344  Nitrogen  as  Nitric  Acid  in  the  Soils 
corn,  being  left  behind  in  the  soil.  We  must  also  take  into 
account  the  presence  of  weeds,  which  doubtless  actively  assimi- 
late the  nitrates  of  the  soil.  It  is  possible,  however,  that  when 
ammonium-salts  are  applied  as  a top-dressing  in  spring  the 
whole  of  the  ammonia  does  not  rapidly  change  into  nitrates, 
either  from  actual  loss  as  ammonia,  or  from  the  presence 
of  conditions  unfavourable  to  rapid  nitrification.  It  will  be 
recollected  that  the  calculations  given  in  the  previous  Report 
(Table  LI.)  showed  a considerable  annual  deficiency  of  nitrogen 
where  400  lbs.  of  ammonium-salts  were  applied  in  the  spring,  but 
none  on  Plot  15,  where  the  salts  were  ploughed-in  in  the  autumn. 
To  return  from  this  necessary  digression.  The  facts  stated 
above  point  to  the  conclusion  that  in  the  case  of  land  growing 
cereal  crops,  no  nitrates,  or  very  small  quantities  only,  will  be 
found  in  the  first  27  inches  of  soil  by  the  end  of  June,  excepting 
in  those  cases  where  the  quantity  of  nitrates  in  the  soil  has 
been  greater  than  the  crop  has  had  power  to  appropriate. 
The  assimilation  of  nitrogen  by  a cereal  crop  practically 
ceases  when  blossoming  is  completed  ; from  this  point  onward, 
therefore,  nitrates  may  begin  again  to  accumulate  in  the  soil. 
If  sufficient  rain  falls  to  keep  the  surface  moist,  we  may  expect 
nitrates  to  appear  in  the  soil  towards  the  end  of  July,  or  in 
August,  and  the  soil  at  harvest  may  contain  nitrates  which  a 
month  previously  had  no  existence.  After  harvest,  if  sufficient 
rain  occurs,  and  especially  if  the  land  is  ploughed,  nitrification 
will  actively  continue.  We  may  suppose  that  generally  the 
maximum  contents  of  nitrates  will  be  reached  about  the  end  of 
September,  and  that  from  this  time  the  quantity  in  the  upper 
layers  of  the  soil  will  in  most  cases  tend  to  diminish,  production 
being  checked  by  the  fall  in  temperature,  while  the  removal  of 
nitrates  by  drainage  will  in  most  seasons  then  become  active. 
The  foregoing  sketch  of  the  course  of  change  of  the  nitrates 
in  a soil  cropped  with  wheat  or  barley  is  founded  mainly  on  the 
evidence  furnished  by  the  analyses  of  the  drainage-waters.  We 
have  now  to  consider  the  evidence  afforded  by  the  analysis  of 
the  soils. 
In  Table  III.  (No.  10)  will  be  found  a determination  of 
nitrates  in  the  soil  of  a poor  wheat-stubble  in  Hoos  Field.  The 
wheat  had  been  grown,  in  alternation  with  fallow,  for  many 
years  without  manure.  The  crop  was  cut  on  Aug.  13,  1878  ; it 
yielded  19f  bushels  of  dressed  corn.  The  soil  was  sampled  on 
Sept.  28.  The  first  18  inches  of  soil  contained  only  2‘6  lbs.  of 
nitrogen  as  nitrates.  We  have  in  the  same  Table  (No.  11)  a 
second  determination  of  nitrates  in  the  same  soil,  made  in 
March  1881.  The  wheat-stubble  of  the  previous  year  had  been 
left  unploughed  up  to  the  time  of  sampling.  The  nitrogen  as 
