﻿THE 
  BACTERIA 
  

  

  205 
  

  

  preceding 
  section 
  the 
  relation 
  of 
  decay 
  to 
  soils 
  was 
  suggested. 
  

   Although 
  the 
  relation 
  of 
  plants 
  and 
  soils 
  is 
  more 
  fully 
  dis- 
  

   cussed 
  in 
  a 
  later 
  chapter, 
  it 
  is 
  advisable 
  at 
  this 
  point 
  to 
  discuss 
  

   briefly 
  the 
  relation 
  of 
  bacteria 
  to 
  soils. 
  Their 
  methods 
  of 
  living 
  

   bear 
  peculiar 
  relations 
  to 
  the 
  nitro- 
  

   gen 
  supply 
  of 
  the 
  soil. 
  

  

  There 
  are 
  at 
  least 
  four 
  groups 
  

   of 
  soil 
  bacteria 
  which 
  are 
  of 
  interest 
  

   in 
  this 
  connection. 
  First, 
  there 
  are 
  

   saprophytic 
  forms 
  which 
  in 
  their 
  

   processes 
  of 
  nutrition 
  make 
  a 
  com- 
  

   pound 
  of 
  nitrogen 
  and 
  hydrogen 
  

   which 
  is 
  known 
  as 
  ammonia. 
  This 
  

   bacterial 
  action 
  is 
  known 
  as 
  ammon- 
  

   ification, 
  which 
  means 
  " 
  ammonia- 
  

   making," 
  and 
  the 
  bacteria 
  which 
  are 
  

   responsible 
  for 
  the 
  action 
  are 
  called 
  

   the 
  ammonification 
  bacteria. 
  Sec- 
  

   ond, 
  there 
  are 
  the 
  so-called 
  nitrite 
  

   bacteria, 
  which 
  in 
  their 
  process 
  

   of 
  nutrition 
  change 
  ammonia 
  into 
  

   other 
  compounds 
  in 
  which 
  there 
  is 
  

   one 
  part 
  of 
  nitrogen 
  to 
  every 
  two 
  of 
  

   oxygen. 
  Such 
  compounds 
  are 
  called 
  

   nitrites. 
  Third, 
  there 
  are 
  the 
  ni- 
  

   trate 
  bacteria, 
  which 
  change 
  nitrites 
  

   into 
  compounds 
  in 
  which 
  there 
  is 
  

   one 
  part 
  of 
  nitrogen 
  to 
  every 
  three 
  

   of 
  oxygen. 
  Such 
  compounds 
  are 
  

   known 
  as 
  nitrates. 
  These 
  last 
  two 
  

   processes 
  are 
  spoken 
  of 
  as 
  nitrifica- 
  

   tion. 
  Fourth, 
  there 
  is 
  a 
  still 
  different 
  and 
  most 
  important 
  

   group 
  of 
  bacteria 
  which 
  enter 
  the 
  roots 
  of 
  certain 
  kinds 
  of 
  

   plants, 
  as 
  clover, 
  soy 
  beans, 
  peas, 
  and 
  alfalfa. 
  When 
  some 
  of 
  

   these 
  bacteria 
  have 
  entered 
  the 
  roots, 
  they 
  become 
  surrounded 
  

  

  1 
  From 
  " 
  Nitrogen 
  Bacteria 
  and 
  Legumes," 
  Bulletin 
  94, 
  S.C. 
  Exp. 
  Sta. 
  

  

  FIG. 
  167. 
  Bacterial 
  tubercles 
  

   on 
  the 
  roots 
  of 
  the 
  cowpea 
  l 
  

  

  A, 
  leaves 
  of 
  the 
  cowpea 
  ; 
  B, 
  part 
  

   of 
  the 
  root 
  system 
  of 
  the 
  cow- 
  

   pea, 
  with 
  numerous 
  tubercles, 
  

   the 
  tubercles 
  being 
  induced 
  by 
  

   bacteria 
  which 
  gather 
  nitrogen 
  

   from 
  the 
  air 
  of 
  the 
  soil. 
  One 
  

   third 
  natural 
  size 
  

  

  