10  The  Production  of  Plant  Food  in  the  Soil. 
longer  or  shorter  duration,  but  it  must  be  added  in  some 
form  or  other  if  the  soil  is  to  be  worked  indefinitely  at  a 
profit.  This  organic  matter  is  one  source  from  which  non- 
leguminous  plants  obtain  their  nitrogenous  food  ; it  is  not, 
however,  the  actual  food,  but  the  raw  material  out  of  which 
the  real  food,  ammonia  and  nitrates,  is  made.  For  long 
the  process  of  manufacture  was  thought  to  be  chemical  ; 
this  was  Liebig’s  view,  and  was  held  from  1840  for  many 
years.  But  advances  in  other  branches  of  science,  and 
particularly  in  bacteriology,  were  slowly  making  this  view 
untenable.  During  the  sixties  and  seventies  bacteria  were 
shown  to  be  the  cause  of  putrefaction,  decay,  and  other 
similar  changes  ; they  were  shown  further  to  produce  disease 
and  to  be  almost  universally  distributed.  And  so  when  in 
1877  Schloesing  and  Muntz  discovered  that  nitrification,  the 
last  stage  of  the  decomposition  process  in  the  soil,  was  also 
the  work  of  bacteria,  the  scientific  world  was  ready  for  the 
discovery,  and  gave  due  heed  to  it.  Before  long  the  work  of 
A.  Koch  and  others  on  the  Continent  had  shown  that  other 
parts  of  the  process  were  brought  about  by  bacteria,  and  had 
thus  broken  down  the  old  chemical  view.  Bacteria  now  stood 
revealed  as  the  makers  of  plant  food  in  the  soil. 
These  new  ideas  were  introduced  into  Great  Britain  by 
Warington,  and  were  much  developed  by  him.  In  one  of  his 
papers  in  1883  he  sets  out  the  changes  brought  about  by 
micro-organisms  in  the  soil  as  follows  : the  organic  matter  is 
broken  down,  giving  off  much  carbonic  acid  and  leaving  a 
residue  richer  in  nitrogen  ; this  also  decomposes,  giving  rise 
first  to  ammonia,  then  to  nitrates.  Subsequent  work  has  fully 
confirmed  the  accuracy  of  these  views,  and  has  revealed  the 
broad  outlines  of  the  changes  taking  place. 
The  organic  matter  ploughed  into  the  soil — whether  it  be 
dung,  stubble,  or  ley — is  attacked  by  insects,  worms,  moulds, 
fungi,  and  bacteria,  and  changed  into  a number  of  products. 
The  most  prominent  is  humus,  the  black,  structureless  material 
playing  so  important  a part  in  getting  a tilth  that  it  is  often, 
and  not  wholly  incorrectly,  regarded  as  the  chief  source  of 
fertility.  But  the  humus  does  not  account  for  the  whole  of  the 
organic  matter  added  ; part  is  dissipated  as  carbonic  acid  gas 
and  water  vapour,  part  is  changed  into  soluble  substances  that 
appear  in  drainage  water  or  in  shallow  well  water,  while  part 
remains  as  non-humic  material  in  the  soil.  Oxygen  is  wanted 
during  the  process,  and  is  absorbed  in  quantity  from  the  air, 
hence  the  necessity  for  sufficiently  aerating  the  soil. 
Turning  now  from  a consideration  of  the  whole  of  the 
organic  matter  to  one  of  its  constituents  : the  nitrogen  under- 
goes several  transformations,  but  after  a time  appears  in  four 
