THE  TROPICAL  AGRICULTURIST 
[Dec.  1,  i8q6 
4)8 
That  the  soil  plays  an  important  part  in  minister- 
ing to  the  growth  of  the  plant  has  always 
been  recognised,  although  many  and  erroneous 
have  been  the  opinions  in  the  past  held  with 
regard  to  what  that  part  is.  Wc  need  only  notice 
the  famous  theory  associated  with  the  name  of 
Jethro  Tull — a theory  which  exercised  such  an  im- 
portant influence  on  the  development  of  modern 
agriculture.  Tull,  who  was  a farmer  in  Berkshire,  was 
led  to  conclude  from  hi  sobservation  of  the  great 
benefit  which  followed  the  application  of  very 
through  cultivation  of  the  soil — such  as  is  pursued 
in  the  case  of  the  culture  of  the  vine  grape — that 
the  food  of  the  plant  was  derived  exclusively  from 
the  soil.  He  consequently  set  himself  to  advocate 
what  he  termed  “ horse-hoe  husbandry,”  a system 
of  husbandi’v  which  he  was  of  the  opinion  should 
facilitate  to  a very  great  extent  the  assimilation  by 
the  plant’s  roots  of  this  food.  The  crops  of  the 
field,  according  to  him,  should  be  shown  in  rows 
or  ridges,  wide  enough  apart  to  admit  of  the 
thorough  tillage  of  the  intervals  by  ploughing  and 
by  hoeing.  The  custom  of  applying  farmyard  manure, 
so  old  and  approved  a practice,  owed  its  success, 
he  held,  to  the  fact  that  in  its  fermentation  it 
helped  in  the  pulverisation  of  the  soil.  A thorough 
system  of  tillage  consequently  obviated  the  necessity 
of  the  application  of  manures. 
With  the  great  advance  in  our  chemical  kmw'- 
ledge  which  took  place  at  the  close  of  last  century, 
truer  ideas  began  te  prevail  on  the  subject  of  the  part 
played  by  the  soil  in  supporting  plant  growth.  Owing 
to  the  researches  of  such  men  as  Priestly,  Bonnet, 
Ingenhousz,  and  others,  the  source  of  the  largest 
organic  constituent  of  the  plant — viz.,  carbon —was 
traced  to  the  atmosphere ; w'hile  the  source  of  hydrogen 
and  oxygen  gradually  came  to  be  recognised  as  water. 
Attention,  therefoi'e,  came  to  be  centred  in  the  mineral 
or  ash  portion  of  the  plant ; and  this  was  shown  by  the 
Swiss  investigator,  De  Saussure,  in  an  investigation 
published  at  the  beginning  of  the  century,  to  come 
from  the  soil.  Sir  Humphrey  D.ivy  devoted  much 
attention  to  the  relation  of  the  soil  to  the  plant,  and 
generally  the  question  of  what  constitutes  fertility  in  a 
soil.  He  studied  what  we  have  already  referred  to  as 
its  physical  or  mechanical  properties,  and  showed  what 
an  important  bearing  these  had  on  the  question  of 
fertility.  He  may  be  said,  however,  to  have 
rather  overrated  the  influence  of  such  pro- 
perties. Moreover,  he  scarcely  recognised  at 
all  that  the  plant  drew  a certain  proportion 
of  its  food  from  the  soil.  In  his  view  the  chief 
function  of  the  soil  was  sinqily  to  act  as  the  me- 
chanical support  of  the  plant.  For  a consider.able 
number  of  years  little  progress  was  made  in  eluci- 
dating the  true  relations  between  the  soil  and  the 
plant,  and  it  i s not  till  lu'arly  the  middle  of  the  pre- 
sent century  that  any  important  advance  has  to  be 
chronicled.  At  that  time  wide.spread  interest  in  the 
subject  was  aroused  by  the  publication  of  Baron  Lie- 
big’s works,  the  first  of  which  appeared  in  1840  in 
the  form  of  an  address  to  the  British  Association, 
By  the  promulgation  of  his  famous  “ mineral  theory” 
he  drew  the  attention  of  the  agricultural  community 
for  the  first  time  to  the  really  important  functions 
discharged  by  the  soil  in  plant  growth.  He  re-stated 
the  doctrine,  taught  by  He  Saussure,  that  the  plant 
drew  the  mineral  matter  whicli  it  contained  from 
the  soil.  He  also  recognised  that  the  plant 
derived  its  nitrogen  to  a large  extent  also  from 
this  source,  but  this  element  he  considered  was 
derived  from  the  air  in  the  form  of  the  ammo- 
nia which  was  washed  down  by  the  rain  in 
sufficiently  largo  quantitie.s  to  supply  the  needs 
of  plants.  Liebig’s  teaeliiiig  was  of  the  highest  value, 
as  it  drew  attention  to  the  need  of  restoring  to  the 
soil  the  mineral  ingredients,  phosphoii;  acid  and 
potash,  the  two  substances  am. >ng  mineral  plant 
foods  which  are  alone  gonei-illy  [iresent  in  deficient 
amount,  and  which  for  thij  reason  vegu’ate  to  a 
largo  extent  the  rate  of  pl.uit  gr.>wt  ;.  lo  this  way 
he  may  be  said  to  have  Ixien  instrumental  in  start- 
ing ttie  practice  of  artificial  inamiring,  a practice 
which  has  done  so  much  to  revolutionise  modern 
agriculture.  He  also  di.scoverod  what  has  proved  to 
bo  a fact  of  enormous  importance  for  modern  agri- 
culture— viz.,  the  action  of  sulphuric  acid  on  phos- 
phate of  lime  in  rendering  it  more,  speedily  avail- 
able to  the  plant  as  food. 
Liebig’s  work  has  been  carried  on  and  developed 
by  a large  number  of  investigators,  among  whom 
may  be  specially  mentioned  Sir  John  B.  Lavves  and 
Sir  J.  Henry  Gilbert,  who  for  a period  of  more  than 
fifty  years  have  devoted  their  lives  to  the  elucidation 
of  many  problems  in  agricultural  chemistry,  and 
chiefly  to  the  inves'.igation  of  the  questions  of  the 
relation  of  tire  soil  to  the  plant  and  the  action  of 
fertilisers. 
Daring  the  last  thirty  years  the  amount  of  atten- 
tion devoted  to  the  subject  of  the  fertility  of  soils 
has  steadily  increased.  These  year.s  have  seen  the 
grow'th  and  development  of  large  numbers  of  agri- 
cultural research  stations,  where  the  nature  and 
functions  of  the  soil  has  formed  the  chief  subject 
of  roseavcli.  The  earliest  of  such  institutions  was 
that  at  Bochelbronn,  in  Alsace,  where  the  distin- 
guished French  agricultural  chemist,  Boussiugault, 
devoted  himself  to  the  question  of  investigating 
problems  connected  with  the  growth  of  the  com- 
moner agricultural  crops.  But  the  most  magnificent 
example  of  a research  station  of  this  kind  is  that 
furnished  by  Bothamsted.  This  research  station 
avas  started  more  than  fifty  years  ago,  and  has  been 
raaiiitaine.l  ever  since,  at  bis  own  private  expense, 
by  Sir  John  B.  Lawe.?,  Bart.  The  name  of  this 
famous  rese.arch  station  is  familiar  to  everyone  who 
takes  any  interest  in  the  scientific  aspect  of  agri- 
culture; and  the  example  of  scientific  collaboration 
furnished  by  the  work  of  Sir  John  Lawos  and  his 
cilleague,  Sir  J.  Henry  Gilbert,  is  quite  unique  in 
respect  of  the  length  of  time  over  wdiich  it  has 
extended. 
On  the  continent  and  in  America  there  are  a 
large  number  of  those  agricultural  experiment  sta- 
tions, where  the  study  of  the  soil  is  made  the 
chief  subject  of  investigation.  The  properties  of  a 
soil  which  influence  the  growtli  of  vegetation  upon 
it  may  bo  grouped  under  three  classes.  The  most 
obvious  function  of  the  soil  is  to  furnish  the  plant 
with  mechanical  support,  and  to  maintain  it  in  an 
upright  position.  It  must,  at  the  same  time,  be  of 
such  a nature  that  the  plant’s  roots  can  draw  from 
it  the  moisture,  nourishment,  and  air  which  they 
require  for  their  proper  growth.  If,  therefore,  it  is 
to  successfully  pertorm  this  function,  it  must  pos- 
sess certain  mechauicil  or  physical  properties,  as 
they  have  been  called,  amoug  the  most  important 
of  whicli  are  a certain  porosity,  and  a capacity  for 
absorbing  water  and  lieat.  Porosity  ii  of  import- 
ance for  several  reasons.  It  is  only  in  a porous 
soil  tliat  the  necessary  supply  of  air  reaches  the 
pl.rut’s  roits.  On  soils  where  tliis  is  not  the  case, 
as  in  water-logged  soils,  for  example,  vegetation 
cannot  flourish — at  any  rate  vegetation  of  a high 
type.  Again,  porosity  is  of  importauce  as  it 
enables  the  plant’s  roots  to  push  their  way  down- 
wards iu  the  soil.  Too  great  compactness  in  the 
soil  prevents  gcow'oh,  since  it  limits  the  spread 
of  the  roots.  The  more  deeply  rooted  and  the  wider 
the  area  over  which  the  roots  of  a plant  extend- 
Ihe  bettor  able  is  such  a plant  to  draw  its  nourishf 
meat  from  the  soil,  and  withstand  the  action  os 
drought  by  obtaining  its  water  from  the  lower  layere 
of  the  soil.  A still  fiuBior  reason  of  the  importance 
of  porosity  is  connected  with  the  niicro  organit 
life  of  the  soil,  since  for  the  proper  developmen- 
of  these  numerous  inicrobos,  many  of  which  dis, 
ch.i.rge  important  functions  iu  preparing  plant  food, 
a supply  of  air  is  a necessary  condition,  Jjastly 
the  rate  of  decomposition  of  the  material  of  the 
soil — inorganic  as  well  as  organic — depends  largely 
on  tli-3  aecoss  of  the  air  to  the  soil  pores.  Not  less 
important  are  the  potvers  posses.seJ  bv  the  soil  of 
absorbing  and  retaining  water  and  beat.  These 
p.)wcrs  are  uocc.isary  for  fertility,  .since  the  fertile- 
toil  must  be  able  to  absorb  and  store  up  water  and 
heat  against  periods  of  drought  and  cold.  The  condi- 
tions wliich  iulluonce  these  properties  are  the  stat 
