20  Some  Secondary  Actions  of  Manures  upon  the  Soil. 
the  ammonia  to  themselves  to  supply  the  nitrogen  they  reqirire 
for  nutrition.  This  explanation  applies,  however,  only  to  the 
Rothamsted  soils  ; the  authors  have  not  examined  the  Woburn 
arable  soils  from  this  point  of  view,  and  indeed  Dr.  J.  A. 
Voelcker  has  indicated  certain  differences  in  the  nature  of  the 
acidity  observed  there.  At  Rothamsted  the  acid  soils  have 
not  been  rendered  absolutely  sterile  ; the  ground  is  still 
covered  by  herbage,  but  it  has  a very  unhealthy  appearance 
and  resembles  in  the  most  interesting  manner  the  vegetation 
of  naturally  acid  soils.  The  grasses  have  a characteristic 
dark  ugly  colour  and  grow  in  tufts  with  bare  spaces  between, 
the  surface  of  the  ground  in  these  bare  spaces  being  occupied 
by  a layer  of  peaty  vegetable  matter,  as  though  the  dead  grass 
had  been  unable  to  decay  in  the  normal  manner.  Half  of 
each  of  the  plots  has  been  limed — 2,000  lb.  per  acre  of  ground 
lime  having  been  applied  in  January,  1903,  and  again  in 
January,  1907.  Table  II.  shows  the  great  increase  of  crop 
which  has  followed  the  liming,  the  effect  of  which  is  also 
seen  in  the  restoration  of  the  herbage  to  a normal  appearance 
and  a close  sward,  accompanied  by  the  disappearance  of  the 
peaty  layer. 
Table  II. — Effect  of  Lime  upon  Rothamsted  Grcoss  Plots. 
Relative  yield  of  hay  on  the  limed  portions,  the  uulimed  part  being  taken 
as  100. 
Plot 
Manuring 
1903 
1904 
1905 
1906 
1907 
1908 
1909 
4/2 
400  lb.  ammonium  salts + super 
124 
111 
1.34 
118 
113 
127 
237 
9 
„ „ „ + complete  minerals 
121 
no 
142 
128 
106 
118 
150 
n/1 
600  1b.  „ „ + 
115 
103 
206 
120 
167 
115 
119 
The  cause  of  the  comparative  infertility  of  the  acid  soils 
must  be  set  down  to  the  fact  that  they  are  permeated  by  the 
micro-fungi  which  can  grow  in  an  acid  medium,  whereas  the 
bacteria  which  normally  people  the  soil  and  require  a neutral 
medium  for  their  growth  are  largely  pushed  out.  The  fungi 
in  question  compete  with  the  higher  plants  like  the  grass  for 
the  manure  applied  to  the  soil,  and  being  active  and  abundant 
they  take  so  much  that  the  crop  suffers.  At  the  same  time 
the  higher  plants  are  doubtless  injuriously  affected  by  the 
suppression  of  many  kinds  of  bacteria  which  are  useful  in 
preparing  food  for  the  crop.  For  example,  the  nitrifying 
bacteria,  which  change  ammonia  in  the  soil  first  into  nitrites 
and  then  into  nitrates,  are  usually  regarded  as  necessary  to  the 
nutrition  of  the  plant,  yet  are  wholly  inhibited  by  a very 
slightly  acid  medium.  A number  of  experiments  were  made 
to  ascertain  if  nitrification  was  still  going  on  in  these  acid 
