Vol.  XV.]  COLOMBO.  NOV  ist,  1895,  [No.  5. 
DO  PLANTS  ABSORB  NITROGEN  ? 
LTHOUGH  the  question  opened 
by  “ Student’s  " query  in  your 
last  issue  (page  231)  would 
demand  a special  article  deal- 
ing completely  with  the  ab- 
sorption of  nitrogen  by  plants, 
a short  note  may  be  of  some 
service. 
Briefly  stated,  plants  have  access  to  nitrogen  in 
cwo  conditions — t.e.  (1)  As  free  uncombined  nitrogen 
in  the  air;  (2)  In  a combined  state  (as  salts  of 
ammonium,  nitrates  and  organic  compounds  in  the 
soil).  A very  large  number  of  carefully  conducted 
experiments  have  shown  that  the  absorption  and 
assimilation  of  nitrogen  in  a free  state  is  confined 
to  the  lower  orders  of  plants— mainly  bacteria — and 
to  leguminous  plants  among  higher  green  vegetation. 
Wheat,  Rye,  Buckwheat,  cruciferous  plants,  and 
many  others  not  belonging  to  the  Leguminosas, 
always  die  of  nitrogen  starvation  when  grown  in 
soil  containing  very  little  or  no  nitrogen,  although 
they  may  be  exposed  to  the  air.  Moreover,  legumi- 
nous plants  die  under  these  conditions  irnless  their 
roots  are  possessed  of  fleshy  “ nodules  ” (well  seen 
on  the  roots  of  ordinary  Broad  Beans).  It  is  from 
the  combined  forms  that  an  ordinary  green  plant— 
Wheat,  for  example— obtains  all  the  nitrogen  it 
possesses,  and  this  it  takes  up  means  of  its  roots- 
hairs  from  the  ground. 
In  the  soil  the  plant  has  access  to  nitrogen  in  (1) 
complex  organic  compounds,  resulting  from  partial 
decay  of  vegetable  dr  animal  remains  ; (2)  ammonium 
compounds  (c.;/.,  ammonium  carbonate,  sulphate ; 
and  also  (3)  nitrates,  chiefly  of  sodium,  potassium, 
magnesium,  and  calcium.  It  was  formerly  supposed 
that  the  ammonium  compounds  supplied  plants  with 
all  the  nitrogen  necessary  for  growth,  but  definite 
experiments  have  shown  that,  although  many  green 
plants  can  be  nourished  by  both  organic  compounds 
and  pure  ammonium  salts,  the  results  are  in  every 
way  inferior  to  those  experiments  where  the  plants 
are  supplied  with  nitrates  to  their  roots.  These 
faxjts,  coupled  with  the  knowledge  that  both  organic 
compounds  and  ammonium  compounds  soon  give  rise 
to  nitrates  in  tbe  soil,  lead  to  the  conclusion  that 
plants  absorb  or  take  up  their  nitrogen  from  nitrates, 
and  seeing  that  nitrate  of  lime  is  most  abundant 
in  the  soil  it  is  concluded  that  this  substance  is 
the  main  source  of  nitrogen  for  plants. 
“ Student  s ” difficulty  lies  in  tbe  assumption,  or 
statement,  that  nitrate  of  Ihne  is  taken  into  the 
plant  as  such — that  is  without  any  change.  If  this 
were  true  we  should,  as  he  remarks,  expect  to  find 
the  lime  (neglecting  the  other  baser,  potash,  soda, 
and  magnesia),  and  nitrogen  in  something  like  the 
same  chemically  equivalent  proporticais  as  met  with 
in  nitrate  of  lime.  This  is  found  not  to  he  the 
case.  There  is  an  excess  of  nitrogen  and  a deficiency 
of  lime,  as  seen  in  the  analysis  quoted,  and  this 
has  led  “ Student’s  ” query  and  suggestion  that  the 
nitrogen  must  have  been  obtained  from  other  sources 
than  nitrates.  The  latter  view  has  been  amply  re* 
futed  by  most  careful  experiments,  and  the  explana* 
tion  of  the  apparent  discrepancy  is  that  the  nitrate 
of  lime  is  split  up  and  decomposed  at  the  very 
threshold  of  entry  into  the  plant — i.e.,  in  the  root- 
hairs  and  in  the  roots  which  are  not  included  in 
the  analyses  given,  which  soon  dry  up  and  remain 
in  the  soil.  The  lime  is  thus  separated  and  practi- 
cally left  in  the  soil,  while  the  nitrogen  enters  into 
new  combination,  and  helps  to  buUd  up  various 
more  or  less  complex  organic  compounds.  The  ratio 
of  the  lime  and  other  bases  to  the  nitrogen  in 
different  plants  grown  even  upon  the  same 
soil,  and  thus  having  equal  access  to  nitrates,  varies 
much. 
The  details  of  the  chemical  changes  which  nitrogen 
undergoes  after  entering  the  plant  and  where  these 
changes  take  place  are  practically  unknown  yet. 
The  changes  are  slow,  compared,  for  example,  with 
those  undergone  by  carbonic  dioxide,  and  difficult 
to  follow.  Different  kinds  of  plants,  growing  with 
equal  access  to  nitrates  in  the  soil,  -show  very 
different  results  as  regards  their  method  of  taking 
up  and  utilising  these  compounds.  In  some  cases 
the  nitrates  can  be  readily  detected  as  such  in  all 
parts  of  the  plant ; in  others  only  in  the  stem,  or 
perhaps  only  in  the  root,  or  in  no  part  at  all.— 
John  Percival.— /ow  nai  of  Ilorticiilture, 
