On  the  Absorbent  Power  of  Soils. 
73 
tirely  deprived  it  of  the  salts  which  were  only  partially  retained 
by  the  first  six  inches  of  soil  it  passed  through.  It  is  therefore 
fair  to  conclude  that  if  the  depth  of  soil  in  the  percolator  had 
been  sufficiently  great,  the  whole  of  the  ammonia  would  have 
been  retained  at  the  first  operation.  Hence  we  see  the  importance 
of  increasing  the  depth  of  cultivated  soil  through  which  the 
soluble  parts  of  manure  have  to  filter  before  escaping  to  the  sub- 
soil. This  does  not  apply  with  equal  force  to  clay  soils,  both  from 
the  much  greater  slowness  with  which  any  liquid  passes  through 
them,  and  from  their  greater  apparent  power  of  retaining  mineral 
matters.  In  Experiment  4 scarcely  any  mineral  substance  was 
obtained  from  the  water,  though  the  clay  through  which  it  had 
filtered  was  highlv  charged  with  ammoniacal  salts.  This  soil, 
from  not  having  been  limed  for  a long  period,  was  expected  to  be 
deficient  in  calcareous  matter,  but  on  examination  lime  was 
found  to  be  present  in  tolerable  quantity.  The  practical  in- 
ferences that  l drew  from  this  inquiry  were  as  follows  : — 
1.  That  clay  soils  might  be  manured  a considerable  time  be- 
fore sowing  without  loss. 
2.  That  light  shallow  soils  should  not  be  manured  long  before 
sowing;  should  not  be  heavily  manured  at  one  time;  and  the 
manure  should  be  kept  as  near  the  surface  as  practicable  without 
leaving  it  uncovered. 
3.  That  it  is  desirable  to  deepen  the  cultivated  soil  of  all 
light  land,  as  it  thus  gives  it  a greater  power  of  holding  ma- 
nure. 
The  power  of  the  cultivated  soil  to  decompose  and  retain  salts 
of  ammonia  has  here  been  pointed  out : no  clue,  however,  has  been 
furnished  to  the  mode  in  which  it  is  accomplished.  It  seemed 
possible  that  it  might  be  due  to  the  carbonic  acid  which  is  taken  up 
by  water  when  passing  through  cultivated  soil, and  which  forming 
and  carrying  with  it  bicarbonate  of  lime  might,  when  coming  in 
contact  with  sulphate  of  ammonia,  decompose  it,  forming  sulphate 
of  lime,  which  filtered  through  with  the  water,  and  carbonate  of 
ammonia,  which  all  soil  has  apparently  the  power  of  absorbing  and 
tenaciously  retaining.  It  is  true  that  this  is  supposing  a reverse 
action  to  take  place  from  that  which  is  observable  when  sulphate 
of  lime  meets  carbonate  of  ammonia  in  solution  (as  in  liquid 
manure  tanks,  where  gypsum  is  used)  and  forms  sulphate  of  am- 
monia and  carbonate  of  lime;  but  unexpected  combinations  so 
frequently  occur  in  chemistry,  especially  where,  as  in  the  soil, 
other  substances  are  present,  that  it  seemed  desirable  to  try  the 
experiment,  and  accordingly  a portion  of  the  soil  used  in  Ex- 
periments 1 and  2 was  deprived  of  its  carbonic  acid  by  being 
spread  out  in  a thin  layer  and  dried  at  a temperature  of  200J. 
The  percolators  were  filled  to  the  depth  of  8 inches  with  the 
