390  The  Woburn  Pot- Culture  Experiments,  1908. 
of  a non-leguminous  crop  like  mustard  than  that  obtained  by 
ploughing  in  a leguminous  crop  like  tares.  Observations  of 
the  field  experiments  had  shown  that  the  tare  soil  was  rendered 
darker  in  colour  but  looser  in  texture  than  the  mustard  soil, 
but  tha+,  though  richer  in  organic  matter  and  nitrogen,  these 
were,  for  some  reason,  not  rendered  available  for  the  use  of  the 
succeeding  corn  crop.  Pot-culture  experiments  in  1907  had 
further  shown  that  the  question  of  water  supply  to  the  crops 
was  a factor  in  the  case,  for,  when  the  tare  soil  was  liberally 
supplied  with  water,  and  not  left,  as  in  the  field,  to  depend 
on  the  rainfall  alone,  then  a better  corn  crop  was  obtained 
after  the  ploughing-in  of  tares  than  of  mustard. 
These  considerations  led  to  the  pot-culture  experiments  of 
1908  taking  the  form  of  seeing  whether  the  addition  of  colloidal 
substances  to  the  soil  would  have  the  effect  of  altering  its 
physical  condition  and  of  enabling  it  to  retain  moisture  better. 
With  this  object  soil  was  taken  direct  from  the  plots  in  Lansome 
Field  on  which  tares  and  mustard  respectively  had  been  grown 
and  ploughed-in  green  in  1907.  Silicate  of  alumina,  silicate 
of  soda,  and  kaolin  were  respectively  added  to  the  soil  in 
quantity  such  that  100  parts  of  the  soil  contained  0’25  parts  of 
each  added  material.  Each  experiment  was  in  duplicate,  and 
each  pot  held  34  lb.  of  soil,  the  materials  being  thoroughly 
incorporated  with  the  whole  of  the  soil.  In  addition  to  the 
use  of  the  colloidal  substances  named,  further  trials  were  made 
with  ground  lime  and  with  magnesia  (MgO),  for  the  purpose 
of  seeing  whether  these  substances  would  show  any  difference, 
on  the  two  kinds  of  soil,  as  regards  their  power  of  breaking 
down  the  organic  matter  and  rendering  the  nitrogen  available 
for  the  use  of  the  corn  crop.  The  quantities  of  these  used 
were  the  same  as  in  the  other  cases,  viz.,  0‘25  per  cent,  on  the 
soil. 
Wheat  was  the  crop  grown,  and  the  principal  differences 
during  the  period  of  growth  were  carefully  noted.  With 
silicate  of  alumina  the  results  were  of  a most  striking  nature  ; 
the  crops  on  both  the  tare  and  the  mustard  soil  assumed  a 
much  darker  colour  than  in  any  other  case,  indicating,  seem- 
ingly, that  nitrogen  had  been  rendered  available.  They  were 
far  in  advance  of  the  others  throughout,  this  being  especially 
the  case  with  the  tare  soil.  Silicate  of  soda  gave  a much 
smaller  increase,  more  marked  with  the  tare  soil,  but  in  the 
case  of  the  mustard  soil  the  surface  seemed  to  “ set  ” hard,  and 
this  prevented  free  growth.  Kaolin  had  in  each  case  an 
indifferent  result.  The  influence  of  lime  was  strikingly  shown 
also,  though  not  as  decidedly  as  with  silicate  of  alumina,  and 
it  seemed  that  here,  too,  the  organic  matter  had  been  broken 
down  and  rendered  available.  The  effect  was  more  marked. 
