  we  have  what  are  called  nit- 
rifying oragnisms.  These  organisms  effect  the  prepa- 
ration of  nitrogen  in  a form  suitable  for  the  plant 
to  absorb,  and  furnish  a very  good  example  of  the 
co-operative  method  in  which  these  minute  denizens 
of  the  soil  carry  on  their  work.  In  the  process 
three  stages  may  be  distinguished,  in  which,  ic  would 
seem,  three  different  classes  of  orgauisras  are  impli- 
cated. The  first  stage  consists  of  the  conversion  of 
nitrogen  in  its  various  organic  forms  into  ammonia. 
Abundant  in  the  air,  in  rain-water,  and  in  the  surface  of 
the  soil,  the  microbes,  active  in  the  process  of  ammonifi- 
cation,  flourish  best  at  temperatures  between  .SO 
degs.  and  90  degs.  Fahr.  Temperatures  near  the 
freezing  point,  or  above  110  degs.  Fahr.,  check  their 
development.  The  second  stage  in  the  process  is 
effected  by  ferments  of  a globular  form,  and  consists 
in  the  conversion  of  ammonia  into  nitrites;  while 
the  final  stage,  which  is  effected  by  ferments  four 
times  as  small  as  those  effecting  the  second  stage, 
consists  in  the  conversion  of  nitrites  into  nitrates. 
Reference  has  already  been  made  to  this  process  in 
an  article  contributed  to  the  “ Scotsman  ” about  a 
year  ago,  so  that  we  need  not  further  refer  to  it, 
except  to  add  that  the  conditions  regulating  this 
process  of  nitrification  are  well  known,  and  throw 
considerable  light  on  the  question  of  the  fertility  of 
the  soil.  We  may  also  add  that  they  are  chiefly 
limited  in  their  occurrence  to  the  superficial  layers 
of  the  soil,  and  have  not  been  met  with  at  a depth 
below  six  feet  ; while  probably  active  fermentation 
is  not  to  be  found  much  below  eighteen 
inches.  The  reason  of  this  is  that  one  of  the 
chief  reasons  of  their  abundant  development  is 
the  presence  of  a plentiful  supplj'  of  air. 
It  is  on  this  account  that  they  are  not  found 
in  water-logged  soils.  Their  susceptibility 
to  poisonous  substances,  such  as  certain  compounds 
of  iron,  which  are  apt  to  be  produced  w’here  the 
soil  is  not  properly  aerated,  and  to  sulphur  com- 
pounds, serve  to  explain  more  clear  than  was  pre- 
viously realised  the  reason  of  the  inimical  action 
of  such  a body  as  fresh  gas-lime  when  applied  as  a 
manure.  And  here  a very  interesting  practical 
question  presents  itself.  Since  the  fertility  of  a 
soil  may  be  said  to  depend  to  no  small  extent  on 
the  abundance  of  these  nitrifying  ferments,  in  the 
case  of  a soil  which  from  some  cause  or  another 
may  have  had  these  valuable  ferments  killed  out, 
is  it  possible — it  may  be  asked— to  re-seed  the 
soil  ■?  To  this  it  may  be  answered  that  numerous 
experiments  have  demonstrated  in  a striking  manner 
the  value  of  inoculating  a sterile  soil  with  nitrify- 
ing ferments.  This  h;os  been  effected  by  strewing 
over  the  fields  some  soil  rich  in  nitrifying  bacteria, 
such  as  an  old  garden  soil.  There  can  be  little 
doubt,  indeed,  that  the  value  of  farmyard  manure, 
to  a certain  extent,  may  be  ascribed  to  the  fact 
that  it  supplies  the  soil  with  abundance  of  such 
organisms.  The  principle  of  soil  inoculation,  as  we 
shall  immediately  point  out,  has  been  more  syste- 
matically worked  out  in  the  case  of  another  class 
of  organisms — viz.,  those  which  fix  free  nitrogen 
from  the  air. 
The  discovery  of  these  organisms  in  certain  ex- 
crescenes  or  nodules  on  tho  roots  of  certain  legu- 
minous plants,  such  as  peas  or  beaus,  was  made  in 
the  year  1880.  It  has  since  been  discovered  that 
it  is  highly  probable  that  quite  a number  of  differ- 
ent kind-)  are  implicated  in  the  process  ; indeed  that 
each  different  kind  of  plant  has  its  own  special  kind 
of  organism.  These  invade  the  roots  from  the  soil, 
and  give  rise  to  the  formation  of  nodules,  where 
they  multiply  with  great  rapidity,  and  stimulate  the 
growth  of  the  plant  cell.  Three  stages  in  the  pro- 
cess may  be  defined.  The  first  is  that  during  which 
the  bacteria  live  as  parasites  as  the  expense  of  the 
plant  cell.  Gradually,  however,  the  struggle  for  ex- 
istence becoming  very  intense,  the  bacteria  become 
passive,  and  the  cells  become  filled  with  bacteriods 
or  bacterium-like  bodies.  It  is  when  this  period  is 
reached  that  the  plant  absorbs  the  contents  of  the 
nodules,  and  loaves  the  cells,  out  of  which  they  are 
formed,  in  a limp  condition.  How  exactly  the  nitrogen 
