Some  Secondary  Actions  of  Manures  upon  the  Soil.  15 
the  soils  taken  from  the  Rothamsted  plots  at  successive  dates 
during  their  history.  At  Rothamsted  the  carbonate  of  lime 
in  the  soil,  the  amount  of  which  varies  from  about  2 to  about 
5 per  cent,  in  different  fields,  is  all  of  artificial  origin  ; for 
though  the  “ red  clay  with  flints,”  the  drift  formation  out  of 
which  has  arisen  the  soil  at  Rothamsted,  rests  on  the  solid 
chalk  rock  at  a depth  of  10  to  15  ft.  below,  yet  both  soil  and 
subsoil  in  a natural  state  are  almost  wholly  lacking  in 
carbonate  of  lime.  Such  natural  soil  may  be  found  on  the 
neighbouring  Harpenden  Common,  which  has  never  been 
cultivated  nor  subject  to  any  improvements  ; and  again  on  the 
grass  land  and  a few  of  the  other  arable  fields  on  the  estate  ; 
in  all  these  cases  analysis  shows  only  about  one  tenth  per  cent, 
of  carbonate  of  lime  in  either  soil  or  subsoil  until  the  under- 
lying chalk  rock  is  reached.  It  is,  however,  on  record  that  up 
to  the  early  years  of  the  nineteenth  century  it  was  a regular 
custom  in  Hertfordshire  farming  on  this  hill  land  to  sink  pits 
through  the  clay  into  the  chalk,  extract  the  chalk  and  spread 
a layer  representing  six  to  ten  tons  per  acre  over  the  arable 
land,  the  process  being  repeated  at  intervals  of  a few  years. 
The  “ dells  ” or  hollows  in  the  fields,  which  represent  the 
fallen-in  pits,  are  evidence  to-day  of  the  old  practice,  and 
much  of  the  friability  and  dryness  of  this  heavy  land,  through 
which  alone  it  has  been  possible  to  keep  it  under  the  plough, 
is  due  to  the  work  thus  done  for  the  present  generation  of 
farmers  during  the  seventeenth  and  eighteenth  centuries  or 
even  earlier.  As  a consequence  the  surface  soil  of  the 
Rothamsted  arable  fields  now  contains  2 per  cent,  or  more  of 
carbonate  of  lime,  visible  to  some  extent  as  tiny  rounded 
fragments  from  the  size  of  a pinhead  to  that  of  a pea,  but 
mainly  present  in  particles  too  small  to  be  seen  ; the  sub- 
soil, however,  contains  none  of  this  carbonate  of  lime,  it  has 
remained  only  in  the  layer  stirred  by  the  plough,  and  has 
never  worked  downwards.  The  old  maxim  that  lime  sinks  in 
the  soil  is  only  true  of  lime  on  pastures,  where  it  is  buried  by 
the  slow  but  persistent  action  of  earthworms  bringing  up 
mould  to  the  surface ; in  arable  land  as  the  Rothamsted  analyses 
prove  the  lime  wastes  but  does  not  sink.  The  very  special 
distribution  of  carbonate  of  lime  in  the  Rothamsted  soil  affords, 
however,  an  exceptional  opportunity  of  studying  the  rate  at 
which  this  important  soil  constituent  is  removed  by  natural 
causes,  and  also  of  how  the  natural  removal  is  decreased  or 
accelerated  by  the  constant  use  of  certain  artificial  manures. 
Samples  of  soil  from  the  Rothamsted  plots  were  not  taken  at 
the  very  beginning  of  the  experiments,  at  least  none  have  been 
preserved  ; the  earliest  which  are  available  date  from  1856, 
and  later  samples,  from  the  same  Broadbalk  field  were  taken 
