and  Subsoils  of  some  of  the  Fields  at  Rothamsted.  353 
of  deciding.  Plot  7 should,  it  is  true,  have  a somewhat  greater 
accumulation  of  crop-residue,  and  should  therefore  yield  rather 
more  nitrates  from  that  source.  But  only  a small  proportion 
of  the  excess  of  nitrogen  as  nitrates,  which  the  figures  as  they 
stand  show  in  the  soil  of  Plot  7 to  the  depth  of  27  inches,  could 
be  so  accounted  for.  It  is  very  probable,  indeed,  that  a portion 
of  the  exdess  of  nitrates  found  in  the  soil  of  Plot  7 is  due  to 
less  free  passage  downwards,  from  whatever  cause. 
In  addition  to  the  various  conditions  above  alluded  to  affecting 
the  amount  of  nitrates  produced,  and  their  tendency  to  remain 
at  higher,  or  to  pass  to  lower  levels  in  the  soil,  the  very  different 
condition  as  to  moisture  of  the  different  plots  after  harvest  must 
not  be  overlooked.  Thus,  it  is  estimated  that  more  water,  equal 
in  amount  to  some  inches  of  rain,  has  been  evaporated  from  the 
soils  yielding  the  heavier  than  from  those  yielding  the  lighter 
crops.  This  implies,  of  course,  that  much  less  subsequent  rain 
will  be  required  to  bring  the  soil  to  an  equal  degree  of  saturation, 
and  so  far  as  it  is  dependent  on  this,  to  cause  an  equal  passage 
downwards,  in  the  case  of  soils  giving  the  poorer  than  of  those 
giving  the  heavier  crops.  Then  there  is  also  the  possibility  of 
a different  mechanical  condition  of  the  soils  of  different  plots. 
On  Plot  19,  manured  during  the  three  preceding  seasons  with 
rape-cake  only,  we  find  34-2  lbs.  of  nitrogen  as  nitrates,  a 
quantity  considerably  larger  than  that  contained  in  the  adjoining 
plots  manured  with  ammonium-salts.  As  the  rape-cake  only 
slowly  decomposes  in  the  soil,  a part  of  the  nitrates  found  will 
in  this  case  be  due  to  the  nitrification  of  a residue  of  the 
manure.  A still  more  striking  example  of  the  production  of 
nitrates  from  organic  manures  is  afforded  by  Plot  2,  which 
receives  annually  fourteen  tons  of  farmyard-manure  : here  the 
quantity  of  nitrate  in  the  soil  amounts  to  52*4  lbs.  of  nitrogen 
per  acre,  exceeding,  in  fact,  every  plot  excepting  that  manured 
with  an  excess  of  nitrate  of  sodium. 
The  origin  of  the  very  considerable  quantities  of  nitrates 
found  in  the  soils  of  Broadbalk  Field  will  require  but  little 
explanation  after  what  has  been  already  stated.  The  nitrates 
found  in  these  soils  in  October  have  been  produced  by  the 
nitrification  of  the  original  nitrogenous  humic  matters  of  the 
soil  ; or  from  the  decay  of  crop-residues  (roots,  leaves,  and 
stubble)  ; or  from  the  nitrification  of  organic  nitrogenous 
manures,  as  rape-cake  or  farmyard-manure  ; or,  possibly  from 
the  oxidation  of  residues  of  ammoniacal  manure  not  previously 
nitrified  ; or  they  consist  in  part  of  residues  of  nitrates  which 
have  remained  in  the  soil  through  the  summer.  Of  the  quantity 
of  nitrates  produced  solely  from  the  oxidation  of  nitrogenous 
humic  matters,  including  crop-residue,  we  have  examples  in  the 
