Some  Secondary  Actions  of  Manures  upon  the  Soil.  19 
together  as  humic  acid ; when  attacked  by  a solution  of 
ammonium  sulphate  or  chloride,  calcium  comes  out  into  the 
solution,  while  an  equivalent  amount  of  ammonium  goes 
into  combination  with  the  humic  acid.  Even  when  the 
mixtures  of  humus  and  ammonium  salts  were  repeatedly 
evaporated  to  dryness  in  a current  of  air  or  carbon  dioxide 
to  represent  in  an  exaggerated  way  conditions  which  might 
occur  in  the  soil,  no  production  of  free  acid  took  place. 
In  consequence  of  these  failures  search  was  then  made 
for  some  living  agency  in  the  soil  which  would  set  free 
acid  from  ammonium  salts,  and  small  quantities  of  the  acid 
soils  from  the  Rothamsted  grass  plots  were  introduced  into 
nutrient  solutions  containing  ammonium  salts  and  organic 
matter.  A clue  was  at  once  obtained  to  the  actions  going  on 
in  the  field,  for  the  soils  were  found  to  induce  in  the  nutrient 
medium  a very  rapid  and  abundant  growth  of  moulds  and 
other  minute  fungi,  the  development  of  which  was  accompanied 
by  an  increasing  acidity  in  the  culture  liquid.  The  moulds 
require  nitrogen  for  their  nutrition,  and  in  order  to  obtain  it 
they  split  up  the  ammonium  salts  and  set  free  the  acid.  It 
was  shown  that  the  degree  of  acidity  thus  generated  corre- 
sponded approximately  to  that  prevailing  in  the  water  in  the 
soil  of  the  acid  plots  at  Rothamsted  soon  after  the  application 
of  the  ammonium  salts  in  the  spring,  being  such  as  could 
be  produced  by  the  liberation  of  the  acids  contained  in 
the  manorial  salts.  The  surface  soil  of  these  plots  was 
found  to  be  swarming  with  microscopic  fungi,  and  several 
species  were  separated  and  identified,  all  of  which  would 
attack  ammonium  salts  and  liberate  acids,  though  to  a 
different  degree.  Further  examination  of  the  Rothamsted 
acid  grass  soils  showed  that  in  addition  to  the  small  quantity 
of  acid  which  could  be  extracted  by  water,  the  amount  of 
which  was  greatest  in  the  spring  soon  after  the  application 
of  the  manures  and  diminished  as  the  year  progressed,  there 
was  present  a much  larger  quantity  of  comparatively  insoluble 
free  humic  acid.  Clearly  this  had  arisen  by  the  action  of  the 
mineral  acids,  set  free  year  by  year  from  the  ammonium  salts, 
upon  the  neutral  humus  or  calcium  humate  originally  present 
in  the  soil,  and  the  humic  acid  had  been  able  to  accumulate 
because  it  is  but  slightly  soluble  in  water.  Without  discussing 
the  other  details  bearing  upon  the  question  (see  Hall,  Miller 
and  Gimingham,  Proc.  Roy.  Soc.  1908,  B.  80,  196),  it  became 
pretty  clear  that  the  acidity  of  the  Rothamsted  grass  soils  had 
arisen  from  the  action  of  various  micro-fungi  upon  the 
ammonium  salts  that  had  be'en  annually  applied  to  these  plots  ; 
such  fungi  have  become  very  abundant  in  the  soil  and  are 
able  to  attack  ammonium  salts  and  set  free  the  acid,  taking 
