Some  Secondary  Actions  of  Manures  upon  the  Soil.  21 
Rotliamsted  soils,  both  by  testing  for  the  organisms  which 
cause  nitrification  and  by  putting  large  samples  of  the  soil 
under  conditions  favourable  to  nitrification  and  seeing  if  any 
nitrates  were  found.  Small  fragments  of  the  acid  soils  rarely 
showed  the  presence  of  the  organisms,  but  the  bulk  samples  in 
all  cases  but  one  did  gain  some  nitrates  during  the  course  of  the 
experiment,  showing  that  the  process  of  nitrification  was  not 
entirely  suspended.  Extracts  from  the  soil,  however,  refused 
to  form  nitrates  even  when  fresh  active  organisms  were 
introduced.  On  the  whole  the  evidence  points  to  the  fact  that 
a little  nitrification  is  going  on  in  the  soils,  because  a few  tiny 
fragments  of  carbonate  of  lime  exist  here  and  there  and 
maintain  a neutral  condition  in  the  soil  with  which  they  are 
in  immediate  contact.  These  nuclei  serve  to  keep  a limited 
number  of  the  organisms  still  active,  but  in  the  main  the 
process  of  nitrification  is  at  a standstill  and  no  nitrates  are 
being  produced.  Now  the  generally  received  opinion  is  that 
such  normal  plants  as  constitute  our  crops  take  in  their 
nitrogen  only  in  the  form  of  nitrate,  so  that  the  freedom  of 
their  growth  is  entirely  dependent  on  the  rate  at  which  the 
nitrifying  organisms  in  the  soil  first  do  the  work  of  manufac- 
turing nitrates.  This  is  probably  too  hard  and  fast  an  opinion. 
Without  doubt  most  plants  feed  on  nitrates  for  choice  and  soils 
contain  very  much  more  nitrates  than  ammonia,  because  as 
fast  as  the  latter  is  set  free  by  the  bacteria  which  split  up  other 
nitrogenous  compounds  in  the  soil  into  ammonia,  it  is  seized 
upon  by  the  nitrifying  bacteria  and  converted  into  nitrates. 
But^  there  are  not  wanting  experiments  to  show  that  many 
plants,  especially  cereals,  are  capable  of  utilising  the  nitrogen 
of  ammonium  compounds  directly,  and  later  experimenters 
(Hutchinson  and  Miller,  J.  Agric.  Sci.,  1909,  3,  179)  have 
succeeded  in  growing  plants  with  ammonium  salts  as  the 
sole  source  of  nitrogen  under  absolutely  sterile  conditions, 
excluding  all  bacteria  which  could  change  the  ammonia  into 
other  compounds.  Most  plants,  however,  prefer  nitrates  to 
ammonia  as  their  source  of  nitrogen,  and  the  reduced  yield  on 
the  acid  soils  may  be  partly  due  to  the  fact  that  the  grass  is 
driven  to  feed  on  ammonium  compounds  instead  of  the  more 
usual  nitrates.  The  cause  of  the  infertility  of  the  acid  soils 
cannot,  however,  be  regarded  as  completely  established  ; the 
absence  of  nitrification  and  the  competition  of  the  fungi  for 
the  manures  are  doubtless  factors  in  the  result,  but  they  would 
hardly  seem  to  account  for  the  total  failure  of  barley  on  the 
Woburn  plots.  It  was  surmised  that  the  fungi  might  produce 
substances  poisonous  to  the  growth  of  the  higher  plants,  but 
experiments  to  test  this  point  have  so  far  yielded  a negative 
result. 
