12  The  Production  of  Plant  Food  in  the  Soil. 
plants.  Under  normal  field  conditions  nitrate  production  or 
“ nitrification  ” is  so  rapid  that  the  plant  probably  gets  all 
its  nitrogen  as  nitrate.  But  this  is  by  no  means  essential  ; 
soils  go  on  producing  crops  even  after  nitrification  has  been 
artificially  stopped  and  only  ammonia  production  allowed  to 
continue.  There  is,  in  fact,  something  to  be  said  against  the 
conversion  of  ammonia  into  nitrates,  since  the  latter  readily 
wash  out  of  the  soil  while  the  former  do  not.  On  the  other 
hand,  ammonia  is  less  effective  as  a food,  giving  a smaller 
increase  of  crop  for  every  pound  of  nitrogen  assimilated  than 
nitrates  would  do. 
The  changes  brought  about  in  the  soil  by  micro-organisms 
may  be  summed  up,  so  far  as  fertility  is  concerned,  in  the  state- 
ment that  comparatively  useless  material  is  changed  into  valuable 
humus,  ammonia,  and  nitrate,  with  a certain  amount  of  waste. 
The  quicker  these  changes  go  on  the  more  productive  is  the  soil. 
Thus  we  arrive  at  an  important  conclusion,  the  raison  d'etre 
of  soil  bacteriology  as  an  agricultural  study  : the  quicker  the 
useful  micro-organisms  work,  the  greater  becomes  the  produc- 
tiveness of  the  soil  ; anything  that  px-omotes  their  activity 
increases  productiveness,  and  anything  that  retards  their 
activity  lessens  productiveness.  On  purely  practical  grounds, 
thexx,  we  can  justify  the  study  of  soil  micx'o-oi’ganisms.  If 
more  were  known  about  them  it  should  become  possible  not 
only  to  increase  their  activity  but  perhaps  even  to  conti’ol 
them  and  thus  to  increase  their  efficiency  as  plant  food 
makers.  As  Hall  has  pointed  out,  the  full  benefit  of  a dressing 
of  dung  is  never  obtained  in  practice,  but  if  only  a small  part 
of  the  waste  could  be  avoided  the  saving  in  the  aggregate 
would  be  enormous. 
As  yet  our  knowledge  of  the  soil  micro-organisms  is  far 
from  complete,  axid  we  can  form  only  a very  dim  and  hazy 
picture  of  what  they  are  actually  doing  in  the  top  six  inches  or 
so  of  earth  ixx  which  they  dwell.  Unfortunately  the  difficulty 
of  working  with  soil  under  the  microscope  is  so  great  that  no 
one  has  actually  been  able  to  watch  them  there,  and  so  recourse 
is  had  to  indirect  methods  of  study.  If  a little  soil  is  put  into 
water  containing  suitable  food  stuffs  the  organisms  multiply  so 
qixickly  that  they  can  be  seexx  in  di’ops  of  the  liquid  mounted 
under  a microscope,  or  they  can  be  studied  by  chemical 
methods.  A gx'eat  deal  has  beexx  leai’ned  in  this  way,  but  some 
skill  is  ixeeded  in  applying  the  kxxowledge  thus  gained  to 
actxxal  soil  coxxditions  for  one  vex-y  important  reason.  Many  of 
the  micro-organisms  can  live  in  two  states  ; as  active  forms, 
feeding,  growing,  multiplying,  and  brixxgixxg  about  their 
appointed  changes  ixx  the  soil  materials,  or  as  “ spore  ” foi’ins, 
ixot  xxxxlike  seeds,  ixx  which  their  vitality  seems  to  be  suspexxded 
