Some  Secondary  Actions  of  Manures  upon  the  Soil.  31 
the-  consideration  of  some  of  the  Rothamsted  plots.  Now  the 
ammonium  sulphate  mixed  with  the  nitrate  of  soda  would 
be  to  some  extent  temporarily  withdrawn  from  solution  by 
the  soil,  so  that  an  application  of  a mixture  of  it  with  nitrate  of 
soda  would  result  in  a less  concentrated  soil  solution  than 
would  be  set  up  by  an  equivalent  amount  of  nitrate  of  soda 
alone.  Yet  the  sulphate  of  ammonia  would  begin  to  nitrify 
very  rapidly,  and  would  thus  provide  food,  for  the  plant  as  the 
nitrate  of  soda  was  beginning  to  get  exhausted.  There  is  also 
some  evidence,  though  so  far  it  depends  on  conclusions  drawn 
from  experiments  on  a pot  scale  only,  that  plants  are  physio- 
logically better  served  by  a mixture  of  nitrate  and  ammonium 
salts  than  by  either  alone.  However  that  may  be,  the  practical 
points  are  that  no  inconveniences  arise  from  making  such 
a mixture,  that  it  is  just  as  effective  and  active  as  the  nitrate 
alone,  and  that  it  sets  up  no  injurious  action,  either  in  the 
direction  of  acidity  or  alkalinity  in  the  soil,  so  that  large 
amounts  can  be  used  without  detriment  either  to  the  tilth  or 
the  health  of  the  soil.  There  can  be  little  doubt  but  that  the 
discredit  which  the  practical  farmer  sometimes  attaches  to 
nitrate  of  soda  as  a stimulant  exhausting  the  soil,  even  as 
a “ scourge  ” as'  it  has  been  called,  is  due  to  its  effect  upon  the 
tilth.  Although  nitrate  of  soda,  when  used  alone,  is  a 
one-sided  manure  that  will  greatly  aid  the  plant  to  remove  the 
available  phosphoric  acid  and  potash  from  the  soil,  it  still 
supplies  the  most  important  element  of  fertility,  and  cannot 
exhaust  the  soil  in  any  real  sense.  At  Rothamsted,  where 
nitrate  of  soda  among  other  single  manures  has  been  used  on 
the  same  plots  for  over  fifty  years,  the  astonishing  thing  is 
the  way  some  sort  of  a yield  is  maintained.  For  example,  the 
average  yield  of  mangolds  from  the  plot  receiving  nitrate 
of  soda  alone  for  twenty-seven  years  has  been  I0j[^  tons  of 
roots  per  acre  against  10  with  rape  cake  alone  ; again,  with 
nitrate  of  soda  alone  the  average  yield  of  barley  for  fifty-one 
years  has  been  as  much  as  30'4  bushels  per  acre.  The 
practical  man,  however,  uses  the  word  exhausted  not  in  its 
strict  sense,  but  as  signifying  any  condition  of  the  soil  which 
lowers  its  crop-producing  capacity  ; for  example,  wheat  is 
spoken  of  as  an  exhausting  crop,  although  it  takes  out  of  the 
land  only  about  a third  of  the  plant  food  that  is  removed  by 
a crop  of  roots.  But  because  wheat  occupies  the  land  for  the 
greater  part  of  the  year,  during  which  time  no  intertillage  can 
be  carried  on,  the  soil  loses  its  texture  and  falls  off  in  its 
mechanical  condition.  The  lack  of  cultivation  may  also  inter- 
fere with  the  availability  of  the  stores  of  plant  food  both  from 
biological  and  physical  causes,  at  any  rate  the  land  after  a wheat 
crop  is  less  productive  because  of  its  lack  of  condition  and  not 
