72 
On  the  Absorbent,  Power  of  Soils. 
that  some  accidental  circumstance  must  have  occurred  to  vitiate 
the  experiment,  and  it  was  repeated  with  every  attention  and  care. 
The  result  is  given  above. 
,My  main  object  in  making  these  experiments  had  now  been 
attained,  and  I had  convinced  myself  that  either  sandy,  clayey, 
or  black  vegetable  soils  possessed  the  power  of  retaining  a much 
larger  amount  of  ammoniacal  salts  than  they  received  in  the  most 
liberal  manuring.* 
It  had  also  been  shown  that  sulphate  of  ammonia  was  quite  as 
easily  retained  by  the  soil  as  the  sesquicarbonate.  A close 
examination,  however,  of  the  foregoing  experiments  will  show  us 
that  great  care  is  requisite  in  their  application  to  practice.  By 
comparing,  for  instance.  Nos.  1 and  3,  we  see  that  though  the 
soil  has  a greater  power  of  retaining  ammonia  than  can,  under 
ordinary  circumstances,  be  required  for  agricultural  purposes, 
still  that  the  extent  to  which  this  power  is  called  into  play  de- 
pends in  great  measure  on  the  mode  in  which  the  ammoniacal 
dressing  is  applied.  In  No.  I,  where  water  was  allowed  to  pass 
once  only  through  a light  sandy  soil,  it  was  found  to  carry  away 
with  it  from  A about  a fourth  part  of  the  sulph.  ainm.,  and 
from  B a considerable  portion  of  the  sesqui-carbonate ; whereas 
in  No.  3,  where  the  water  after  passing  through  the  same  de- 
scription of  soil  was  twice  returned  upon  it,  the  whole  of  the  am- 
moniacal salts  were  retained,  though  the  soil  was  washed  with 
repeated  applications  of  water. 
If  with  these  results  before  us  we  try  to  arrive  at  what  actually 
takes  place  when  manure  is  applied  to  land  of  this  description, 
we  should  have  to  bear  in  mind  that  though  the  quantity  of  am- 
monia per  acre  is  much  less  in  the  manure  than  in  the  experi- 
ments above  described,  yet  that  in  consequence  of  the  practical 
impossibility  of  distributing  manure  uniformly  through  the  culti- 
vated soil,  some  portions  of  the  land  receive  much  more  than  the 
average  dressing,  and  in  each  small  lump  of  farmyard  manure, 
guano,  &c.,  a considerable  amount  of  soluble  salts  will  be  found. 
It  must  also  be  remembered  that  as  manure  is  usually  ploughed 
in  to  the  depth  of  3 or  4 inches  at  least,  the  solution  of  the  above- 
mentioned  salts  when  washed  by  rain  has  a very  limited  depth 
of  cultivated  soil  to  filter  through  before  it  arrives  at  the  subsoil : 
when  there,  it  is  very  problematical  whether  the  roots  of  plants 
would  be  able  to  reach  it,  even  if  the  subsoil  should  contain  the 
requisite  ingredients  for  arresting  its  downward  progress.  It  has 
been  shown  that  returning  the  filtered  water  upon  the  soil  en- 
* The  weight  of  the  column  of  soil  operated  on  (8  inches  deep)  was  2 lbs.,  and  to 
this  10  grains  of  the  carbonate  or  sulphate  of  ammonia  were  added,  which  is  equivalent 
to  an  application  of  18£  cwt.  per  acre. 
