Some  Secondary  Actions  of  Manures  upon  the  Soil.  23 
after  rain,  and  dry  with  a hard  glazed  surface  that  marks  off 
the  plots  to  the  eye  from  a considerable  distance.  In  either 
wet  or  dry  weather  the  nitrate  plots  can  at  once  be  dis- 
tinguished on  walking  over  them  by  their  tread  and  feel  to  the 
sole  of  the  foot.  It  is  unnecessary  to  multiply  instances,  as 
the  effect  is  pretty  generally  recognised  ; usually  it  has  been 
explained  as  due  to  the  attraction  of  nitrate  of  soda  for 
moisture.  Nitrate  of  soda  is  always  damp  because  of  its 
fondness  for  water,  and  a bag  of  the  salt  left  standing  in 
an  ordinary  damp  manure  shed  will  sometimes  be  found 
standing  in  a pool  of  liquid,  a solution  formed  by  the  water 
which  the  contents  have  absorbed  from  the  atmosphere.  Such 
an  explanation  of  the  wetness  of  soil  dressed  with  nitrate  of  soda 
is  entirely  inadequate,  because  the  extra  quantity  of  water 
retained  by  the  soil  from  such  a cause  would  be  imperceptible. 
Suppose  as  much  as  a ton  of  nitrate  of  soda  was  applied,  that 
it  absorbed  its  own  weight  of  water,  and  again  remained  wholly 
in  the  surface  layer  of  the  land  nine  inches  deep,  a state 
of  things  which  could  only  last  for  a short  time  before  the 
crop  was  growing  ; then  because  such  a layer  over  one 
acre  weighs  just  about  1,000  tons,  the  water  retained  by 
the  whole  ton  of  nitrate  of  soda  would  not  amount  to  more 
than  one  in  a thousand  of  soil,  and  could  not  cause  the 
slightest  difference  to  the  texture.  Moreover,  determinations 
have  been  made  of  the  water  actually  present  in  the 
Rothamsted  soils  on  the  mangold  plots,  and  no  differences  that 
could  affect  the  behaviour  of  the  soil  have  ever  been  detected. 
The  altered  appearance  and  the  greater  apparent  wetness  must 
therefore  be  due  to  some  other  cause.  Mechanical  analyses 
were  next  made  of  the  soils,  i.e.,  the  soils  were  separated  at 
first  by  sieves,  and  then  by  processes  of  sedimentation  in 
water  into  fractions  containing  particles  successively  finer  and 
finer  in  grade.  The  greater  the  proportion  of  fine  and  very 
fine  particles  a soil  contains  the  heavier  to  work  and  the 
more  retentive  of  water  will  it  be  ; true  clay  soils  possess  a 
considerable  proportion,  up  to  nearly  half  their  weight,  of 
material  scientifically  defined  as  clay  and  composed  of 
particles  so  small  as  to  be  barely  visible  as  units  under  the 
highest  powers  of  the  microscope.  It  seemed  possible  that  the 
greater  stickiness  of  the  nitrated  soils  might  be  due  to  a general 
disintegration  of  the  soil  into  finer  particles  which  w^e  might 
suppose  could  be  brought  about  by  the  long  continued  action 
of  the  fertiliser.  But  it  was  surprising  to  find  that  the  nitrated 
soils  were  distinctly  and  regularly  coarser,  that  is  to  say,  they 
had  been  deprived  of  some  of  their  finer  particles.  Table  III. 
gives  the  average  mechanical  analysis  of  five  pairs  of  plots 
from  the  different  Rothamsted  fields ; in  each  pair  there  was 
