Some  Secondary  Actions  of  Manures  upon  the  Soil.  33 
heavy  land  which  had  not  long  before  been  limed,  so  destroyed 
the  texture  of  the  soil  that  the  ploughman  knew  at  once  when 
he  entered  upon  the  plot  in  question  because  of  the  heavier 
draught  of  the  plough.  Accepting  then  deflocculation  as  an 
explanation  of  the  injurious  effects  of  potash  salts  upon  clay 
soils  the  question  that  remains  is  the  origin  of  the  alkali,  for 
alkali  it  must  be  that  has  brought  about  the  deflocculation.  The 
first  investigation  of  the  action  of  potash  salts  upon  soil  was 
carried  out  by  the  late  Dr.  A.  Voelcker  and  the  results  were  pub- 
lished in  this  Journal  in  1864  ; he  showed  that  an  interchange 
of  bases  takes  place  similar  to  that  which  takes  place  between 
ammonium  salts  and  the  zeolites  of  the  clay,  potash  goes  into 
the  zeolite  and  becomes  insoluble,  and  an  equivalent  amount  of 
lime,  magnesia,  and  soda  leaves  the  zeolite  to  combine  with  the 
acid  of  the  potash  salt.  These  exchanges  would  count  for 
nothing  in  the  problem,  because  no  substance  is  formed  which 
would  interfere  with  the  character  of  the  soil. 
The  carbonate  of  lime  in  the  soil  next  suggested  itself  as  a 
possible  reacting  substance,  and  a series  of  experiments  have 
showed  that  when  weak  solutions  of  potash  salts  remain  in 
contact  with  carbonate  of  lime  a small  quantity  of  free 
carbonate  of  potash  is  produced.  The  figures  showing  the 
extent  of  the  reaction  have  not  as  yet  been  published  but  the 
action  is  one  of  those  cases  where  the  amount  of  chemical 
change  that  is  set  up  depends  upon  the  relative  quanti- 
ties of  the  reacting  bodies,  so  that  in  the  soil,  where  the 
carbonate  of  lime  would  generally  be  in  great  excess,  the 
proportion  of  the  potash  salts  that  could  be  converted  into 
carbonate  would  be  comparatively  large.  The  investigation 
has  not  yet  been  completed,  because  it  involves  some  further 
considerations  of  the  part  played  by  carbon  dioxide  which 
is  also  present  in  the  soil  and  also  of  the  results  of  partial 
washing  of  the  soil  as  by  rain,  but  the  central  fact  has 
been  established  that  soluble  potash  salts  and  carbonate  of 
lime  will  react  so  as  to  produce  carbonate  of  potash,  which 
like  other  alkalis  will  bring  about  deflocculation  of  the  clay. 
Moreover  common  salt  has  exactly  a similar  action,  and  this  at 
once  provides  us  with  an  explanation  of  the  many  unintel- 
ligible and  often  contradictory  reports  of  the  action  of  salt  as  a 
fertiliser.  It  has  variously  been  reported  as  enabling  the  soil 
to  retain  more  moisture,  as  injuring  the  soil,  especially  when 
the  land  has  been  flooded  with  sea  water,  as  sometimes  increas- 
ing but  occasionally  as  diminishing  the  crop.  These  reports 
coincide  with  those  concerning  the  action  of  potash,  and  the 
effect  in  both  cases  may  be  set  down  to  the  deflocculation 
brought  about  by  the  small  trace  of  carbonate  of  soda  or  potash 
that  is  formed  by  the  action  of  the  carbonate  of  lime  in  the 
soil  upon  the  soluble  potash  or  soda  salt. 
VOL.  70. 
D 
