814 
Supplement  to  the  “ Tropical  AgricuUuristr 
[May  1,  1897. 
most  of  the  potash  is  absorbed  in  the  form  of 
sulirhates  and  phosj)hates,  and  probably  also  as 
silicate.  In  whatever  form  it  is  absorbed  it  has 
at  any  rate  been  iiroved  beyond  doubt  to  be 
absolutely  necessary  for  the  growth  of  cro]is. 
It  is  a generally  accepted  principle  of  manur- 
ing that  potiish  manures  need  not  be  applied  to 
cliiy  soils  which  tire  considered  to  always  contain 
a sulliciencey  of  the  constituent,  .\ctual  analyses 
and  experiments  on  the  Held  h.ive  ^hown,  how. 
ever,  that  this  reasoning  is  not  quite  reliable 
Though  the  jirestnce  of  a fair  jiroiiortion  of  clay 
in  a soil  may  be  ticcepted  as  a guarantee  of  the 
presence  of  potash,  it  i'  noi  always  so.  There  are 
soils  containing  much  clay  that  do  not  contain 
much  potash.  It  has  been  found  by  exiieriments 
that  potash  manures  produced  a large  increase  of 
crop,  though  the  field  to  which  the  manures  were 
applied  wa.'^  of  a stiff  and  cl  lyey  nature,  such  as 
would  be  described  as  a stiff  clay  loam. 
Again,  potash  may  be  present  without  being 
effective.  The  ordinary  methods  of  chemical 
analysis  can  determine  the  amount  of  potash  in 
the  soil,  but  of  that  amount  it  may  happen  that 
the  greater  jjropotion  exists  in  a condition  in 
which  crops  are  unable  to  make  u.se  of  it. 
The  ordinary  solvent  agencies  in  (he  soil  have 
but  a limited  power  of  dissolving  insoluble  com- 
pounds in  which  it  cliielly  exists,  and  that  too 
only  on  the  limited  region  of  the  soil  particles 
with  which  they  come  into  the  most  intimate 
contact.  Hence  a soil  may  be  rich  in  p )tash, 
and  yet  that  potash,  or  the  greater  part  of  it, 
may  be  ([uite  unavailalile,  or  be  av.iilable  in 
quite  insullicieiit  quantity  for  the  needs  of  the 
growing  crop  which  migiit,  therefore,  largely 
benefit  by  the  apj)lication  of  an  artificial  potash 
manure,  llefore  it  can  be  safely  concluded,  from 
its  composition  only,  that  a soil  will  not  be 
benefited  by  p'otash  manures,  it  is  necessary  to 
know  not  only  that  the  soil  contains  much 
potash,  but  that  it  contains  it  in  a form  in 
which  it  is  readily  availalile  lor  the  u.se  of  plants. 
Some  soils  cnntaiu  as  much  as  d or  4 per  cent 
and  more  of  potash,  wdiile  the  soils  >1411011  are 
generally  poorest  in  this  constituent  are  sandy 
and  peaty  soils,  and  these  latter  are  generally 
found  to  produce  a considerate  increase  of  croji 
by  the  use  of  potash  manures. 
The  variation  in  the  amount  of  potash  .=olul)le 
in  water  in  a soil  usually  ranges  between 
a thou.sandth  and  a huudreth  par  cent.  The 
remainder  is  usually  pre.-eut  as  hard  insoluble 
compounds  which  are  not  available  for  ihi  plant’s 
needs,  and  only  very  gradually,  wilJi  the  lap.se 
of  time,  become  so.  Tliere  are,  however,  certain 
compounds  which,  while  not  solulile  in  water, 
are  believed  to  lie  more  or  less  avail.ible  for 
the  plant.  Tlie  water,  in  tlie  soil  l).v  virtue  of 
carbonic  aci'l  and  ciu-taiu  otlier  organic  acids 
which  are  dissolved  in  it,  has  a greater  solvent 
power  on  miner.d  substances  than  pure  water, 
and  this  solvent  power  is  further  aided  by  the 
acid  nature  of  the  juices  in  plant  roots.  A 
striking  instance  of  the  j)Ower  of  roots  to  dissolve 
insoluble  substance's  can  be  S(;en  in  nature  by 
tracing  the  roots  of  such  plants  as  ferns  that 
appear  to  cling  to  the  bare  surface  of  rocks. 
Dr.  llernard  Dyer,  who  has  biam  endeavoring  to 
estimate  the  di.ssolving  ]iower  of  .^oil  water  and 
plant  roots  on  mineral  substances  such  as  potash, 
has  come  to  tl.e  conclusion  after  a large  number 
of  determination.s.  that  the  sap  acidity  of  roots 
is  equal  to  a one  per  cent  solution  of  citric  acid. 
Testing  certain  soils  wdth  a solution  of  this 
strength  he  has  found  that  the  amount  of  potash 
available  is  very  much  greater  than  that  indicated 
by  testing  it  in  pure  water.  Thus  in  two  soiks, 
in  which  the  total  amount  of  potash  present  was 
about  14  per  cent,  the  available  potash  was  four 
thousandth  and  three  hundredth  of  a per  cent 
respectively.  This  is  much  more  than  could  be  dis- 
solved by  pure  water,  yet  this  result  also  shows 
that  only  a very  small  proportion  of  the  total 
potash  in  the  soil  may  be  regarded  as  immediately 
available  to  plants.  Of  this  small  proportion 
available,  it  must  be  noted  also  that  a loss  by 
drainage  of  the  soluble  potash  in  the  soil  is  con- 
stantly going  on  to  a greater  or  less  extent. 
In  our  next  issue  we  shall  continue  our  notice  of 
the  interesting  pamphlet  referred  to  by  Dr. 
Aikraan  and  Prof.  Wright,  so  as  to  place  the  latest 
im formations  with  reference  to  potash  in  soils 
and  potash  manures  at  the  disposal  of  our  readers. 
lUIBBPR  GROWING. 
The  Australian  T ropiculturisf  for  March  has  a 
special  article  on  this  subject,  refer>ing  to  the  in- 
dustry as  a new  fortune-promising  one  and  calling 
upon  tlie  Government  to  plant  np  all  available 
costal  areas,  since  “ flat  alluvial  river  lands  are 
regarded  as  the  best  soil  for  Indiariibber  trees.’’ 
Regarding  Castilloa,  it  is  said  that  the  tree 
thrives  best  in  moist  but  not  marshy  forests, 
on  a warm  sandy  clay.” 
“ For  each  plant  a hole  should  be  dug  3 ft. 
in  diameter,  and  1 ft.  deep,  and  filled  with  fine 
loamy  soil  to  which  a little  sand  has  been 
added.  Tlie  lui.xture  should  be  well  trodden 
down  and  watered  night  and  morning  for  two 
day.s,  when  it  is  ready  for  the  young  Castilloa, 
which  must  be  placed  in  its  new  bed  at  exactly 
the  same  depth  as  in  the  nursery ; if  it  is 
weak,  a stake  support  is  very  desirable.” 
To  this  might  be  added  that  the  young  plants 
should  be  at  least  1.5  feet  apart.  But  it  cannot 
be  reiterated  too  often  that  the  locality  must 
be  a sheltered  one. 
A curious  fact  about  the  Castilloa  is  that 
frequently  natural  ropes  of  rubber  are  found 
hanging  from  the  tree  formed  by  tapping 
of  the  bark  by  wood-peckers,  the  exudation 
of  the  creamy  sap  tliat  coagulates  as  it  trickles 
slowly  down. 
The  method  of  harvesting  the  sap  is  as  follows  : — 
Immediately  below  the  first  brancli  a horizontal 
canal  is  cut,  V shaped;  from  the  point  of  the 
\ downwards  a perpendicular  canal  is  cut  till 
it  joins  auotlier  liorizontal  V,  and  so  on  down 
to  the  ground.  The  sap  is  guided  into  a receiver, 
and  the  water  in  it  evai)oratos.  Evaporation 
may  be  assisted  by  artificial  heat,  and  coagulation 
is  forced  by  an  agent.  Then  Indiariibber  is 
formed.  The  canals  should  bo  jilastered  with 
mud  or  clay,  to  protect  the  life  of  the  tree. 
Tlie  quantity  ot  rubber  produced  from  tlie  sap 
depends  largely  on  the  coagulating  agent  used. 
Sixty  ])er  cent,  oiujht  to  be  converted  into  rubber. 
An  ounce  of  alum  in  a pint  of  water  (not 
lm])erial  pint)  is  a good  agent ; a weak  alcoholic 
solution  is  better,  bccau-se  epoudior. 
