22  'The  Production  of  Plant  Food  in  the  Soil. 
is  now  readily  explicable ; we  may  suppose  that  the 
moisture,  the  warmth,  and  the  soluble  organic  matter 
in  the  greenhouse  soil  are  favourable  to  the  harmful 
organisms  e.g.  the  protozoa,  which,  after  a time,  become  so 
numerous  that  they  prevent  the  bacteria  from  making  plant 
food  sufficiently  rapidly  for  the  grower’s  purpose.  In  order 
a b 
Fia.  3— Buckwheat  growing  on  poor  soil  without  manure-  a.  soil  heated  to  212°  F. : b,  untreated 
(Russell  & .Darbishire,  Journal  of  Agricultural  Science,  Vol.  2,  page  305.) 
to  kill  the  insect  pests  the  practice  arose  in  America  of  heating 
the  soil  by  blowing  in  steam,  or  by  other  means  ; before  long 
it  was  observed  that  the  crop-producing  power  had  been 
increased.1  This  practice  has  been  introduced  into  England 
1 See,  for  instance,  Stone  and  Smith,  Massachusetts  Experiment  Station 
Report,  1898  and  1903. 
