18  Some  Secondary  Actions  of  Manures  upon  the  Soil. 
a soil  becoming  acid  through  a particular  course  of  treatment. 
That  the  acidity  had  developed  upon  the  Wobuim  plots  and  not 
at  Rotbamsted  where  the  same  manuring  had  been  continued 
for  even  longer  periods,  was  obviously  due  to  the  fact  that  the 
soil  at  Woburn  contained  practically  no  carbonate  of  lime  at 
the  beginning  of  the  experiments.  Analyses  show  less  than 
one-tenth  per  cent.,  and  even  this  minute  proportion  may  be 
largely  over-estimated,  because  there  is  evidence  that  the 
organic  matter  in  these  acid  soils  always  splits  up  on  treat- 
ment with  acid  and  yields  some  carbon  dioxide  which  would 
be  reckoned  as  carbonate  of  lime  in  the  process  of  analysis 
adopted.  Thus  in  the  Woburn  soil  there  is  no  base  present 
to  maintain  the  neutrality  should  any  agency  arise  to  produce 
acid,  whereas  in  the  Rothamsted  arable  soils,  as  we  have  seen, 
there  has  always  been  sufficient  carbonate  of  lime  to  keep  up 
a neutral  condition  and  put  out  of  action  any  acid  as  fast  as  it 
was  produced.  However,  it  was  observed  later  that  one  of  the 
Rothamsted  fields  did  contain  plots  on  which  the  soils  had 
become  acid  through  the  application  of  ammonium  salts  year 
after  year  for  a long  period  ; this  was  the  Park  grass  field 
which  is  cut  for  hay  every  year.  Now  there  is  no  record  of 
the  Park  having  ever  carried  anything  but  grass,  and  analyses 
of  the  soil  at  the  margins  of  the  plots  where  no  experimental 
treatment  had  been  given  showed  that  this  was  one  of  the 
pieces  of  land  which  had  never  received  the  regular  chalkings 
to  which  allusion  has  been  made  earlier.  The  soil,  therefore, 
of  the  grass  plots  had  started  with  but  a small  proportion  of 
carbonate  of  lime,  an  amount  comparable  with  that  present  in 
the  Wobnrn  soil  at  the  outset  of  the  experiments  there,  and  the 
acidity  has  developed  itself  on  this  soil  just  as  it  has  at 
Woburn. 
It  is  not  clear  at  first  sight  how  free  acid  can  arise  by  the 
interaction  of  ammonium  salts  with  any  of  the  constituents 
of  the  soil  ; ammonium  sulphate  and  chloride  are  both 
neutral  salts  in  which  acid  and  base  are  firmly  combined  at 
ordinary  temperatures.  Certain  physical  and  chemical  possi- 
bilities had  been  suggested,  and  these  were  first  examined  in 
some  detail  (see  Hall  and  Gimingham  loc.  cit.),  rising  clay, 
sand,  humus,  and  other  soil  constituents  separately,  but  with- 
out detecting  any  process  which  would  give  rise  to  free  acid. 
On  sand  the  ammonium  salts  had  no  action  ; with  clay  an 
interchange  of  bases  between  the  salt  and  the  zeolites  took 
place  as  already  described,  bnt  the  resulting  liquid  remained 
perfectly  neutral,  as  would  indeed  be  expected  on  purely 
chemical  grounds  ; with  humus  a similar  interchange  took 
place,  also  giving  rise  to  no  free  acid.  The  humus  of  normal 
soils  consists  of  calcium  salts  of  the  indefinite  acids  grouped 
