Fi, ration  of  Free  JS’itro(jen. 
115 
The  phosphoric  acid  soluble  iii  citric  acid  shows  uo  great 
change  except  in  one  instance,  so  we  may  conclude  that  the 
mere  exposure  to  weather  does  not  itself  alter  the  condition 
of  the  phosphoric  acid  reserves  in  the  subsoil.  But  with 
potash  the  difference  is  great ; after  weathering,  the  citric  acid 
soluble  potash  is  increased,  sometimes  two  or  three  fold,  the 
soil  originally  poor  in  available  potash  being,  after  weathering, 
as  rich  in  this  constituent  as  the  best  arable  soils.  The  average 
of  twenty  determinations  on  these  and  other  soils  shows  a rise 
in  the  citric  acid  soluble  potash  from  ()‘0113  per  cent,  before 
weathering  to  0'018b  per  cent,  after  exposure.  While  it  has 
always  been  contended  that  the  well-known  practical  advantage 
gained  by  laying  up  land  roughly  for  the  winter  is  due  to 
the  bringing  into  solution  of  the  mineral  reserves  of  the  soil,  in 
Mr.  Mason’s  exj)eriments  this  exjierience  was  for  the  first  time 
reduced  to  scientific  form,  and  shown  to  be  due  to  the  passing 
over  into  a more  soluble  form  of  the  jiotash  compounds  and  of 
those  only. 
The  Field  Exderiments. 
It  has  already  been  stated  that  the  underlying  ideas  in 
Mr.  Mason’s  woi’k  were  the  utilisation  on  the  one  hand  of 
the  subsoil  mineral  reserves  of  the  soil,  and  on  the  other  of 
the  power  of  leguminous  plants  to  gather  nitrogen  from  the 
atmosphere.  Before  his  eyes  were  opened  to  the  potentialities 
of  the  latter  action  by  Hellriegel  and  Wilfarth’s  }iaper,  and  by 
the  continuous  discussion  that  ensued  with  Lawes  and  Gilbert, 
he  had  been  ex[)erimenting  on  deep  cultivation,  accompanied 
by  wide  drilling  and  thin  seeding,  without,  however,  obtaining 
any  results  of  economic  importance.  Other  field  ex])eriments 
dealt  with  the  advantages  to  be  derived  from  sowing  nitrate  of 
soda  in  successive  small  doses,  and  with  the  value,  or  otherwise, 
of  bringing  a little  subsoil  to  the  surface  at  each  ploughing. 
Dee{)  working  of  the  subsoil  was  expensive,  but  brought  almost 
immediately  a i-eturn  in  the  increased  and  earlier  dryness  of  the 
land  after  rain,  so  that  more  days  in  the  year  were  available  for 
cultivation.  The  possibilities  of  nitrogen  fixation,  however, 
opened  up  a new  [)ath,  and  Mr.  Mason  pre}>ared  to  alter  his 
scheme  of  farming  to  utilise  it  to  the  full.  The  first  plan  of 
• croj)ping  thus  devised  was  described  in  the  Agricultural  Gazette 
for  1891  (Vol.  34,  page  445),  fitnn  the  article  in  which  Table 
II.  is  transcribed. 
