20  The  Production  of  Plant  Food  in  the  Soil. 
The  bacteria,  however,  have  the  great  advantage  that  the  soil 
conditions  are,  on  the  whole,  more  favourable  to  them  than 
to  their  enemies,  and,  as  they  have  a remarkable  power  of 
multiplying,  they  can  keep  their  numbers  up.  Beyond  a 
certain  limit,  however,  they  cannot  go  because  of  the  harmful 
organisms  ; we  can  thus  explain  the  well-known  fact  that  the 
numbers  of  bacteria  in  soils  are  lower  than  would  be  expected 
and  never  rise  beyond  a certain  amount  under  natural 
conditions.  If  now  the  soil  is  heated,  or  treated  with  volatile 
antiseptics,  or  dried  for  a long  time  under  proper  conditions, 
or  treated  in  certain  other  ways,  the  destructive  organisms, 
being  the  more  sensitive,  are  killed.  Many  if  not  all  the  active 
forms  of  the  bacteria  are  also  killed,  but  not  the  spores,  and  these 
soon  give  rise  to  a bacterial  flora  which,  being  freed  from  their 
enemies,  can  multiply  to  an  extent  previously  impossible,  and 
can  make  more  plant  food,  thus  increasing  the  productive 
power  of  the  soil  in  spite  of  the  fact  that  the  partial  sterilisation 
process  has  rather  weakened  them. 
Sooner  or  later  all  the  results  of  scientific  investigations  in 
agriculture  must  find  a place  somewhere  in  practice,  and 
already  this  new  work  has  offered  satisfactory  explanations 
of  phenomena  long  recognised  by  the  cultivator  of  the  soil, 
but  hitherto  inexplicable.  It  has  been  known  from  time 
immemorial  that  heating  the  soil  improved  its  productive 
power.  Virgil  records  the  fact  in  the  following  lines  in  his 
Georgies,  written  nearly  two  thousand  years  ago  : “ Often,  too, 
it  has  proved  good  to  burn  barren  fields,  and  to  set  fire  to  the 
stubbles  with  crackling  flames.  Either  the  soil  thus  acquires 
some  hidden  strength  and  rich  nourishment,  or  else  every  fault 
is  baked  out  of  it  and  the  useless  moisture  sweats  out,  or  the 
heat  opens  up  passages  and  hidden  pores  by  which  the  juices 
find  their  way  into  the  young  shoots,  or  else  heat  strengthens 
the  soil  and  contracts  the  gaping  cracks  lest  the  penetrating 
showers  should  harm  them,  or  the  fierce  power  of  the  scorching 
sun  or  the  searching  cold  of  the  north  wind  should  sear  them.” 
(Book  1,  lines  84  et  seq .) 
Heating  the  soil  is  to  this  day  practised  by  the  natives  of 
India.  In  the  rice-growing  districts  of  Bombay  a layer  of 
grass,  branches,  or,  best  of  all,  a plaster  made  of  cow  dung  and 
chopped  straw  is  spread  on  the  soil  and  slowly  burnt  before 
the  monsoon  ; this  is  a regular  practice  and  is  known  as 
rah d It  is  mentioned  in  some  of  the  ancient  literature  of 
1 F.  Fletcher:  Nature , April,  1910,  page  156.  Mr.  Fletcher  shows  that 
the  effect  is  neither  to  supply  plant  food  from  the  ashes  nor  to  improve  the 
physical  condition  of  the  soil. 
