356  Nitrogen  as  Nitric  Acid , in  the  Soils 
The  facts  shown  by  the  other  columns  of  Table  VII.  are 
nearly  related  to  those  just  mentioned.  When  we  see  that  the 
average  of  30  years’  produce  of  corn  and  straw  on  the  un- 
manured plots  stands  to  the  amount  of  nitrogen  oxidised  in 
this  soil  to  nitric  acid  as  1000  : 7-6  ; that  on  Plot  6a,  with  nearly 
double  the  produce,  it  stands  as  1000  : 7’3  ; and  on  Plot  7a, 
with  more  than  two  and  a half  times  the  produce,  as  1000  : 7*0, 
it  is  surely  evident  that  the  capacity  for  producing  nitrates  rises 
side  by  side  with  the  increase  of  crop ; that,  in  fact,  the  crop- 
residue  annually  left  in  the  soil  constitutes  in  most  cases  the 
chief  material  out  of  which  nitrates  are  produced.*  Recent  crop- 
residues  are  not,  however,  in  any  case  the  sole  source  of  soil 
nitrates  ; if  this  were  so,  it  is  clear  that  the  figures  given  in  the 
Table  would  be  in  all  cases  identical,  while  in  fact  the  pro- 
portion of  nitric  acid  produced  per  1000  of  crop  decreases  some- 
what with  each  additional  increment  of  crop.  This  is,  however, 
what  must  naturally  be  expected.  The  nitric  acid  which  is 
produced  is  derived  partly  from  the  old  nitrogenous  capital  of 
the  soil,  and  partly  from  comparatively  recent  crop-residues  ; in 
the  case  of  the  unmanured  plots  the  influence  of  the  old  nitro- 
genous capital  of  the  soil  is  shown  to  the  largest  extent,  while  on 
the  plots  of  heaviest  produce  (9a  and  8a)  it  has  least  influence 
on  the  result.  On  Plots  2 and  9b  we  need  hardly  remark  that 
the  manure  furnishes  a considerable  source  of  nitrate  over  and 
above  the  crop-residue. 
Before  leaving  the  results  furnished  by  Broadbalk  Field,  we 
must  look  for  a moment  at  the  general  relation  of  the  nitrate  con- 
tained in  the  subsequent  drainage  to  that  shown  by  analysis  as 
existing  in  the  soil.  Taking  an  average  of  all  the  plots  in  which 
the  composition  of  the  subsequent  drainage  admits  of  estimation, 
we  find  that  for  100  lbs.  of  nitrate  existing  in  the  soil  in  Octo- 
ber, 60*2  lbs.  have  been  removed  in  the  course  of  3^  months’ 
drainage  (8*2  inches),  or  have  passed  below  the  level  of  the 
drain-pipes.  The  amounts  of  nitrates  passing  into  the  subsoil 
will  really  be  in  excess  of  these  estimates,  but  they  are  the  only 
figures  at  command. 
VVe  may  now  pass  to  the  determinations  of  nitric  acid  made 
in  the  soil  of  various  plots  in  the  Experimental  Barley-field. 
The  manuring  of  the  plots  with  which  we  are  at  present 
concerned,  and  their  average  produce  during  30  years,  1852-81, 
have  been  already  given  in  Table  I.  (p.  335).  The  crop  of  1881 
was  cut  on  August  5 and  8,  and  carried  on  August  18.  The  land 
was  then  scarified,  and  afterwards  ploughed  in  the  beginning 
* The  vegetable  matter  resulting  from  dead  weeds  will  participate  with  the 
residue  of  the  crop  iu  affording  material  for  nitrification  ; this  residue  of  wocds 
must  always  be  included  in  the  idea  of  crop-residue. 
